Unit 2.8 - Organic chemistry - alcohols and halogenoalkanes

Revision - IUPAC system for naming organic compounds

You should remember from unit 1 that there is a set of rules, set by IUPAC, for classifying and

naming organic compounds. As we will be studying further organic chemistry in this unit now is a

good time to remind yourself of those rules:

PrefixNumber of Carbons in

chain

Position of functional groups in the chain  

   

meth-  1. The 'parent' chain is the ___________ possible chain   

eth-  2. The carbon with the functional group attached is given a ____________.  

prop-     3. Numbering starts from the end of the chain that gives

but-   the carbon being considered the ____________ number.       

pent-  Alkyl groups

   

hex-     

methyl 

hept-   1. Position in chain is

ethyl 

oct-   indicated by ___________ possible

propyl dec-   number.

   

dodec-  2. If more than 1 side chain is attached then the name of the compound includes the side groups in

eicos-   _________________ order, regardless of which carbon atom they are found on.

 

Try these examples:

4-methylpent-1-ene E-hex-2-ene

1. Alcohols

Introduction

Ethanol is the compound generally referred to as alcohol and found in

alcoholic drinks. It is most commonly produced through the action of yeast

on natural sugars otherwise known as fermentation. Many other alcohols

do exist; in chemistry ‘alcohol’ refers to the homologous series of

molecules containing the hydroxyl (-OH) functional group. The reactivity

of alcohols is based on the hydroxyl group. They are industrially important as

raw materials in the synthesis of other compounds and as solvents.

Naming

Naming alcohols is relatively simple - the name of the alkyl group containing the ______________

number of carbons is used and the suffix ______ is added. Here are some examples:

Name Displayed formula Skeletal formula

Methanol

Ethanol

Propan-1-ol

________pentan-2-

ol

Primary, secondary or tertiary?

There are three main types of alcohol - primary, secondary and tertiary; they are defined by the

number of alkyl (-C) groups attached to the carbon with the hydroxyl (-OH) group:

Primary alcohols

Ethanol

Primary alcohols have the general structure RCH2OH. There are two hydrogens and one alkyl group

attached to the same carbon as the hydroxyl group

Secondary alcohols

Propan-2-ol

Secondary alcohols have the general structure RR1CHOH. There is one hydrogen and two

__________ groups attached to the same carbon as the hydroxyl group

Tertiary alcohols

2-methylpropan-2-ol

Tertiary alcohols…

NB These differences in structure are very important as they affect the way that the alcohol reacts!

Reactions of Alcohols - combustion, reduction, oxidation and substitution

a. Combustion

Alcohols burn readily in air or even more readily in pure oxygen. Many burn with an almost

unperceivable blue flame. Combustion of alcohols produces carbon dioxide and _________

CH3CH2OH(l) + ___O2(g) ___CO2(g) + ___H2O(l)

CH3CH2CH2CH2OH(l) + ___O2(g) ___CO2(g) + ___H2O(l)

b. Reaction with sodium

Sodium reacts vigorously with water to produce _________________ and ____________.

The sodium is less dense than water so it floats, fizzes and moves around. Water acts as

an acid in this reaction by losing a proton

The reaction with alcohols is _________vigorous and the sodium _________ in the ethanol

as it is _________ dense than the alcohol

2Na(s) + 2CH3CH2OH(l) 2CH3CH2O-Na+(alc) + H2(g)

sodium ethoxide

The products are _________________ and ___________, the reaction is less vigorous

because ethanol is a _____________ acid than water

c. Oxidation

Oxidation reactions are a useful test to find out if an alcohol is primary, secondary or tertiary

as they each give different products. Common oxidising agents such as acidified potassium

dichromate(VI) and potassium manganate(VII) can be used…

Primary alcohols - Are readily oxidised to form aldehydes, which then rapidly oxidise to

form carboxylic acids:

Primary alcohol Aldehyde Carboxylic acid*

*The most common product of this oxidation is the carboxylic acid; however, the aldehyde can

be separated during the reaction

Secondary alcohols - Are readily oxidised to form ketones with no further oxidation

occurring:

Propan-1-ol Propanal Propanoic acid

+ H2O

Tertiary alcohols - Are not oxidised by any of the common oxidising reactions, there is

simply no reaction

Oxidation as a test for primary, secondary or

tertiary

Add acidified potassium dichromate (VI)

Primary -

Secondary -

Tertiary -

Chromium(III) compounds are dark green

Distillation - separating the products of oxidation

Distillation can be used to prepare a ketone from a secondary alcohol:

Compound Boiling temp (C)Butanone 80Butan-2-ol 95Only the ketone will be collected if the temperature is kept between 80 C & 94 C

Or to separate the products of oxidation of a primary alcohol:

Reflux - preparing carboxylic acids from primary

alcohols

Heating under reflux ensures that the oxidation reaction is completed

Compound Boiling temp (C)Ethanol 78Ethanal 21Ethanoic acid 118The aldehyde can be collected between 21 C & 70 C

Propan-2-ol

Secondary alcohol

Propanone

Ketone

d. Substitution to form halogenoalkanes

Alcohols react with phosphorous(V) chloride (PCl5) to produce a chloroalkane and hydrogen chloride

gas…

CH3CH2OH(l) + PCl5(s) CH3CH2Cl(l) + HCl(g)

NB this reaction can be used as a test for alcohols - if you add PCl5 to an unknown liquid the

production of HCl gas is evidence for the presence of an -OH group.

Preparation - halogenoalkanes from alcohols

e.g. the preparation of 1-bromobutane from butan-1-ol…

Step 1 - Sodium bromide reacts with conc. sulphuric to produce hydrogen bromide

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

Step 2 - Hydrogen bromide reacts with alcohol to produce 1-bromobutane

CH3CH2CH2CH2OH(l) + HBr(g) CH3CH2CH2CH2Br(l) + H2O9(l)

Step 3 - The products are distilled to yield a mixture, which separates into 2 layers - aqueous

and organic, a separating funnel is used to collect the organic layer

Step 4 - The aqueous layer is discarded and the organic layer is redistilled to produce 1-

bromobutane, which has a boiling temperature of 102 C

2. Halogenoalkanes

Any aldehyde or unreacted primary alcohol

evaporates, condenses here in the

_______________ and falls back into the

reaction vessel where it can it oxidised

The oxidation reaction takes place here. First the

aldehyde is produced then, after further oxidation, the

________________. Heat causes the products to

_____________.

Introduction

Halogenoalkanes are saturated hydrocarbons with one or several halogen functional groups. The

halogens are found in group ___ and consist of __________, ___________, __________ and

____________.

Uses of halogenoalkanes (p214-p217)

Until 1986 chlorofluorocarbons (CFCs) were routinely used as

refrigerants in fridges and freezer and propellants in aerosols;

however, it was discovered that they undergo radical reactions

when exposed to UV radiation high in the atmosphere, which lead

to the removal of ozone (O3) from the ozone layer…

CCl2F2 CClF2 + Cl

Cl + O3 ClO + O2

ClO + O3 Cl + 2O2 Net reaction: 2O3 3O2

As the ozone layer protects us from harmful UV radiation, the

international community chose to ban the use if CFCs. Unfortunately many CFCs are very unreactive

and will persist in the atmosphere for hundreds of years.

Naming

Naming halogenoalkanes is relatively simple and follows the IUPAC convention…

Prefix with fluoro, chloro, ____________ or _____________ depending on the halogen

More than one halogen group can be indicated by adding di, tri or tetra

Numbers are used to indicate the position of the halogen group in the chain

_________________ __________________ __________________ __________________

Primary, secondary or tertiary?

Just like alcohols, halogenoalkanes can be either primary, secondary or tertiary depending on the

number of alkyl groups attached to the carbon with the halogen group…

Primary halogenoalkanes

Chloroethane

Primary halogenoalkanes have the general structure RCH2X. There are two hydrogens and one

alkyl group attached to the same carbon as the halogen group

Secondary halogenoalkanes

2-chloropropane

Secondary halogenoalkanes have the general structure RR1CHX. There is one hydrogen and two

__________ groups attached to the same carbon as the halogen group

Tertiary halogenoalkanes

2-bromo-2-methylpropane

Tertiary halogenoalkanes…

Primary, secondary and tertiary halogenoalkanes react very differently. Luckily there is a test, which

can be used to identify which is present using silver nitrate solution. We will cover the test in the

reactions section under hydrolysis.

Reactions - hydrolysis, elimination and substitution

As mentioned on the previous page, the position of the halogen in the hydrocarbon chain affects

reactivity, as does the type of halogen present…

Bond Average bond energy (kJmol-1)

C-F 467

C-Cl 346

C-Br 290

C-I 228

a. Hydrolysis - ‘splitting with water’

Halogenoalkanes react with water or hydroxide (OH-) ions in a reaction known as hydrolysis. The

products are an alcohol and a hydrogen halide: RX + H2O ROH + HX

RX + OH- ROH + X-

e.g. CH3Cl + H2O _____ + _____

CH3CH2CH2Br + H2O _____________ + _____

NB Reactivity depends on the H-X bond strength as shown in the table above. Fluorocarbons are the

least likely to undergo hydrolysis

Test for primary, secondary or tertiary - silver nitrate

Procedure

Add 1 cm3 of silver nitrate and 1 cm3

ethanol (__________) to each tube

and warm

Add 5 drops of the different

halogenoalkanes and observe

Explanation

Halogenoalkanes react with water

to form an _____________ and

some _____________ ions

Silver nitrate (AgNO3) reacts with

the halide ions to form an insoluble

_______________

Rate of reaction:

_________ > _________ > _________

b. Reaction with potassium hydroxide

1-bromobutane 2-bromobutane 2-bromo-2-

methylbutane

Primary

C-X bonds become ______________ down

the group, meaning that fluorocarbons are

very _______________ and iodocarbons are

relatively ______________

Potassium hydroxide (KOH) can react with halogenoalkanes in one of two ways depending on

whether it is dissolved in water or ethanol:

Conditions Type of reaction Product

i) Reflux heating with aqueous

KOH (dissolved in _________)Substitution Alcohol

ii) Reflux heating with alcoholic

KOH (dissolved in __________)Elimination Alkene

For example:

i) CH3CHBrCH3(l) + KOH(aq) ____________ + ______

2-bromopropane aqueous KOH

ii) CH3CHBrCH3(l) + KOH(alc) ____________ + ______

2-bromopropane alcoholic KOH

c. Reaction with alcoholic ammonia

There is a ________ pair of electrons on the ____________ atom of

ammonia (NH3), this means it can act as a nucleophile and replace the

halogen atom in a halogenoalkane:

i) CH3CH2I + NH3 CH3CH2NH2 + HI

ethylamine

Ethylamine also has a lone pair of electrons on nitrogen so it can react further with iodoethane and

there is a chain reaction*:

ii) CH3CH2I + CH3CH2NH2 (CH3CH2)2NH + HI

iii) CH3CH2I + (CH3CH2)2NH (CH3CH2)3N +HI

iv) CH3CH2I + (CH3CH2)3N (CH3CH2)4N+ I-

tetraethylammonium iodide

*Skeletal formulae for the amine products reactions i-iv respectively:

Spider diagram of A-level organic chemistry reactions

Task - Produce your own spider diagrams to represent the reactions of alcohols and halogenoalkanes that we have covered. Be sure to include:

Reactants Conditions Products