topic 8 chemistry of carbon compounds 31

92

Topic 8 Chemistry of Carbon Compounds

3131UnitUnit

31.1 Introduction

Synthesis of carbon compounds is a major activity in industrial research laboratories because it allows chemists to make new molecules, which may have useful properties, from readily available molecules.

Suppose we need to plan a synthetic route to prepare propanone from 2-bromopropane.

Typical reactions of selected functional groups

HH

2-bromopropane propanone

C C H

H

H

O H

C

H

H

C

Br

H

C

H

H

C

H

H

?

The synthesis may take two steps as follows:

H C C C

H

H

H H H

Br H

2-bromopropane propan-2-ol

H C C C

H

H

H H

OH H

propanone

H C C C

H

H

H H

O H

H

To be able to design such a synthesis, we require a good knowledge of typical reactions of the functional groups concerned (i.e. –Br,

–OH and C

O

) and the conversion of one functional group into another.

In this unit, we are going to discuss important reactions of members of some homologous series. This will enhance your knowledge of typical chemical reactions of selected functional groups.

93

Unit 31 Typical reactions of selected functional groups

31.2 Important reactions of alkanes

We have discussed two important reactions of alkanes in Topic 7 Fossil Fuels and Carbon Compounds. These are combustion and substitution reactions.

Combustion of alkanes

In a good supply of oxygen, alkanes undergo complete combustion to produce carbon dioxide and water, and release much heat. The following equation represents the complete combustion of methane, CH4.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Reaction with halogens — substitution reactions

The reaction of an alkane with chlorine occurs when a mixture of the two is irradiated with ultraviolet light or heated. For example, methane reacts with chlorine to yield a mixture of carbon compounds. The reaction of an alkane with a halogen is a substitution reaction, called halogenation.

CCl4(l) + HCl(g)

CHCl3(l) + HCl(g)

CH2Cl2(l) + HCl(g)Cl2

Cl2

Cl2

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)UV light

or heat

UV lightor heat

UV lightor heat

UV lightor heat

combustion 燃燒 substitution reaction 取代反應 halogenation 鹵化作用

Alkanes react with bromine in a similar manner.

The products of the reactions between alkanes and halogens are known as haloalkanes.

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Chlorination of methylbenzene takes place in the gas phase in the presence of ultraviolet light or when heated.

methylbenzene (chloromethyl)benzene (dichloromethyl)benzene (trichloromethyl)benzene

CH3 CH2Cl CHCl2 CCl3

(g)UV lightor heat

Cl2(g)

UV lightor heat

Cl2(g)

UV lightor heat

Cl2(g)

31.3 Addition reactions of alkenes

In Topic 7, we learnt that the most characteristic reaction of compounds containing carbon-carbon double bonds is an addition reaction. When a symmetrical reagent X — X adds to a carbon-carbon double bond, each X atom becomes attached to one carbon atom of the double bond. The double bond changes to a single bond.

When an unsymmetrical reagent X — Y adds to a carbon-carbon double bond, the X atom becomes attached to one carbon atom of the double bond and the Y atom becomes attached to the other.

C C

X X

addition reactionXX+CC

C C

X Y

YXaddition reaction

+CC

Addition of hydrogen and halogens are examples of addition of a symmetrical reagent. Addition of hydrogen halides are examples of addition of an unsymmetrical reagent.

addition reaction 加成反應

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31.5 Addition of halogens to alkenes

In Topic 7, we have already seen that when we mix bromine (dissolved in an organic solvent) with ethene, they react immediately to form 1,2-dibromoethane. Chlorine also undergoes addition reaction with ethene.

C C

Cl Cl

Cl Cl(g)

H H

H H

H H(l)

H H

(g) +

1,2-dichloroethane

C C

31.4 Addition of hydrogen to alkenes

In the presence of a catalyst, hydrogen adds across a carbon-carbon double bond of ethene to form ethane which is a saturated compound.

If a platinum or palladium catalyst is used, the process takes place under normal laboratory conditions. Nickel is a cheaper but less efficient catalyst. It needs to be finely powdered and the gases need to be at 150 °C and 5 atmospheric pressures for hydrogenation to occur.

hydrogenation 加氫作用

(g) + C C

H H

Pt catalystH H(g)

H H

H(g)H

ethaneethene

H H

H H

C C

Ethene reacts explosively with fluorine, but the reaction with iodine is rather slow.

96


Test for unsaturation

Often the test for an alkene involves shaking the substance under test with aqueous bromine rather than pure bromine. Decolorization of the aqueous bromine occurs if an alkene is present. In the case of ethene, a mixture of 2-bromoethanol, 1,2-dibromoethane and hydrogen bromide is obtained.

BrBr

C C

Br Br

C C

Br OH

H H +

H H

2-bromoethanol

H H

H H

H H

H H

1,2-dibromoethane

in water

in organic solvent

C C

Br Br

H H + HBr

H H

1,2-dibromoethane

+C C

If the aqueous bromine is dilute, 2-bromoethanol will be the main product of the reaction. This does not affect what you see — the aqueous bromine still changes from yellow-brown to colourless (Fig. 31.1).

Fig. 31.1 Decolorization of aqueous bromine by ethene

Remember that we can also use the addition reaction of an alkene with cold acidified dilute potassium permanganate solution as a test for alkenes. The purple permanganate solution changes from purple to colourless quickly when shaken with an alkene.

97


31.1 Studying the addition reactions of alkenes

Write an equation for each of the following reactions and name the product in each case.

a) Methylpropene with bromine (in organic solvent)

b) Cyclopentene with hydrogen

c) Cyclohexene with bromine (in organic solvent)

Solution

a)

b)

c)

C CH H (in organic solvent)

H

Br Br

C

H

H1,2-dibromo-2-methylpropane

C CH H(g) + Br2 (in organic solvent)

H

C

H

H

CH3 CH3

Pt catalyst

cyclopentane

(l) + H2(g) (l)

Br

1,2-dibromocyclohexane

(l) + Br2 (in organic solvent)

Br

(in organic solvent)

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31.6 Addition of hydrogen halides to alkenes

Ethene reacts with gaseous hydrogen halides at room temperature.

1 Identify A, B and C in the following reactions.

a)

b)

c)

2 Write an equation for each of the following reactions:

a) hexa-1,4-diene with excess hydrogen; and

b) cyclohexa-1,3-diene with bromine (in organic solvent).

Br2 (in organic solvent)CH3CH2CH AC

CH2CH3

CH2CH2CH3

H2 / PtB

CH2CH3

C OH

CH2BrCH2

Ethene also reacts with concentrated aqueous solutions of hydrogen halides in the cold.

Br(g) C C

bromoethane

H H

H H

(g) + H H H(l)

H H

H Br

C C

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When a molecule HA adds to an asymmetric alkene, the major product is the one in which the hydrogen atom attaches itself to the carbon atom already carrying the larger number of hydrogen atoms.

Some asymmetrical alkenes are shown below.

When a hydrogen halide adds to an asymmetrical alkene, two products are possible.

For example, the addition of hydrogen bromide to propene could give either 1-bromopropane or 2-bromopropane.

In fact, the product is almost entirely 2-bromopropane. The Russian chemist, Markovnikov, formulated a rule for predicting the major addition product formed. Markovnikov’s rule says that:

Markovnikov’s rule 馬科尼科夫規則

C2H5CH3

CH3

C

H

C

H

H

propene

CH3

C C

H

2-methylpent-2-ene 1-methylcyclohexene

CH3

CH3

H H

H

(g) + H Br(g)

C C

1-bromopropane

H H(l)

H H

H Br

C

H

H

C C

2-bromopropane

H

H H

Br H

C

H

H

H(l)

C C

100


Thus, for the reaction between propene and hydrogen bromide, 2-bromopropane is the major product.

For the reaction between 2-methylpent-2-ene and hydrogen bromide, 2-bromo-2-methylpentane is the major product.

For the reaction between 1-methylcyclohexene and hydrogen bromide, 1-bromo-1-methylcyclohexane is the major product.

CH3

Br(g)

H

H H

(g) + H C C

major product

H H(l)

H H

Br H

C

H

H

C C

minor product

H

H H

H Br

C

H

H

this carbon atom carries more hydrogen atoms

+C C H(l)

CC

HCH3

C2H5CH3

(l) + Br(g)H C C

HCH3

Br H

CH3 C2H5(l)

this carbon atom carries morehydrogen atom

major product

+ C C

HCH3

H Br

CH3 C2H5(l)

minor product

(l) + Br(g)H

this carbon atom carries morehydrogen atoms

major product minor product

CH3

(l) +

CH3

Br

CH3

H

Br

(l)

101


31.2 Predicting the major products of the addition of hydrogen halides to alkenes

Predict the major product of each of the following reactions:

a) b) c)?

CH2 HBr?

HCl

(D is deuterium, an isotope of hydrogen)

CC

C2H5H

CH3H

?DBr

Fig. 31.2 summarizes the addition reactions of alkenes.

RCH CH2

alkene

RCHBrCH2Br

RCHCH3

RCHCH2

Br

OH OH

diol

R H

C C

H H

polyalkene

bromoalkane

dibromoalkane

RCH2CH3

alkane

RCHCH2Br

OH

bromoalcohol

Br2

(in organic solvent)

H2(g)Pt catalyst

polymerization in the plastics industry

cold alkalinedilute potassiumpermanganatesolution

HBr(g) Br2(aq)

n

Fig. 31.2 A summary of the addition reactions of alkenes

102


Solution

a) b)

c)

31.3 Preparing a haloalkane from an alkene

What alkene would you start with to prepare the following haloalkane? There may be more than one possibility.

CH2HBr

CH3

Br


HClCl

CC

C2H5H

CH3H


DBrC

H

H

D Br

C

C2H5

CH3

? CH3CH2CCH2CH2CH3

CH3

Cl

CH3

CH3CH CCH2CH2CH3 or CH3CH2C CHCH2CH3 or CH3CH2CCH2CH2CH3

CH3

CH3

CH3CH2CCH2CH2CH3

Cl

HCl

CH2

Solution

The haloalkane can be prepared by the reaction of an alkene with HCl. The carbon atom bearing the Cl atom in the haloalkane must be one of the carbon atoms in the carbon-carbon double bond of the reactant.

There are three possibilities, any one of them could give the desired product.

103


1 Predict the major product of each of the following reactions.

a) b) CH3CH2CH2CH=CH2 + HCl

2 Consider the reactions of 1-ethylcyclopentene shown below:

a) Give the reagent(s) and condition(s) for the conversion of 1-ethylcyclopentene to ethylcyclopentane.

b) Draw the structural formula of the product X.

3 Draw the structural formula of an alkene you would start with to prepare each of the following haloalkanes.

a) b)

C CH2

CH3

CH3

+ Hl

CH2CH3ethylcyclopentane

major product XHBr1-ethylcyclopentene

?

CH3CH2CHCH2CH2CH3

Br CH2CH3

I

hydrolyzed 被水解

31.7 Substitution reactions of haloalkanes

Substitution reactions are typical reactions of haloalkanes. For example, a substitution reaction takes place between a haloalkane and hydroxide ions, in which the haloalkane is hydrolyzed to form an alcohol. Take 1-bromobutane, for example:

CH3CH2CH2CH2Br(l) + OH–(aq) CH3CH2CH2CH2OH(aq) + Br–(aq)

104


Haloalkanes are usually hydrolyzed by heating under reflux with sodium hydroxide solution (Fig. 31.3). Often ethanol is added to act as a solvent. These substitution reactions can be used to prepare alcohols.

In the reflux condenser, all the vapour is condensed back into the flask. This prevents any loss of the reaction mixture*.

For a given alkyl group, the iodo compound reacts most readily, the bromo compound less so and the chloro compound reacts less readily. This is because the carbon-halogen bond becomes progressively stronger from I to Cl.

Order of reactivity: RI > RBr > RCl

The C–F bond is so strong that the fluoroalkanes are extremely unreactive.

Fig. 31.3 Apparatus used for heating an haloalkane with sodium hydroxide solution under reflux

R X

haloalkane

R OHNaOH(aq)

alcoholreflux

The boiling points of haloalkanes are not high. We have discussed this in Unit 29.

105


31.4 Comparing the rate of hydrolysis of haloalkanes

A student carried out an experiment to compare the rate of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane.

A general equation for the hydrolysis is:

OH + H+ + X–X + H2OR R

The experiment consisted of three steps.

Step 1 Three solutions (each containing 2 cm3 of ethanol and 1 cm3 of dilute silver nitrate solution) were put into different test tubes.

Step 2 The test tubes were placed in a water bath at 60 °C.

Step 3 5 drops of 1-chlorobutane, 1-bromobutane and 1-iodobutane were added separately to the test tubes.

The student found that a silver halide precipitate appeared in each test tube.

silver chloride produced by

1-chlorobutane

silver bromide produced by

1-bromobutane

silver iodide produced by 1-iodobutane

106


31.8 Reactions of alcohols

There are three main categories of alcohols: primary (1°), secondary (2°) and tertiary (3°). This classification is based on the number of alkyl groups attached to the carbon bearing the hydroxyl group (–OH). A primary alcohol has one alkyl group attached, a secondary alcohol has two alkyl groups attached and a tertiary has three alkyl groups attached.

For convenience sake, methanol (CH3OH) is classified as a primary alcohol, even though it has no alkyl group attached to the carbon bearing the –OH group.

R1

R2

R1

OH OH

C OH

H

R

H

C

H

R

secondary (2°) alcohol

CR

tertiary (3°) alcohol

C OH

H

H

H

primary (1°) alcohol*

a) What was the purpose of adding silver nitrate solution to each reaction mixture?

b) The student found that the precipitate appeared first in the test tube containing 1-iodobutane, followed by 1-bromobutane and then 1-chlorobutane. Explain this order.

Solution

a) The silver nitrate solution was added to follow the rate of hydrolysis of the haloalkanes. Halide ions produced from the hydrolysis would form a precipitate with the silver ions.

Ag+(aq) + X–(aq) AgX(s)

b) The rate of hydrolysis of haloalkanes is related to how easily the C–X bond breaks. As the C–I bond is weaker than the C–Br and C–Cl bonds, the C–I bond breaks most readily. Hence the rate of hydrolysis of 1-iodobutane is the highest. The precipitate appeared first in the test tube containing it.

As the C–Cl bond is the strongest, the bond is the most difficult to break. Hence the rate of hydrolysis of 1-chlorobutane is the lowest.

107


Substitution reactions of alcohols with halides

Alcohols react with hydrogen halides (HI, HBr and HCl) to form haloalkanes. This reaction involves the cleavage of the carbon-oxygen bond of the alcohol.

Draw the structural formula for each of the following alcohols and state whether it is primary, secondary or tertiary.

a) Propan-1-ol b) 2-methylbutan-2-ol

c) Cyclohexanol

OH + HXR X + H2OR

Examples of different categories of alcohols are shown below:

Although all alcohols behave similarly in some reactions, the primary, secondary and tertiary alcohols react differently in certain reactions.

C

H

H

H

H

H

C OH

ethanol(1° alcohol)

HC C

HH

H OH

H C

H

H

propan-2-ol(2° alcohol)

HC C

CH3H

H OH

H C

H

H

methylpropan-2-ol(3° alcohol)

2-methylcyclopentanol(2° alcohol)

CH3

OH

The order of reactivity of alcohols in substitution reactions with halides is 3° > 2° > 1°.

The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF does not react).

108


Reaction with hydrogen chloride

Bubble dry hydrogen chloride through the anhydrous alcohol in the presence of anhydrous zinc chloride as a catalyst.

We can also prepare chloroalkanes by refluxing concentrated hydrochloric acid using zinc chloride as a catalyst.

ZnCl2 catalystCH3CH2CH2OH(l) + HCl(g) CH3CH2CH2Cl(l) + H2O(l)

1-chloropropane

Fig. 31.4a An experimental set-up for the reaction between hydrogen bromide and alcohol

refluxCH3CH2CH2OH(l) + HCl(aq) CH3CH2CH2Cl(l) + H2O(l)

ZnCl2 catalyst1-chloropropane

Tertiary alcohols react reasonably rapidly with concentrated hydrochloric acid without a catalyst. For example, methylpropan-2-ol reacts with concentrated hydrochloric acid at room temperature. 2-chloro-2-methylpropane is formed in the reaction.

Reaction with hydrogen bromide

Normally, the reaction is carried out by refluxing together a mixture of alcohol, sodium bromide and concentrated sulphuric acid (Fig. 31.4a).

C CH3(l) + HCl(aq)

CH3

OH

CH3 C CH3(l) + H2O(l)

CH3

Cl

CH3

room temperature

2-chloro-2-methylpropane

109


Concentrated sulphuric acid reacts with sodium bromide to produce hydrogen bromide* which reacts with the alcohol.

The mixture is then warmed to distil off the bromoalkane (Fig. 31.4b).

CH3CH2OH(l) + HBr(g) CH3CH2Br(l) + H2O(l)

bromoethane(boiling point 38 °C)

reflux

Fig. 31.4b A laboratory set-up for separating bromoalkane from the reaction mixture

Reaction with hydrogen iodide

In this case, the alcohol is reacted with a mixture of sodium iodide and concentrated phosphoric acid*. Concentrated phosphoric acid reacts with sodium iodide to produce hydrogen iodide* which reacts with the alcohol. The iodoalkane is distilled off.

refluxCH3CH2OH(l) + HI(g) CH3CH2I(l) + H2O(l)

iodoethane(boiling point 72 °C)

Phosphoric acid is used instead of concentrated sulphuric acid because sulphuric acid oxidizes iodide ions to iodine and produces very little hydrogen iodide.

Concentrated sulphuric acid and sodium bromide react according to the following equation:

H2SO4(l) + NaBr(s) NaHSO4(s) + HBr(g)

Concentrated phosphoric acid and sodium iodide react according to the following equation:

H3PO4(l) + NaI(s) NaH2PO4(s) + HI(g)

110


Reaction with phosphorus halides

We can also convert alcohols to haloalkanes by reaction with phosphorus halides.

To prepare a bromoalkane or an iodoalkane, the alcohol is usually heated under reflux in a water bath with a mixture of red phosphorus and bromine or iodine (Fig. 31.5). Phosphorus first reacts with bromine or iodine to give phosphorus trihalide. The halide then reacts with the alcohol to give the corresponding haloalkane which can be distilled off.

3CH3CH2OH(l) + PBr3(l) 3CH3CH2Br(l) + H3PO3(l)

bromoethane phosphorousacid

reflux

Phosphorus pentachloride reacts with alcohols in the cold*.

roomtemperature

CH3CH2CH2OH(l) + PCl5(s) CH3CH2CH2Cl(l) + POCl3(l) + HCl(g)

1-chloropropane phosphorustrichloride oxide

The formation of hydrogen chloride fumes upon reaction with phosphorus pentachloride is a test for the presence of a hydroxyl group in a compound.

Fig. 31.5 A laboratory set-up for the reaction of ethanol with PBr3

(the reaction should be carried out inside a fume cupboard)

111


Reaction with sulphur dichloride oxide (thionyl chloride)

Sulphur dichloride oxide (thionyl chloride) has the molecular formula SOCl2. It reacts with primary and secondary alcohols to produce chloroalkanes. Sulphur dioxide and hydrogen chloride are given off.

Fig. 31.6 A summary of the substitution reactions of alcohols with halides

31.5 Studying the reactions of alcohols with halides

Predict the product of each of the following reactions:

a)

b)

c)

SOCl2

reflux?OH

PBr3

reflux?CH2OH

HBr

reflux?CH3CH2CH2CH2OH

refluxCH3CH2CH2OH(l) + SOCl2(l) CH3CH2CH2Cl(l) + SO2(g) + HCl(g)

Fig. 31.6 summarizes the substitution reactions of alcohols with halides.

R OHalcohol

R Clchloroalkane

R Brbromoalkane

R Iiodoalkane

• reflux with conc. HCl + ZnCl2 catalyst; or• mix with PCl5; or• reflux with SOCl2

• reflux with NaI + conc. H3PO4; or• reflux with red P + I2

• reflux with NaBr + conc. H2SO4; or• reflux with red P + Br2

112


Solution

a)

b)

c)

31.6 Deducing the structure of a compound

Compound X has the molecular formula C4H8O.

• When phosphorus pentachloride, PCl5, was added to a dry sample of X, steamy fumes were observed.

• When aqueous bromine was shaken with a sample of X, the aqueous bromine turned colourless.

• Compound X has a geometrical isomer.

Use the information above to draw the structural formulae of the two isomers which could be compound X.

Solution

• X reacted with PCl5 to give steamy fumes (HCl). This shows the presence of a hydroxyl group in X.

• X decolorized aqueous bromine. This shows the presence of a carbon-carbon multiple bond in X.

The structural formulae of the two isomers which could be X are as follows:

SOCl2

refluxOH Cl

CH2OHPBr3

refluxCH2Br

HBr

refluxCH3CH2CH2CH2BrCH3CH2CH2CH2OH

CC

CH2OHCH3

HH

CC

CH2OHH

HCH3

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Elimination reactions: dehydration of alcohols to form alkenes

Most alcohols dehydrate and form alkenes when they are heated in the presence of a strong acid.

1 Predict the products of each of the following reactions:

a) reflux pentan-2-ol with sodium bromide and concentrated sulphuric acid;

b) reflux cyclohexanol with red phosphorus and iodine;

c) reflux propan-1-ol with concentrated hydrochloric acid and zinc chloride catalyst.

2 How would you carry out each of the following conversions?

a)

b)

CH3CH2CH2CHCH3

OH

?CH3CH2CH2CHCH3

Br

CH3CH2CH2CHCHCH3

CH3

OH

?CH3CH2CH2CHCHCH3

CH3

l

C

OH

C

H

CCstrong acid

heat+ H2O

These reactions are described as dehydration since they involve the removal of a water molecule from a molecule of the reactant. Dehydration is an example of elimination reaction.

In an elimination reaction, atoms or groups of atoms are removed from two adjacent atoms (usually carbon atoms) of the reactant molecule.

dehydration 脫水作用 elimination reaction 消去反應

114


The experimental conditions — temperature and acid concentration — that are required to bring about dehydration are closely related to the structure of individual alcohols.

Dehydration of primary alochols

Primary alcohols are the most difficult to dehydrate. For example, we can carry out the dehydration of ethanol in the laboratory by heating ethanol at 180 °C with excess concentrated sulphuric acid. Ethene is evolved and can be collected over water (Fig. 31.7).

+ H2O(l)conc. H2SO4C

OH

C

H

H H(l)

excessH H

H H

(g)

ethanol (1° alcohol)

HH

C C180 °C

ethene

We can regard the concentrated sulphuric acid as a dehydrating agent, removing water from the ethanol.

Fig. 31.7 Experimental set-up for the dehydration of ethanol using concentrated sulphuric acid

115


Aluminium oxide at 300 °C can also act as a dehydrating agent (Fig. 31.8).

Dehydration of secondary alcohols

Secondary alcohols usually dehydrate under milder conditions.Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165 – 170 °C.

Fig. 31.8 Experimental set-up for the dehydration of ethanol using hot aluminium oxide

(l) + H2O(l)85% H3PO4

OH(l)

165 – 170 ˚C

cyclohexenecyclohexanol(2˚ alcohol)

Dehydration of tertiary alcohols

Tertiary alcohols are usually so easily dehydrated that extremely mild conditions can be used. For example, methylpropan-2-ol dehydrates in 20% sulphuric acid at a temperature of 85 °C.

H(g) + H2O(l)

CH3CH3

C

H

C

H

H H(l)85 °C

C

OH

HH

20% H2SO4 CC

H

H C

HH


methylpropene

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The relative ease with which alcohols undergo dehydration shows the following order:

Ease of dehydration: 3° > 2° > 1° alcohol

With some alcohols, several alkenes may be produced, but normally the most-substituted alkene is the major product.

Consider the dehydration of butan-2-ol. But-1-ene and but-2-ene are formed.

But-2-ene is the major product of the dehydration.

But-1-ene has only one alkyl group attached to the carbon atoms of the C=C bond while but-2-ene has two. But-2-ene is the more substituted alkene and hence the major product of the reaction.

OH

H C

H

H

C

H

C

H

H

C

H

H

H C

H

H

C

H

C

H

H

C

H

H

H

but-1-ene(minor product)

remove

dehydration

OH

H C

H

H

C

H

C

H

H

C

H

H

H H C

H

H

C

H

C

H

C

H

H

H

but-2-ene*(major product)

remove

dehydration

Remember that but-2-ene has two geometrical isomers. We have discussed this in Unit 30.

117


31.7 Testing an unknown gas

A mixture of ethanol and excess substance X was heated to 180 °C in flask A as shown below. The gas G liberated passed through tube B containing sodium hydroxide solution and then tube C containing acidified dilute potassium permanganate solution. The excess gas liberated at D was ignited. The purple colour of the solution in tube C faded when gas G passed through it.

a) i) Identify substance X.

ii) Name gas G formed in flask A.

iii) Write an equation for the formation of gas G.

b) What was the purpose of passing gas G through the sodium hydroxide solution in tube B?

c) Explain the observable change for acidified dilute potassium permanganate solution in tube C when gas G passed through it.

Solution

a) i) Concentrated sulphuric acid

ii) Ethene

iii)

b) To remove any traces of acidic vapours.

c) Ethene underwent an addition reaction with acidified dilute potassium permanganate solution. The permanganate solution became colourless.

excess conc. H2SO4CH3CH2OH(l) C2H4(g) + H2O(l)

180 °C

118


31.8 Dehydration of alcohol to alkene

a) Draw the structural formulae of alkenes that can be made by dehydration of 2-methylpentan- 2-ol.

b) Which of the alkenes drawn in (a) is likely to be the major product? Explain your answer.

Solution

a)

b) 2-methylpent-1-ene has two alkyl groups attached to the carbon atoms of the C=C bond while 2-methylpent-2-ene has three. 2-methylpent-2-ene is the more highly substituted alkene and hence the major product.

conc. H2SO4 or H3PO4

C3H7

CH3

C2H5

CH3

C

H

C

H

H

H

H

C

HH

OH

remove

2-methylpent-1-ene

CCdehydration

heat

C2H5

conc. H2SO4 or H3PO4

C2H5

CH3

C

H

C

H

Hdehydration

heat

H

C

HH

OH2-methylpent-2-ene

remove

CH3

CH3

1 Draw and name the major dehydration product of the following compound:

Tetrahydrofuran (THF) is a common solvent.OH

CH3?

H2SO4

THF*, 50 °C

O

119


Oxidation of alcohols

Consider the treatment of the following three alcohols with acidified potassium dichromate solution.

2 Alcohol A has the following structure:

When A was treated with hot concentrated sulphuric acid, three substances X, Y and Z were produced. X, Y and Z were isomers of each other. Only a small amount of Z was produced. Both X and Y reacted with bromine to give 1,2-dibromo-1-phenylpropane.

a) Draw the structure of 1,2-dibromo-1-phenylpropane.

b) Suggest structures of X, Y and Z.

c) What is the isomeric relationship between X and Y?

CH3CH2CH2OH CH3CHCH3

CH3CH3

CH3

OH

C


OH



Fig. 31.9 shows the results when the three alcohols are heated with acidified potassium dichromate solution. Propan-1-ol and propan-2-ol are oxidized by the dichromate solution. The orange dichromate solution turns green as the dichromate ions are reduced to green chromium(III) ions. Methylpropan-2-ol does not react with acidified potassium dichromate solution.

Oxidation of the molecule of a carbon compound usually corresponds to increasing its oxygen content or decreasing its hydrogen content*.

Fig. 31.9 Results obtained when propan-1-ol, propan-2-ol and methylpropan-2-ol are heated with acidified potassium dichromate solution

methyl-propan-2-ol(3° alcohol)



We have discussed this in Topic 5 Redox Reaction, Chemical Cells and Electrolysis.

OH

C

H

H

C

H

C

H

H

H

120


Table 31.1 Reactions between alcohols and oxidizing agents

Alcohol Reaction

Primary

Secondary

Tertiary

We can oxidize an alcohol to yield a carbonyl compound — the opposite of the reduction of a carbonyl compound to yield an alcohol:

Notice that in this reaction, two hydrogen atoms are being removed: one from the oxygen atom and one from the carbon atom. Hence oxidation of the –OH group will not take place unless there is a hydrogen atom on the carbon atom bearing the –OH group.

Upon oxidation, primary alcohols yield aldehydes or carboxylic acids, secondary alcohols yield ketones. Tertiary alcohols do not normally react with most oxidizing agents.

C

OH

an alcohol

H

reduction

oxidation

a carbonyl compound

O

C

C

H

OH

R H CR H[O]

O

an aldehyde

[O]

a carboxylic acid

C

O

R OH

R1

R1

C

OH

R H CR[O]

O

a ketone

R1

R2C

OH

R[O]

no reaction

A number of oxidizing agents can be used in the oxidation process, such as acidified potassium dichromate solution, acidified or alkaline potassium permanganate solution.

121


Oxidation of primary alcohols to aldehydes

Acidified potassium dichromate solution can oxidize propan-1-ol to propanal.

To prepare propanal, we must choose the reaction conditions to prevent any subsequent oxidation of propanal to propanoic acid. We can either

• carry out the reaction at room temperature and add the oxidizing agent to the alcohol so that the oxidizing agent is never in excess; or

• heat the reaction mixture to distil off the propanal once formed so that it will not be further oxidized (Fig. 31.10).

[O]

propan-1-ol(boiling point 97 °C)

propanal(boiling point 49 °C)

+ +C H(aq)

OH

H

CH3CH2 CH3CH2 C

O

H(l) H2O(l)

where [O] represents an oxidizing agent

Fig. 31.10 An experimental set-up for the oxidation of propan-1-ol to propanal

122


Oxidation of primary alcohols to carboxylic acids

We can oxidize propan-1-ol to propanoic acid by refluxing it with acidified potassium dichromate solution (Fig. 31.11a).

In the reflux condenser, all the vapour is condensed back into the flask. This prevents any loss of the reaction mixture and favours the oxidation of propan-1-ol to propanoic acid rather than to propanal.

2[O]


propanoic acid(boiling point 141 °C)

+ +C H(aq)

OH

H

CH3CH2 CH3CH2 C

O

OH(l) H2O(l)

Fig. 31.11a An experimental set-up for the oxidation of propan-1-ol to propanoic acid

To separate propanoic acid from the reaction mixture, transfer the mixture to the apparatus as shown in Fig. 31.11b. On heating, propanoic acid and water distil out to give an aqueous solution of propanoic acid.

123


Acidified potassium permanganate solution is too powerful an oxidizing agent to stop at the aldehyde; it oxidizes primary alcohols to acids and oxidizes secondary alcohols to ketones.

In the following activity, you are going to oxidize ethanol to ethanoic acid and test the ethanoic acid produced.

Fig. 31.11b An experimental set-up for separating propanoic acid from the reaction mixture

Oxidizing ethanol to ethanoic acid and testing the ethanoic acid produced.

31.1

Oxidation of secondary alcohols to ketones

Secondary alcohols can be oxidized to give ketones. For example, we can oxidize propan-2-ol to propanone by refluxing it with acidified potassium dichromate solution.

[O]


propanone(boiling point 56 °C)

+C H(aq)

OH

CH3

CH3 CH3 C

O

CH3(l) H2O(l)+

124


Fig. 31.12 summarizes the common reactions of propan-1-ol.

We will discuss this reaction later in this unit.

Fig. 31.12 A summary of the common reactions of propan-1-ol

31.9 Studying the oxidation of alcohols

What alcohol would give each of the following products on oxidation with acidified potassium dichromate solution?

a)

b) O

CH3

OHH C

H

H

C

H

H

C

H

H

C

H

H

C

H

C

O

• reflux with conc. HCl +

ZnCl2 catalyst; or

• mix with PCl5; or

• reflux with SOCl2

• reflux with NaBr + conc. H2SO4; or• reflux with red P + Br2

• reflux with NaI +

conc. H3PO4; or

• reflux with red P + I2

CH3COOH +conc. H2SO4*

• K2Cr2O

7 / H3O +, reflux; or

• KMnO4 / H

3O +, reflux

K2Cr2O7

/ H3O+

distil off th

e propanal • excess conc. H2SO4, 180 °C; or • Al2O3, 300 °C

C3H7Cl1-chloropropane

C3H7Br1-bromopropane

C3H7I1-iodopropane

CH3CH CH2

propene

CH3COOC3H7

propyl ethanoate

CH3CH2CHOpropanal

CH3CH2COOHpropanoic acid

C3H7OHpropan-1-ol

125


Solution

a) The product is a carboxylic acid. It is formed from the oxidation of a primary alcohol.

b) The product is a ketone. It is formed from the oxidation of a secondary alcohol.

31.10 Converting an alkene to an alcohol

Propene can be converted into propan-2-ol by the following route:

a) Give the reagents and conditions for reaction 1.

b) Name compound A.

c) The propan-2-ol can be converted back to propene.

Choose the type of reaction that is involved from the following terms:

addition elimination reduction substitution

d) Reaction 2 is a slow reaction. Give the reagent(s) and condition(s) for reaction 2.

CH3

OHH C

H

H

C

H

H

C

H

H

C

H

H

C

H

C

O

2-methylhexanoic acid

K2Cr2O7 / H3O+

reflux

OHCH3

H C

H

H

C

H

H

C

H

H

C

H

H

C

H

C

H

H

2-methylhexan-1-ol

K2Cr2O7 / H3O+

refluxcyclopentanol cyclopentanone

OHH

O

Br

CH3CH3 CH3CH3

OH

C

H

propan-2-ol

reaction 2C

H

compound A

reaction 1propene

126


Solution

a) React with hydrogen bromide at room temperature.

b) 2-bromopropane

c) Elimination reaction

d) Reflux with sodium hydroxide solution.

31.11 Deducing structures of compounds

Suggest a possible structure for each of the compounds X, Y and Z. Explain briefly your deductions.

a) X (C4H6O) gives Y (C4H6O2) upon oxidation.

b) Z (C4H10O) can exist as a pair of enantiomers, and reacts with phosphorus pentachloride to give hydrogen chloride.

Solution

a) The oxidation of X adds one more oxygen atom to each molecule of X. Thus, the reaction is an oxidation of an aldehyde to carboxylic acid.

or

X

C

O

HC C

HH

H

CH

H

Y

C

O

OHC C

HH

H

CH

H

X

C

O

HC C

HH

H

CH

H

Y

C

O

OHC C

HH

H

CH

H

127


b) Z contains an –OH group as it reacts with phosphorus pentachloride to give hydrogen chloride.

The chiral carbon is marked above. Z has a chiral carbon so that it can exist as a pair of enantiomers.

PCl5 C CC C

HH

H

H H H H

Cl H

H HCl+C* CC C

HH

H

H H H H

OH H

H

1 Give the carbon compounds obtained from butan-1-ol in the following reaction scheme.

2 What alcohol would give each of the following products on oxidation?

a) b)

3 Explain why each of the following methods of preparation are NOT appropriate. In each case, suggest an appropriate method for the preparation.

a) Prepare CH3CH2I by reacting CH3CH2OH with a mixture of NaI(s) and concentrated H2SO4.

b) Prepare CH3CH2CHO by heating CH3CH2CH2OH with acidified Na2Cr2O7(aq) under reflux.

A

C

D

conc. H2SO4

heat

reflux with red P + Br2

K2Cr2O7 / H3O+

reflux

CH3CH2CH2CH2OH

BHBr

CH3

CH3CHCHOCH3

C

O

128


Principles and applications of an alcohol breathalyzer*

Task

To know whether a driver has committed drink driving, a policeman can test the breath of a driver on the spot using a breathalyzer.

The oxidation of ethanol forms the basis of a breathalyzer. Search for information about the principles and applications of an alcohol breathalyzer. Then write a short essay of not more than 1 000 words on the topic.

Reference websites

1 You may start your search from the following websites:

• Essay titled ‘How Breathalyzers Work’ http://www.howstuffworks.com/breathalyzer3.htm

2 Extend your search using search engines and appropriate keywords.

• http://www.metacrawler.com • http://www.google.com • http://www.yahoo.com • http://www.altavista.com

breathalyzer 呼氣分析儀

We will discuss this further in Topic 16 Analytical Chemistry.

31.9 Reactions of aldehydes and ketones

Many reactions of aldehydes and ketones are similar because they

both contain the carbonyl group C

O

. However, they differ in their reactions with oxidizing and reducing agents.

129


Oxidation of aldehydes and ketones

Aldehydes are easily oxidized to yield carboxylic acids, but ketones are generally inert towards oxidation. This difference is a consequence of structure: aldehydes contain a hydrogen atom next to the carbonyl group. This hydrogen atom is readily oxidized to –OH. Hence aldehydes are readily oxidized.

Aldehydes can be oxidized by acidified potassium dichromate solution. For example, when we warm acidified potassium dichromate solution with ethanal, ethanal is oxidized to ethanoic acid. The solution changes from orange to green as the dichromate ions are reduced to chromium(III) ions.

On the other hand, ketones are not oxidized by acidified potassium dichromate solution.

Reduction of aldehydes and ketones

Reduction of the molecule of a carbon compound usually corresponds to increasing its hydrogen content or decreasing its oxygen content. For example, converting a carboxylic acid to an aldehyde is a reduction because the oxygen content of the carboxylic acid molecule is decreased.

CR

O

H

an aldehyde

hydrogen atom here

C

O

R OH

a carboxylic acid

[O]R1CR

O

a ketone

alkyl or aryl group here

CH3 CH3

K2Cr2O7 / H3O+

O

C C

O

OHheat

H + [O]

O

C

O

C R H[H]*

reductionR OH

a carboxylic acid an aldehyde

oxygen content decreases

Fig. 31.13 Left: Acidified potassium dichromate solution

Right: After the oxidation of ethanal, the solution becomes green

We use [H] to represent a reducing agent.

130


Converting an aldehyde to an alcohol is also a reduction.

OH

H

HC

O

C RreductionR H

[H]

an alcohol

an aldehyde

hydrogen content increases

Sometimes, when devising routes of synthesis of carbon compounds, it is necessary to reduce aldehydes and ketones back to alcohols. This requires a powerful reducing agent. Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.

OH OH

R1

R1R C

O

H[H]

an aldehyde

R C

H

H

a primary alcohol

R C

O

[H]

a ketone

R C H

a secondary alcohol

In the laboratory, reduction is usually effected by the use of a metal hydride. Lithium tetrahydridoaluminate (lithium aluminium hydride) (LiAlH4)* is used in anhydrous ethoxyethane*. The reaction with aldehydes is very vigorous.

For example,

CH3CH2CH2CH

O

CH3CH2CH2CH

OH

H

butan-1-ol(a primary alcohol)

butanal(an aldehyde)

1 LiAlH4 / ethoxyethane

2 H3O+

CH3CH2CH2CCH3

O

CH3CH2CH2CCH3

OH

H

pentan-2-ol(a secondary alcohol)

pentan-2-one(a ketone)


2 H3O+

Reduction of aldehydes and ketones can also be carried out with milder reducing agents such as sodium tetrahydridoborate (sodium borohydride) (NaBH4).

Lithium tetrahydridoaluminate reacts violently with water. Therefore reductions with this reagent must be carried out in anhydrous condition.

131


The reduction reactions of aldehydes and ketones are essentially the reverse of the oxidation reactions of primary and secondary alcohols.

The hydride reagent LiAlH4 does not affect carbon-carbon double bonds. Hence it can convert unsaturated aldehydes and ketones into unsaturated alcohols.

For example,

O OHH

cyclohex-2-enol(a secondary alcohol)

cyclohex-2-enone(a ketone)


2 H3O+

31.12 Question concerning three compounds of the same molecular formula

Compound X, Y and Z have the same molecular formula, C4H8O.

a) Suggest how X can be converted into Y. Give the reagent(s) used for each step and the structure of the intermediate compound(s).

b) Suggest a chemical test to distinguish between Y and Z.

Solution

a)

b) Warm Y and Z with acidified potassium dichromate solution separately. Y is oxidized by the dichromate solution. The solution changes from orange to green as the dichromate ions are reduced to chromium(III) ions. Z shows a negative result.

CH2 CHCH2CH2OH CH3CH2CCH3CH3CH2CH2CHO

O

ZYX

H2 / PtCHCH2CH2OHCH2 CH3CH2CH2CH2OH

K2Cr2O7 / H3O+

distil off the productCH3CH2CH2CHO

132


1 Compound A can be converted into two different carbon compounds as shown below. Give the structural formulae of the new compounds B and C.

2 Draw the structural formulae of compounds X and Y in the following synthesis.

3 For each of the following reactions, suggest a possible reactant.

a)

b)

c)

CH3CH2CHO

A

B

C

1 LiAlH4 / ethoxyethane2 H3O

+

K2Cr2O7 / H3O+

heat

CHCH3CH3CH2CCH1 LiAlH4 / ethoxyethane

2 H3O+

X YH2

Pt catalyst

O

A CH2Clconc. HCl

ZnCl2 catalyst

CH2CH3 CH2CH3+Bconc. H2SO4

heat

OH

CH3


2 H3O+

C

133


A condensation reaction is a reaction in which two or more molecules react together to form a larger molecule with the elimination of a small molecule such as water.

31.10 Reactions of carboxylic acids

Esterification

Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form esters. The acid and alcohol are boiled under reflux with the concentrated acid.

R1 +R1

+

C O H + H O C O

+

R R

O O

alcoholcarboxylic acid ester water

H2O

Because water is produced, just like when we breathe on a cold surface, the process is named ‘condensation’.

The reversible reaction of a carboxylic acid with an alcohol to form an ester through a condensation reaction is known as esterification. Esterification reactions are acid catalyzed. They proceed very slowly in the absence of strong acids.

For example,

The trivial name for ethyl ethanoate is ethyl acetate.

condensation reaction 縮合反應 esterification 酯化作用

conc. H2SO4

CH2CH3 + H2OCH2CH3CH3 CH3

ethanoic acid

C O H + H O C O

ethanol ethyl ethanoate*

O O

CH3 + H2OCH3

conc. H2SO4O H + H

benzoic acid

C

O

O O

methanol methyl benzoate

C

O

134


Fig. 31.14 shows an experiment* in which ethanol and ethanoic acid are warmed with concentrated sulphuric acid as a catalyst. An insoluble layer of ethyl ethanoate forms on the water.

An ester is also formed by heating an alkanol with an alkanoic acid and concentrated sulphuric acid under reflux.

Fig. 31.14 Ethyl ethanoate is formed when ethanol and ethanoic acid undergo esterification

In the following activity, you are going to study the reaction between ethanol and ethanoic acid in the presence of a catalyst.

Studying the reaction between ethanol and ethanoic acid.

31.2

Uses of esters

Esters have pleasant, sweet and fruity smells. The flavours and fragrances of many fruits and flowers are due to a mixture of natural esters.

Esters have many uses.

1 Perfumes, cosmetics and the artificial flavourings present in foods and drinks contain esters made by chemists.

2 Esters are good solvents for many carbon compounds. For example, nail varnish remover and whiteboard marker pens may contain ethyl ethanoate as a solvent.

Fig. 31.15 The fragrances of many flowers are due to a mixture of natural esters

artificial flavouring 人造香料

135


For example,

CH3

CH3

CH3

2 H3O+


methylpropanoic acid methylpropan-1-ol

CH2OHCH3C COOH C

H H

COOH

benzoic acid


2 H3O+

CH2OH

phenylmethanol

2 H3O+

1 LiAlH4 / ethoxyethaneC

O

OHR CR OH

H

H

Reduction

Reduction of carboxylic acids can be accomplished with the powerful reducing agent lithium tetrahydridoaluminate (LiAlH4). It reduces carboxylic acids to primary alcohols in excellent yields.

Fig. 31.17 Nail varnish remover and food flavouring contain esters

Fig. 31.16 Chewing gum contains esters as flavourings

136


Amides from carboxylic acids

Carboxylic acids react with aqueous ammonia to form ammonium salts.

an ammonium carboxylate

O– NH4+CR

O

OH + NH3CR

O

If we evaporate the water and subsequently heat the dry salt, dehydration produces an amide.

an amide

O– NH4+CR

O

NH2 + H2OCR

O

heat

2CH3COH(l) + (NH4)2CO3(s)

O

2CH3CO– NH4+(s) + CO2(g) + H2O(l)

O

We can also produce ethanamide from ethanoic acid by adding solid ammonium carbonate to an excess of concentrated ethanoic acid.

When the reaction is complete, the mixture is heated under reflux. The ammonium salt dehydrates and produces ethanamide.

The excess ethanoic acid is to prevent the dissociation of the ammonium salt* before it dehydrates.

The mixture is distilled at about 170 °C to remove the excess ethanoic acid and water — leaving almost pure ethanamide in the flask.

O O

CH3CO– NH4+(s) CH3CNH2(s) + H2O(l)

ethanamide

The ammonium salt tends to split into ammonia and the parent acid on heating, and recombining on cooling.

CH3COONH4(s) CH3COOH(l) + NH3(g)

The presence of the excess ethanoic acid helps to prevent this from happening by shifting the position of equilibrium to the left.

We will discuss chemical equilibrium in Topic 11 Chemical Equilibrium.

137


31.13 Deducing structural formulae of alcohols and acids from which esters are derived

Esters have strong sweet smells which are often floral or fruity. Two examples are given below.

a) Write down the names of the alcohols and carboxylic acids from which they are derived.

b) Draw the structural formulae of these alcohols, carboxylic acids and esters.

Solution

Ester Fragrance

Ethyl 2-methylbutanoate pear

Phenylmethyl ethanoate jasmine

Ester Alcohol Carboxylic acid

Ethyl 2-methylbutanoate

CH3

CH3CH2CH CH2CH3C

O

O

ethanol

OHH C

H

H

C

H

H

2-methylbutanoic acid

OH

CH3

CH3CH2CH C

O

Phenylmethyl ethanoate

CH2 CH3O

O

C

phenylmethanol

OHCH2

ethanoic acid

OHCH3 C

O

138


31.14 Predicting products of reactions of a compound

The structure of compound A is shown below:

From the structure of the compound, predict the reaction that occurs when A reacts with each of the following reagents.

a) Br2 (in organic solvent);

b) H2 (in the presence of a Pt catalyst); and

c) CH3COOH (in the presence of a concentrated H2SO4 catalyst).

Draw the structural formula of the product in each case.

Solution

There are two functional groups in compound A: the –OH group and the C=C bond.

a) The reaction with bromine results in the addition of bromine to the C=C bond.

b) The reaction with hydrogen also results in the addition of hydrogen to the C=C bond.

c) The ethanoic acid reacts with the –OH group to form an ester and water.

compound A

CH3

H3C

HO

C8H17

CH3

H3C

HOBr

Br

C8H17

CH3

H3C

HO

C8H17

H3C C

O

O

CH3

H3C

C8H17

139


1 Write balanced equations for the reactions that occur when the following pairs of compounds are heated under reflux with a few drops of concentrated sulphuric acid as a catalyst.

a) Propan-2-ol and propanoic acid

b) Ethanoic acid and ethane-1,2-diol

2 An ester A is used as a solvent for paints and varnishes. The structural formula of A is shown below.

CH3COOCH(CH3)CH2CH3

Ester A

a) Ester A can be manufactured by heating an alcohol under reflux with ethanoic acid and a catalyst.

i) Suggest a suitable catalyst for this reaction.

ii) Explain why the reaction is carried out under reflux.

b) Draw the structural formula of the alcohol used to make ester A.

c) Apart from being a good solvent, suggest another use of ester A.

3 This question is about a compound X and two of its reactions.

a) When X reacts with ethanol under suitable conditions, ester A is formed. Write a balanced equation for the reaction which occurs.

b) X can be reduced to compound B by LiAlH4 under suitable conditions. Draw the structural formula of B.

CH2OH

COOHethanol

LiAIH4

ester A

compound Bcompound X

140


31.11 Hydrolysis of esters

Esterification is a reversible reaction. Therefore an ester can react with water to produce alcohol and carboxylic acid. This reaction is called hydrolysis of ester.

OHR1R1R C

O

O R C

O

O H +hydrolysis

For example, hydrolysis of methyl ethanoate produces ethanoic acid and methanol. However, the reaction of the ester with water is very slow by itself. A catalyst is needed to speed up the reaction. The catalyst can be either a dilute mineral acid (such as sulphuric acid) or a dilute sodium hydroxide solution.

In acidic solution

OH(aq)H(aq) + CH3CH3 CH3(l) + H2O(l)C O CH3 C

O O

methyl ethanoate ethanoic acid methanol

H3O+

O

In alkaline solution

When dilute sodium hydroxide solution is used, the ethanoic acid formed reacts with it to give a sodium salt, sodium ethanoate.

CH3 H(aq) + NaOH(aq)C O O– Na+(aq) + H2O(l)CH3 C

O O

ethanoic acid sodium ethanoate

The overall reaction thus becomes:

CH3 CH3(l) + NaOH(aq)C O O– Na+(aq) + CH3OH(aq)CH3 C

O O

methyl ethanoate sodium ethanoate methanol

hydrolysis 水解作用

141


Salt formation removes the ethanoic acid from the reaction mixture. The hydrolysis reaction therefore goes to completion and so alkaline hydrolysis is usually preferred.

In the laboratory, first heat a mixture of an ester and an aqueous alkali under reflux (Fig. 31.18a). Then separate the products by fractional distillation (Fig. 31.18b). The alcohol is obtained as distillate. The salt of carboxylic acid remains in the solution as it is less volatile. We can obtain the free carboxylic acid by adding excess mineral acid. For example:

Fig. 31.18a Alkaline hydrolysis of an ester in the laboratory

CH3COO–(aq) + H+(aq) CH3COOH(aq)

ethanoate ion(from salt)

hydrogen ion(from mineral acid)

ethanoic acid

Fig. 31.18b Separating alcohol from the products formed in the hydrolysis of ester

142


Heat the amide under reflux with an aqueous acid or alkali. Moderately concentrated solution of the acid or alkali is usually used.

The products depend on whether an acid or alkali is used in the hydrolysis. If an acid is used in the hydrolysis, the product contains ammonium ions. If an alkali is used, the carboxylic acid loses H+ ions and the product contains carboxylate ions.

For example, when boiling hydrochloric acid is used for hydrolysis, ethanamide forms ethanoic acid and ammonium chloride.

31.12 Hydrolysis of amides

Just as esters, the reaction of an amide with water is extremely slow, but the reaction can be catalyzed by an acid or an alkali. In the presence of a strong acid or alkali, amides are hydrolyzed to carboxylic acids or their salts respectively.

In acidic solution

NH2 + H2O + H+CR

O

OH + NH4+CR

O

In alkaline solution

NH2 + OH–CR

O

O– + NH3CR

O

CH3COOH(aq) + NH4Cl(aq)CH3CONH2(aq) + H2O(l) + HCl(aq)heat

When ethanamide is refluxed with sodium hydroxide solution, ammonia gas is given off and you are left with a solution containing sodium ethanoate.

CH3COO– Na+(aq) + NH3(g)CH3CONH2(aq) + NaOH(aq)heat

We can obtain ethanoic acid by adding excess mineral acid to the solution containing sodium ethanoate.

143


31.15 Predicting the hydrolysis products of a compound

The structure of compound X is shown below:

a) How many functional groups are there in X? Name all of these functional groups.

b) Write the structures of carbon compounds formed when X is heated with excess sodium hydroxide solution.

Solution

a) There are 4 functional groups in X:

• carbon-carbon double bond;

• amine group;

• amide group; and

• ester group.

b)

CH3COO– CH3CH2OH

CH3

NH

NH2

C CH2CH3

O O

C

O

O–

NH2

NH2

C

O

144


31.16 Distinguishing different compounds using test tube reactions

Describe, by giving reagent(s) and stating observations, how you could distinguish between compounds in each pair using a simple test tube reaction.

a) CH3CH2CH2OH and CH3CH2CHO A B

b)

and

C D

c) CH3CONH2 and CH3COOH E F

Solution

a) Warm each compound with ethanoic acid in the presence of concentrated sulphuric acid.

A reacts with ethanoic acid to form an ester with a sweet smell while there is no observable change for B.

b) Add aqueous bromine to each compound.

D decolorizes the aqueous bromine immediately while there is no observable change for C.

c) Add sodium hydroxide solution to each compound and heat.

E gives a gas that turns moist red litmus paper blue (ammonia) while there is no gas evolved for F.

Another possible test is to add sodium hydrogen carbonate solution to each compound. F gives a gas that turns limewater milky (carbon dioxide) while there is no gas evolved for E.

CH3

CH3

CH3

CH3

C C

145


1 Draw the structural formulae of A, B, C and D in the following reactions.

a)

b)

c)

2 The structure of compound A is shown below:

Draw structural formulae to show the two carbon compounds formed by the alkaline hydrolysis of A.

C

O

OHCH3CH2 NH3+ A B H2O+heat

C

O

NH2CH3(CH2)16 NaOH+ C NH3+

C

O

NH2CH2 H2O+ D NH4Cl+HCl+

CH2

CH3

CH3

H

C

O

O

compound A

C C

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146

1 Reactions of alkanes

a) Combustion

CH4 + 2O2 CO2 + 2H2O

b) Substitution reactions with halogens

Cl2 CH4 CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl UV light or heat

2 Addition reactions of alkenes

CH3

CH3

CH3

CH3

CH3

CH3

H C

H

H

C

H

CH3 hydrogenation

H C

H

X

C

X

CH3

H C

H

H

C

X

CH3

major product

H+ C

H

X

C

H

CH3

Markovnikov’s rule is followed

minor product

H2 / Pt catalyst

X2

X2 = Cl2(g) orBr2 (in organic solvent)

HX

X = Cl, Br or I

C

H

H

C

3 Markovnikov’s rule for addition reaction of an asymmetric alkene:

When a molecule HA adds to an asymmetric alkene, the major product is the one in which the hydrogen atom attaches itself to the carbon atom already carrying the larger number of hydrogen atoms.

4 Substitution reactions of haloalkanes — alkaline hydrolysis of haloalkanes

R OHR XNaOH(aq)

reflux

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147

5 Substitution reactions of alcohols with halides

a) Order of reactivity of alcohols is 3° > 2° > 1°

b) Reactions with halidesH

H C

H

H

C

H

Cl

H

H C

H

H

C

H

Br

H

H C

H

H

C

H

OH

H

H C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

I

NaBr, conc. H2SO4

reflux

conc. HCl

ZnCl2 catalyst, reflux

Nal, conc. H3PO4

reflux

c) Reactions with phosphorus halidesH

H C

H

H

C

H

Cl

H

H C

H

H

C

H

Br

H

H C

H

H

C

H

OH

H

H C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

I

red P + Br2

reflux

PCl5

room temperature

red P + I2

reflux

d) Reactions with sulphur dichloride oxide

H

H C

H

H

C

H

Cl

H

H C

H

H

C

H

OH C

H

H

C

H

H

SOCl2

reflux

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148

6 Dehydration of alcohols to form alkenes

CH3

CH3

CH3

CH3

CH3

CH3

CH3

OH

H

H C

H

H

C

H

OHexcess conc. H2SO4

180 °C

(1° alcohol)

C

H

H

C

H

H

H

H C

H

H

Cconc. H3PO4

165 – 170 °C

(2° alcohol)

C

H

H

C

H

C OH20% H2SO4

85 °C

(3° alcohol)

C

H

H

C

mild

er cond

itions

7 Oxidation of alcohols

OH

CH3

CH3

CH3

H

OH

H

H C

H

H

C

H

OHK2Cr2O7 / H3O

+

gentle heat, distil

(1° alcohol)

H

H C

H

H

C

H

H

CK2Cr2O7 / H3O

+

reflux

(2° alcohol)

C OH no reactionK2Cr2O7 / H3O

+

reflux

(3° alcohol)

H C

H

H

C

O

Hreflux

further oxidationH C

H

H

C

O

H C

H

H

C

O

C

H

H

H

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149

8 Oxidation and reduction of aldehydes and ketones

a) Oxidation of aldehydes

OHK2Cr2O7 / H3O

+

heatR C

O

R C

O

H

R1

R1

OH

R C H

2° alcohol

OH

R C

H

H

1° alcohol

ketone

aldehyde


2 H3O+


2 H3O+

R C

O

H

R C

O

b) Reduction of aldehydes and ketones

9 Reactions of carboxylic acids

a) Esterification

b) Reduction

c) Amide synthesis

H2OR1OR1H+ OOHconc. H2SO4

R C

O

+R C

O

OHOHR C

O

R C

H

H


2 H3O+

O– NH4+NH3+ H2O+OHR C

O

R C

O

NH2R C

O

heat

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150

10 Hydrolysis of esters

a) Acid hydrolysis

b) Alkaline hydrolysis

R1 OHO H +R C

O

H3O+

H2OR1O +R C

O

O– Na+ R1 OH+R C

O

NaOHR1O +R C

O

11 Hydrolysis of amides

a) Acid hydrolysis

b) Alkaline hydrolysis

NH2 OH NH4++R C

O

H2O+ H++R C

O

NH2 OH–+R C

O

O– NH3+R C

O

12 The following table summarizes typical reactions of members of some homologous series.

Homologous series

Reaction Reagent(s) and condition(s) Products

Alkanes

combustion good supply of oxygen carbon dioxide and water

substitution reactions with

halogens

halogens

UV light or heathaloalkanes

Continued on next page

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151

Homologous series

Reaction Reagent(s) and condition(s) Products

Alkenes

hydrogenationhydrogen

Pt as catalystalkanes

addition of halogens

chlorine or bromine haloalkanes

addition of hydrogen

halidesHCl / HBr / HI haloalkanes

Haloalkanes hydrolysis reflux with NaOH(aq) alcohols

Alcohols

substitution reactions with

halides

reflux with conc. HCl + ZnCl2 catalyst; or mix with PCl5; or reflux with SOCl2

chloroalkanes

reflux with NaBr + conc. H2SO4; or reflux with red P + Br2

bromoalkanes

reflux with NaI + conc. H3PO4; or reflux with red P + I2

iodoalkanes

dehydration

excess conc. H2SO4, 180 °C for 1° alcohols;

conc. H3PO4, 165 – 170 °C for 2° alcohols;

20% H2SO4 85 °C for 3° alcohols

alkenes

oxidation K2Cr2O7 / H3O+

1° alcohol aldehyde carboxylic acid

2° alcohol ketone

3° alcohol no reaction

Aldehydesand ketones

oxidation K2Cr2O7 / H3O+ aldehyde carboxylic acid

reduction1 LiAlH4 / ethoxyethane

2 H3O+

aldehyde 1° alcohol

ketone 2° alcohol

Carboxylic acids

esterification conc. H2SO4 as catalyst esters

reduction1 LiAlH4 / ethoxyethane

2 H3O+

alcohols

amide synthesis

reaction with NH3(aq), heat amides

Esters hydrolysis reflux with acid or alkalialcohols and carboxylic acids

(or their salts)

Amides hydrolysis reflux with acid or alkalicarboxylic acids(or their salts)

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152

Note: The symbol indicates the level of difficulty of a question.

Part I Knowledge and understanding

1 Complete the flow chart that includes reactions among the following carbon compounds: alkane, alkene, haloalkane, alcohol, aldehyde, ketone, carboxylic acid, ester and amide.

alkene

alcohol

aldehyde

amide

ester

R: H2 / Pt catalystT: addition

Key:R ReagentT Reaction type

R:

T:

R: HCl / HBr / HI

T:

R: HCl / HBr / HI

T:

R: OH–(aq)

T:

R:

T:

R:

T:

R: K2Cr2O7 / H3O+

T:

then to

R:

T:

R: K2Cr2O7 / H3O+

T:

R:

T:

R:

T:

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153

2 From the information given, draw the structural formula of each carbon compound. All of the compounds consist of molecules which have four carbon atoms.

a) A hydrocarbon that rapidly decolorizes aqueous bromine.

b) A compound that is oxidized to a ketone.

c) An ester that is formed from ethanoic acid.

d) An aldehyde that gives butan-1-ol upon reduction.

e) An amide.

3 Draw structural formulae of the products formed from the reactions of but-2-ene.

Br2 (in organic solvent) Br2(aq)

H2 / Ni catalyst MnO4– / OH–

CC

CH3H3C

HH

HBr(g)

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154

4 Draw structural formulae of the products formed from the reactions of propan-1-ol.

NaBr + conc. H2SO4

conc. H2SO4

as catalyst

CH3CH2CH2OH

excess conc. H2SO4HCl / ZnCl2 as catalyst

NaI + conc. H3PO4 K2Cr2O7 / H3O+

then to

Structural type Structural formula of the isomerStructural formula for the

compound (if any) formed by complete oxidation of the alcohol

Primary

Secondary

Tertiary

5 a) A primary alcohol, a secondary alcohol and a tertiary alcohol are isomers with molecular formula C4H9OH. Each was heated under reflux with potassium dichromate in dilute sulphuric acid. Complete the table below.

155


155

b) Propan-1-ol, CH3CH2CH2OH, can be converted to CH3CH2CH2I using red phosphorus and iodine.

i) Name the compound CH3CH2CH2I.

ii) State the conditions needed to react propan-1-ol with red phosphorus and iodine.

iii) This halogenation of propan-1-ol is brought about by an intermediate compound produced from the reaction between red phosphorus and iodine. Suggest the formula of this intermediate.

(Edexcel Advanced Subsidiary GCE, Unit Test 2, Jun. 2006)

Part II Multiple choice questions

6 What type of reaction occurs when the following compound reacts with sodium hydroxide solution?

CH2CH2Cl

A Addition B Dehydration C Elimination D Substitution

7 Many alcohols are oxidized by warming with acidified potassium dichromate solution.

Which of the following alcohols resists this oxidation?

A B

C

C OH

CH3

CH3

CH3

D

C OH

CH3

H

CH3CH2

CH2OHCH2OH

C H

CH3

CH2OH

CH3 C H

CH3

CH2OH

CH3

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156

8 Refer to the reaction scheme shown below.

Step 1 Step 2 CH3CH2CH2CHO CH3CH2CH2CH2OH CH3CH2CH2CH2Br

Which of the following reagents can bring about the reaction indicated?

Step 1 Step 2

A LiAlH4 Br2(aq) B LiAlH4 NaBr / conc. H2SO4

C K2Cr2O7 / H3O+ Br2(aq)

D K2Cr2O7 / H3O+ NaBr / conc. H2SO4

9 An alcohol and a carboxylic acid are heated in the presence of concentrated sulphuric acid under reflux. The following compound is obtained.

C O

H

H

CH

H

C H

H

C

HH

H H

O

C

Which of the following combinations is correct?

Alcohol Carboxylic acid

A Ethanol ethanoic acid B Ethanol propanoic acid C Propanol ethanoic acid D Propanol propanoic acid

10 The compound CH3CH(OH)CH2COOCH3 is found in marshmallows. Which of the following statements concerning the compound are correct?

(1) Its systematic name is ethyl 2-hydroxybutanoate. (2) It has one chiral carbon. (3) It turns warm acidified K2Cr2O7 (aq) from orange to green.

A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3)

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157

Part III Structured questions

11 Give the structural formula(e) of the major product(s) expected from each of the following reactions.

CH2CH2OH

Na2Cr2O7 / H3O+

heat

COOHH3C1 LiAIH4 / ethoxyethane

2 H3O+

CH3CH2 CH2CH3OC

O

NaOH(aq)

CH3

NaOH(aq)NH2C

O

CH CHCH

O

1 LiAIH4 / ethoxyethane

2 H3O+

CH3

Cl2

UV light

OCCH3

O

COOH

OH–(aq)

heat

CH3CH2CH2OHexcess conc. H2SO4

180 °C

a)

b)

c)

d)

e)

f)

g)

h)

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158

12 Hydrocarbon G reacts with HBr to give J as the major product.

CC

CH2CH3H3CH2C

HH3CG

+ HBr J

Give the structure of J and its systematic name. (HKALE 2004)

13 Consider the following reaction of 1-bromobutane.

C

H

H

C

H

H

C

H

H

H C Br

H

H

OH–(aq)

heatA

a) Draw the structural formula of A.

b) i) State the reagent(s) and condition(s) required to convert A back to 1-bromobutane.

ii) Name the type of reaction that takes place.

c) A student attempted to prepare A using the same reagents and conditions but using 1-fluorobutane in place of 1-bromobutane.

Suggest why the reaction would proceed at a different rate.

159


159

14 Ethanol, C2H5OH, can be produced by the fermentation of glucose, C6H12O6.

a) Ethanol has a relatively high boiling point. This can be explained in terms of intermolecular hydrogen bonds.

Draw a second molecule of ethanol alongside the one drawn below and show how a hydrogen bond could be formed. Clearly show any relevant dipoles and lone pairs of electrons.

O

CH2CH3

H

b) When ethanol is heated with acidified potassium dichromate solution, it can be oxidized to form either ethanal, CH3CHO (Fig. A), or ethanoic acid, CH3COOH (Fig. B).

The boiling points of ethanol, ethanal and ethanoic acid are given in the table below.

CH3CH2OH CH3CHO CH3COOH

Boiling point (°C) 78 21 118

Use this table of boiling points to explain

i) why the organic product is likely to be ethanal if the apparatus shown in Fig. A is used;

ii) why the organic product is likely to be ethanoic acid if the apparatus shown in Fig. B is used.

c) Write a balanced equation for the oxidation of ethanol to ethanoic acid. Use [O] to represent the oxidizing agent.

(OCR Advanced Subsidiary GCE, Chains and Rings, Jan. 2005)

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160

15 X, Y and Z are carbon compounds. The flow diagram below shows the conversion of X to Z.

acidified K2Cr2O7(aq)

heatconc. H2SO4

heat

X Y

methanol

Z

a) Z has a pleasant smell and its molecular formula is C4H8O2. Draw the structure of Z.

b) To which homologous series does Y belong?

c) Give the systematic name of X.

d) State the expected observation when X reacts with acidified potassium dichromate solution.

e) State the function of concentrated sulphuric acid in the reaction of Y with methanol. (HKCEE 2006)

16 The following reaction scheme shows some of the reactions of butan-2-ol.

solid XCH3CH(OH)CH2CH3

butan-2-oI CH2 CHCH2CH3

A

B

Na2Cr2O7(aq) and H2SO4(aq)

a) Why is butan-2-ol classified as a secondary alcohol?

b) Compound A can be prepared from butan-2-ol by passing its vapour over a heated solid, X.

i) Give the name of the carbon compound A.

ii) Name the solid X.

iii) What type of reaction is taking place?

iv) Draw a labelled diagram of the apparatus you would use to prepare and collect gas A from butan-2-ol.

c) Give the structural formula and the name of compound B.

d) Butan-2-ol can be used to clean plastic materials, such as CDs and DVDs. Suggest ONE precaution which should be taken when using butan-2-ol in this way.

(Edexcel GCE (Nuffield) Unit Test 1, Jun. 2005)

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161

17 Ethanol, C2H5OH, can be converted into ethanal, CH3CHO, if it is heated with an acid and sodium dichromate solution, provided that the ethanal is immediately distilled off.

A possible arrangement of apparatus for this experiment is shown below. However, it is incompletely labelled and the diagram contains some errors. You may assume that the apparatus is correctly clamped.

a) What are the names of the three items labelled A, B and C?

b) List THREE errors in this diagram.

c) Which acid should be used?

d) What type of reaction is the conversion of ethanol to ethanal? Justify your answer by considering their formulae.

e) i) What is the formula of the dichromate ion in sodium dichromate?

ii) What colour change would you expect to see as the reaction proceeded?

f) If the mixture is refluxed first before being distilled, what is the name and formula of the carbon compound formed?

(Edexcel Advanced Subsidiary GCE (Nuffield), Unit Test 1, Jan. 2007)

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162

18 Geraniol (C9H15CH2OH) is a naturally occurring compound that contributes to the smell of roses. The skeletal formula of geraniol is shown below.

OH

a) Mild oxidation of geraniol gives an aldehyde X.

i) Draw the skeletal formula of aldehyde X.

ii) Complete the equation for the oxidation of geraniol to aldehyde X.

C9H15CH2OH + [O]

b) Reaction of geraniol with ethanoic acid can be used to make ester Y, which is used in chewing gum and desserts.

i) Suggest why esters are used in the manufacture of foods.

ii) State the conditions needed to make ester Y from geraniol and ethanoic acid.

iii) Complete the equation for the formation of ester Y.

+ C9H15CH2OH

(OCR Advanced GCE, Chains, Rings and Spectroscopy, Jan. 2005)

19 a) A carbon compound, W, with molecular formula C4H10O reacts with phosphorus pentachloride to give compound X, C4H9Cl.

When W is heated with potassium dichromate solution and dilute sulphuric acid there is no colour change.

i) Identify the functional group present in W.

ii) Draw the structural formulae of W and X.

iii) When a structural isomer of W is heated under reflux with acidified potassium dichromate solution, it produces compound Y, C4H8O2.

Suggest a possible identity for Y.

b) Propene, C3H6, reacts with hydrogen bromide, HBr.

Draw the structures of the two possible products and indicate which is the major product.

(Edexcel Advanced Subsidiary GCE, Unit Test 2, Jan. 2007)

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163

20 In a certain experiment, a mixture of methyl propanoate and sodium hydroxide solution was heated under reflux for some time. The resulting mixture M was then transferred to flask Y and heated as shown below.

a) i) Write a chemical equation for the reaction between methyl propanoate and sodium hydroxide solution.

ii) Name the type of reaction taking place.

b) i) Name apparatus X.

ii) What was the function of apparatus X?

c) Name the distillate collected in flask Z.

d) What was the purpose of using anti-bumping granules?

e) Which one of the following hazard warning labels should be displayed on a bottle of methyl propanoate?

A B C D

f) Draw the structure of another ester which has the same molecular formula as methyl propanoate, and give its systematic name.

164


164

21 Describe, by giving reagent(s) and stating observations, how you could distinguish between compounds in each pair using a simple test tube reaction.

a)

CH2CH2CHO CCH2CH3

O

and

b)

CH3CCH3

Oand CH2=CHCH2OH

c) CH3CH2COOH and CH3COCH2OH

22 Compound A is used to add the flavour of mushrooms to foods.

CH2

CC

OCH3C

O

CH3H

compound A

a) i) Apart from the benzene ring, name the TWO functional groups in compound A.

ii) Draw the skeletal formula of compound A.

iii) Deduce the molecular formula of compound A.

b) Compound B is a stereoisomer of compound A.

Explain what is meant by the term ‘stereoisomerism’. Use compounds A and B to illustrate your answer.

c) If the food is cooked for a long time, naturally occurring acids catalyze the hydrolysis of compound A.

Draw structures to show the TWO carbon compounds formed by the acid hydrolysis of compound A.

(OCR Advanced GCE, Chains, Rings and Spectroscopy, Jan. 2006)

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165

23 Oseltamivir is an antiviral drug against the avian virus H5N1. It is also known by the brand name Tamiflu®.

a) Mark each chiral centre with an asterisk ‘*’ on the structure of oseltamivir shown below.

CH2CH3

CH3

CH3CH2

CH2CH3

CHNH2

C

NH

O

OC

OO

oseltamivir

b) Besides the ether linkage, how many functional groups are there in oseltamivir? Name TWO of these functional groups.

c) Given that ether linkages are NOT affected by alkalis, write the structure of the products formed when oseltamivir is heated with excess NaOH(aq).

(HKASLE 2007)

24 Compound X is an aldehyde with the molecular formula C5H8O. It is known that X contains the following functional group as well.

CC

a) X shows no geometrical isomerism. Give TWO possible structures of X, neither of which is chiral.

b) Select ONE of your structures for X. This reacts with HBr to form two products, one of which is chiral.

i) Give the structures of the two products, indicating which one is chiral.

ii) Indicate which of the two products is likely to be in a greater yield.

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166

25 Aromatic compounds P, Q and R are esters with the same molecular formula C8H8O2.

a) A mixture of P and aqueous NaOH was heated under reflux for an hour. Excess dilute H2SO4 was then added to the resulting mixture and a white precipitate (C7H6O2) was formed. Suggest the structure of P and write an equation for the reaction of P with aqueous NaOH.

b) A mixture of Q and aqueous NaOH was heated under reflux for an hour. Excess dilute H2SO4 was then added to the resulting mixture. Upon warming, a smell of vinegar was detected. Deduce the structure of Q with the help of chemical equations.

c) Propose one possible structure of R. (HKASLE 2006)

26 The rates of hydrolysis of chloroethane, bromoethane and iodoethane are different.

• Describe how you would monitor the reaction rates.

• Explain why chloroethane, bromoethane and iodoethane react at different rates.

(For this question, you are required to give answers in paragraph form. Use equations, diagrams and examples where appropriate.)

(OCR Advanced Subsidiary GCE, Chains and Rings, Jun. 2005)

topic 8 chemistry of carbon compounds 31

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