new way chemistry for hong kong a-level book 3a new way chemistry for hong kong a-level 3a 1...

New Way Chemistry for Hong Kong A-Level Book 3A New Way Chemistry for Hong Kong A-Level

3A

1

Alkenes Alkenes and and

Electrophilic AdditioElectrophilic Additionn


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Preparation Preparation of Alkenesof Alkenes


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CrackinCrackingg• Prepared by the cracking of alkanes

of high molecular masses

• Give alkenes of low molecular masses

A. Industrial preparation


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CrackinCrackingg

e.g.

2CH3CH3 CH2 = CH2 + 2CH4600 oC

2CH3CH2CH3

CH3CH = CH2 + CH2 = CH2 + CH4 + H2

600 oC


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Elimination ReactionsElimination Reactions

• Involve removal of atoms or groups of atoms from adjacent carbon atoms in the reactant molecule

• Formation of a double bond between carbon atoms

B. Synthetic preparation


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1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols• Removal of a water molecule from

a reactant molecule

• By heating the alcohols in the presence of a dehydrating agent.

E.g. Alumina(Al2O3), conc. H2SO4,

conc. H3PO4

• Give alkenes and water as the products


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1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols

CH3CH2OHalumina

350oCC C

H

H

H

H

+ H2O

CH3CH2OH

conc. H2SO4

up to 200oCC C

H

H

H

H

+ H2O


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• Experimental conditions (i.e. temperature and concentration of concentrated sulphuric acid)

is closely related to the structure of the individual alcohol.



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• Primary alcohols generally required concentrated sulphuric acid and a relatively high temperature



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• Secondary alcohols are intermediate in reactivity

• Tertiary alcohols dehydrate under mild conditions (moderate temperature and dilute sulphuric acid)



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• The relative ease of dehydration of alcohols generally decreases in the order:

Tertiary alcohol

Secondary alcohol

>

Primaryalcohol

>



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• Intramolecular vs intermolecular

H C C OH

HH

H H

conc. H2SO4

170oCC C

H

H

H

H

+ H2O

Substitution

H C C OH

H

H

H

H

H O C C H

H

H

H

H

+

C2H5 O C2H5 + H2O


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Intramolecular dehydration is favoured at higher temperatures because it involves breaking of strong C – H bonds.


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Q.29(a)

C

CH3

H3C

CH3

OH

H3C C OH

CH H

H

CH3

C C

H3C

H3C

H

H

+ H2O


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15

Q.29(b)OH

OH

H

+ H2O


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16

Q.29(c)C C C CH3

OH

H

H

H

H

H

H

C C

H

H

H

C2H5

+ H2OC C C CH3

OH

H

H

H

H

H

H

C C C CH3

OH

H

H

H

H

H

H C C

H

H3C

H

CH3

+ H2O

C C

CH3

H

H

H3C


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• Secondary and tertiary alcohols may dehydrate to give a mixture of alkenes

• The more highly substituted alkene is formed as the major product



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2. Dehydrohalogenation of haloalkanes2. Dehydrohalogenation of haloalkanes

• Elimination of a hydrogen halide molecule from a haloalkane

• By heating the haloalkane in an alcoholic solution of KOH

H C C H

H

X

H

H

OH-

C2H5OH, heatC C

H

H

H

H

+ H2O + X-


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H C C H

H

X

H

H

OH-

C2H5OH, heatC C

H

H

H

H

+ H2O + X-

C2H5OH is a co-solvent for both RX and OH


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e.g.

2. Dehyhalogenation of haloalkanes2. Dehyhalogenation of haloalkanes


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• Dehydrohalogenation of secondary or tertiary haloalkanes can take place in more than one way

• A mixture of alkenes is formed



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Q.30(a)H2C C Br

CH3

CH3

H3C NaOH

C2H5OH, heat

C C Br

CH3

CH3

NaOH

C2H5OH, heat

H

H3C

H

C C

CH3

CH3

H

H3C

C C Br

CH3

C

NaOH

C2H5OH, heat

H

H3C

H

C C

H

H

H3C

C2H5

H

H H

major

minor


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Q.30(b)Cl

CH3

NaOH

C2H5OH, heat

Cl

CH3

NaOH

C2H5OH, heat

H

CH3

Cl

C

NaOH

C2H5OH, heat

H

H

H

C

H

H

major

minor


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• The ease of dehydrohalogenation of haloalkanes decreases in the order:

Tertiary haloalkane

Secondary haloalkane

>

Primaryhaloalkane

>



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• The relative stabilities of alkenes decrease in the order:


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Relative Stability of Alkenes in Relative Stability of Alkenes in Terms of Enthalpy Changes of Terms of Enthalpy Changes of HydrogenationHydrogenation

• Hydrogenation of alkenes is exothermic

• From enthalpy changes of hydrogenation

predict the relative stabilities of alkenes


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Enthalpy changes of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene


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Relative Stability of Alkenes in Relative Stability of Alkenes in Terms of Enthalpy Changes of Terms of Enthalpy Changes of HydrogenationHydrogenation

• The pattern of the relative stabilities of alkenes determined from the enthalpy changes of hydrogenation:


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Addition Addition ReactionsReactionsHydrogenation of alkynesHydrogenation of alkynes

• Alkenes can be prepared by hydrogenation of alkynes

Depend on the conditions and the catalyst employed


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HydrogenationHydrogenation

• Lindlar’s catalyst is metallic palladium(Pd) deposited on calcium carbonate

further hydrogenation of the alkenes formed can be prevented


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Reactions of Reactions of AlkenesAlkenes

An An IntroductionIntroduction


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• Alkenes are more reactive than alkanes

• Undergoes addition reaction rather than substitution


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• Presence of C=C double bond

• C=C double bond is made up of a bond and a bond

• Addition reactions only involve breaking of weaker bonds of alkenes


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• The electrons of the bond are

diffuse in shape

less firmly held by the bonding carbon

nuclei

Susceptible to the attack by electrophiles


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Electron-deficient species

Attack electron-rich center e.g. C=C bond

Examples :

Cations : H+, Br+, R+,… (lead to heterolysis)

Free radicals : H, Cl, R,…(lead to homolysis)

Electrophiles : -


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Electron-rich species

Attack electron-deficient site

e.g. carbonyl carbon, C=O

Examples :

anions : OH, Br, RO,…

molecules : H2O, ROH, NH3

Nucleophiles : -

All have lone pairs for donating to the reaction sitesAll lead to heterolytic fissions


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Reactions of Reactions of AlkenesAlkenes

ExamplesExamples


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Catalytic Catalytic HydrogenationHydrogenation• Alkenes react with hydrogen in the

presence of metal catalysts (e.g. Ni, Pd, Pt) to give alkanes

Lower temperatures can be used with Pd or Pt


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Catalytic Catalytic HydrogenationHydrogenation

e.g.

cis-addition, refer to notes on ‘chemical kinetics’, pp.36-37)


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O

H3C CH3

CH2H2 / Pt

25oCCH3

CH3

H3C

O

Under mild conditions, C=O and benzene ring are unaffected.


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Q.31

C5H10

C5H10

H2 / Pt

H2 / Pt

C5H12

no reaction

BA

C


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42

A / B


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43

A / B


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44

C

** *

***


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Application : - hardening of plant oils

Plant oil (polyunsaturated liquid with low m.p.)

Margarine (soft unsat’d solid with higher m.p.)

Animal fat (hard sat’d solid with still higher m.p.)

Partial hydrogenation

Complete hydrogenation


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• Fats and oils are organic compounds called triglycerides

triesters formed from glycerol and carboxylic acids of long carbon chains


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• Saturated fats

solids at room temp

usually come from animal sources

long carbon chains are zig-zag and easily packed


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• Unsaturated oils

liquids at room temp

usually come from plant sources

lower m.p. due to cis-arrangement (kinked shape)


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• Fats are stable towards oxidation by air

• More convenient to handle and store


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Catalytic Catalytic HydrogenationHydrogenation• Advantages:

higher m.p. ideal for baking

turning rancid much less readily than unsaturated oils


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Application : - hardening of plant oils

Plant oil (polyunsaturated liquid with low m.p.)

Margarine (soft unsat’d solid with higher m.p.)

Animal fat (hard sat’d solid with still higher m.p.)

Partial hydrogenation

Complete hydrogenation


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H2C

CH

H2C

O

O

O

O

O

O

HC

O

OO

O

H2C

H2C

O

O

H2 / Ni150°C, 5 atm

trans-fat coronary heart disease


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Hydrogenation of vegetable oils produces margarine


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54


• Margarine and butter do not have sharp m.p. because they are NOT pure substances.

• They are mixtures containing different triesters.


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Electrophilic Addition Reactions(AdElectrophilic Addition Reactions(AdEE))

• Addition of electrophiles to the C=C double bond of alkenes


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• Electrophiles that attack the C=C double bond include

protons (H+)

neutral species in which the molecule is polarized, e.g. bromine


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(a) Addition of halogens in non-aqueous solvents(a) Addition of halogens in non-aqueous solvents

X = Cl, Br or I

CH3CCl

3

Occurs with or without light

Addition is preferred to substitution

Reaction mechanism is not required


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C C

Br

Br

+

bromonium ion

C C

Br

+ Br


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59

C C

Br

Br-

C C

Br

Br-

C C

Br

Br

C C

Br

Br

trans-addition


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61

e.g.



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62

• The decolourization of bromine in 1,1,1-trichloroethane is a useful test for unsaturation

A drop of bromine

dissolved in 1,1,1-

trichloroethane is added to

an alkene

The reddish brown

colour of bromine is decolourize

d



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(b) Addition of halogens in aqueous solutions(b) Addition of halogens in aqueous solutions

C C + X2(aq)

C C

OH

X

(major)

C C

X

X

(minor)

-OH comes from H-OH which is in excess.

Reaction mechanism is not required.


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64

e.g.

• The consequent decolourization of the reddish brown colour of bromine water is also a test for unsaturation

(b) Addition of halogens in aqueous solutions(b) Addition of halogens in aqueous solutions


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65

C C

Br

Br

+

bromonium ion

C C

Br

+ Br


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66

C C

Br

O

H H

C C

Br

O

H H

O

HH

C C

Br

OH

+ H3O+

bromohydrin


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67

Q.32

NaCl(aq)

C C + Br2

C C

Br

Br

C C

Br

OH

+ C C

Br

Cl

+


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68

C C

Br

Cl

C C

Br

Cl


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69

Q.32

NaI(aq)

C C + Br2

C C

Br

Br

C C

Br

OH

+ + C C

Br

I


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70

C C

Br

I

C C

Br

I


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71

Q.32

NaNO3(aq)

C C + Br2

C C

Br

Br

C C

Br

OH

+ + C C

Br

ONO2


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72

C C

Br

NO3

C C

Br

ONO2


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73

(c) Addition of H – X (c) Addition of H – X

X = Br, Cl, OSO3H, OH, etc.

Mechanism required

H-Br

conc. H-OSO3H

H-OH H3O+

C C

H

H

H

H

H C C H

HH

H Br

H C C H

HH

H OSO3H

H C C H

HH

H OH

Acid-catalyzed hydration


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Addition of Hydrogen BromideAddition of Hydrogen Bromide• A molecule of HBr adds to the C=C

double bond of an alkene

• Give a bromoalkane


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Reaction Mechanism: Electrophilic Reaction Mechanism: Electrophilic Addition Reactions of Hydrogen Addition Reactions of Hydrogen Bromide to AlkenesBromide to Alkenes

sp2 hybridized carbonium ion

Br is a nucleophile

rate-determining step

C

H

H

H

C Br-C C

Br

H

+C C

BrH

fast


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76

sp2 hybridized trigonal planar

C

H

H

H

C Br-C C

Br

H

+C C

BrH

fast

50%

50%

50%50%

If the resulting C is chiral

* *

racemic mixture


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77

Q.33


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78

(a)one bond and one bond are broken

(b) two bonds are formed

(c) Heat evolved during bond formation >Heat required during bond breaking

Addition reactions are usually exothermic

Q.33

view movie


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79

Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule

CH2=CH2 & CH3CH=CHCH3 are symmetrical alkenes.

CH3CH=CH2 is an asymmetrical alkene.


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• A hydrogen halide can add to an asymmetrical alkene in either of the two ways

• The reaction proceeds to give a major product preferentially

the reaction is said to exhibit “regioselectivity”


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the addition of HBr to ethene produces bromoethane as the only product



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• When but-2-ene reacts with HBr

2-bromobutane is formed as the only product



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83

• When propene reacts with HBr

the major product is 2-bromopropane

the minor product is 1-bromopropane



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H is given to the rich


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85


Markovnikov’s rule states that in the addition of HX to an asymmetrical alkene, the hydrogen atom adds to the carbon atom of the carbon-carbon double bond that already has the greater number of hydrogen atoms


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• The products formed according to this rule are known as Markovnikov products


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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Carbocations are a chemical species that

contains a positively charged carbon

• Very unstable

• Exist transiently during the reaction

• Classified as primary, secondary or tertiary

according to the number of alkyl groups that are directly attached to the positively charged carbon


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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s RuleThe more stable the carbocation

the more stable the transition state

the lower the activation energy

the faster its formation


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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• The stability of the carbocations

increases in the order:


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Alkyl groups stabilize the positively charged carbocation by positive inductive effect


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• A greater number of alkyl groups

release more electrons to the positively charged carbon

increase the stability of the carbocation


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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Consider the addition of HBr to

propene:


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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• The hydrobromination of propene

involves two competing reactions:


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Since the formation of carbocation is the rate-determining step,

the overall reaction is faster if it involves the formation of a more stable carbocation.


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Q.34(a)

C C

H

H

H3C

H3CHX

H3C C C H

H

H

CH3

X

CH2H

H3C

H3C

> H3C C

CH3

H

C

H

H

X = Cl, Br, or I


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Q.34(b)

C C

H

H

H

H3C

H O S OH

O

O

++

cold

H C CH3

CH3

OSO3H

heatC CH3

H

H3C

> C2H5 C

H

H


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• On heating, alkyl hydrogensulphates form alkenes and sulphuric acid


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Separation of a mixture containing an alkane and an alkene.

Alkane / Alkene

conc. H2SO4 / cold

no reaction

Alkane

addition

C C

H OSO3H

dissolved in conc. H2SO4

insoluble in conc. H2SO4

Separated by separating funnel


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Separation of a mixture containing an alkane and an alkene.

Alkane / Alkene

conc. H2SO4 / cold

no reaction

Alkane

addition

C C

H OSO3Hheat

Alkene

dissolved in conc. H2SO4

insoluble in conc. H2SO4


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100

Alkyl hydrogensulphates can be easily hydrolyzed to alcohols by heating with water

conc. H2SO4 + H2O dilute H2SO4

acid-catalyzed hydration


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(c) Addition of H – X (c) Addition of H – X

X = Br, Cl, OSO3H, OH, etc.

Mechanism required

H-Br

conc. H-OSO3H

H-OH H3O+

C C

H

H

H

H

H C C H

HH

H Br

H C C H

HH

H OSO3H

H C C H

HH

H OH



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C C

H

H

H

H3C

H O S OH

O

O

++

C CH3

H

H3C

O

H

H

C

CH3

CH3

H

O

H

H


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103

C

CH3

CH3

H

O

H

H

O

H H

C

CH3

CH3

H

HO

+ H3O+The acid catalyst is regenerated



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104

Q.34(d)

(d)H2O / H+

OH

H

>

3 2


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105

Q.34(c)

C C

H3C

H3C

H

H

I – Cl +

H3C C C H

H

I

CH3

Cl

C CH2I

H3C

H3C

> CC

H

HCH3

H3C

I

3 1EN : C = I = 2.5

electron-donating


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106

Q.34(e)

C CH3

H

F3C

< CC

H

HCF3

H

H

More destabilized by negative inductive effect

H C

H

CF3

C

H

H

ClH

H

H

F3CHCl

(e)H – Cl


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107

C CH3

H

Cl

Q.34(f)

H

H

H

ClHCl

(f)H – Cl

Cl C C

H

Cl

H

H

H

C CH3

H

Cl

The resonance effect more than compensates the negative inductive effect of Cl

C C

H

HCl

H

H

>


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108

Q.34(g)

H

H

H

HBr(g)

H – Br C C

H

Br

H

H

H

C C

H

HH

H

C CH3

H

>

21


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109

C CH3

H

C CH3

H

C CH3

H

C CH3

H

The +ve charge is shared by the benzene ring by resonance effect.

Stabilized by resonance effect as well as inductive effect(2)


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110

C CH3

H

benzylic carbocation

More stable than 3 carbocation


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111

Q.34(h)

H2C CH

CH

CH2

excess H – F

H C C C C

H

F

H

H

H

F

H

H

H


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112

CCH

H

CH2H

H2C > C C

H

HH

H

CH

H2C

21

C C

H

CH2H

H

H2C

Allylic carbocation is stabilized by resonance effect.

Stabilized by resonance effect as well as inductive effect

Benzylic > allylic > 3 > 2 > 1 > CH3

+

Stability : -


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113

CH

C

H

CH2H

H2C F- C FCH

H2C

CH2H

H

H – F

C C C C H

H

H

H

F

H

H

F

H

H

C C C C H

H

H

H

F

H

HH

H

> C C C C H

H H

F

H

H

H

H

H

less destabilized by –ve I-effect of F


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114

Effect of substituents on the reactivity of AdE

1. Electron-donating groups increase the reactivity by

(a) the electron density of C=C bond, thus making it more susceptible to electrophilic attack.

(b) Stabilizing the carbocation intermediate/T.S. by +ve I-effect and/or resonance effect, thus lowering the Ea for the rate-determining step.


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Effect of substituents on the reactivity of AdE

2. Resonance effect > inductive effect

3. Electron-withdrawing groups lower the reactivity by working in the opposite

ways.


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116

Q.35

C

C

H H

H

>C

C

H3C CH3

H3C CH3

>C

C

H CH3

H CH3

>C

C

H H

H H

>C

C

H CF3

H H


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117

Oxidation of alkenes

(a) Combustion

C C + O2 CO2 + CO + C + H2O

More sooty and luminous than that of corresponding alkanes due to higher carbon contents


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118

(b) Reaction with KMnO4

C O

R2

R1

O C

R4

R3

+C C

R4

R3

R2

R1

KMnO4, H+ or OH

heat

C C

R4

R3

R2

R1

KMnO4, H+ or OH

coldR2 C C R4

R1

OH

R3

OH

Used as a test for alkenes

carbonyl products


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119

C O

R2

R1

O C

R4

R3

+C C

R4

R3

R2

R1

KMnO4, H+ or OH

heat

If all R groups are alkyl groups,

ketones will be the final products.

C C

CH3

CH3

H3C

H3C

KMnO4, H+ or OH

heat

C O

H3C

H3C

2


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KMnO4, H+ or OH

heatNo reaction

C O

H3C

H3C


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121

C O

R2

R1

O C

R4

R3

+C C

R4

R3

R2

R1

KMnO4, H+ or OH

heat

If either R1 or R2 is H / either R3 or R4 is H,

further oxidation of aldehydes to carboxylic acid will occur.

C C

H

CH3

H

H3C

KMnO4, H+ or OH

heatC O

H

H3C

2


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122

KMnO4, H+ or OH

heatC O

H

H3C

C O

HO

H3C

aldehyde carboxylic acid


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123

C O

R2

R1

O C

R4

R3

+C C

R4

R3

R2

R1

KMnO4, H+ or OH

heat

If both R1 & R2 are H / both R3 & R4 are H,

further oxidation to first methanoic acid and then CO2 will occur.

C C

H

H

H

H

KMnO4, H+ or OH

heatC O

H

H

2


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124

C O

H

H

2KMnO4, H+ or OH

heat2CO2 + 2H2O

C O

HO

H

2

methanoic acid


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125

(c) Ozonolysis

+ O3

CH3CCl3

< 20oCO O

O

unstable ozonide

O O

O

+ H2OZn dust

CH3COOH2 O + H2O2

Further oxidation of aldehyde to carboxylic acid by H2O2 is inhibited using Zn dust and CH3COOH


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126

(c) Ozonolysis

1. O3

2. Zn dust / H2OC O

H

H3C

CO

H

CH3

+C C

H

CH3

H

H3C

C C

H

H

H3C

H3C

C O

H3C

H3C

CO

H

H

+

1. O3

2. Zn dust / H2O


3A

127

Oxidative cleavage can be used to locate C=C bond in an unknown sample


3A

128

Q.36

C C

H

CH3

H

C

CH3

H2C

C C

CH3

H

H

C

CH3

H2C

(3Z)-2-methylpenta-1,3-diene

(3E)-2-methylpenta-1,3-diene


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Q.36CH3

H

H3C

H

CH2

CH3H

H3C

H

CH2

(3E)-3-methylpenta-1,3-diene

(3Z)-3-methylpenta-1,3-diene


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Q.37


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Q.37


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Q.37


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The END


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28.4 Preparation of Alkenes (SB p.173)

Classify the following alcohols as primary, secondary or tertiary alcohols.

(a) CH3CHOHCH2CH3

(b) CH3CH2CH2OH

(c) (CH3)2COHCH2CH2CH3 Answer(a) It is a secondary alcohol.

(b) It is a primary alcohol.

(c) It is a tertiary alcohol. Back


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Classify the following haloalkanes as primary, secondary or tertiary haloalkanes.

(a) (c)

(b)

Answer

(a) A secondary haloalkane

(b) A primary haloalkane

(c) A tertiary haloalkane

Back28.4 Preparation of Alkenes (SB p.173)


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28.5 Reactions of Alkenes (SB p.177)

Of the isomeric C5H11+ carbocations, which one

is the most stable?Answer

The more stable C5H11+ carbocation is the tertiary

carbocation as shown below:

Back


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Both alkanes and alkenes undergo halogenation. The

halogenation of alkanes is a free radical substitution

reaction while the reaction of alkenes with halogens is

an electrophilic addition reaction. Can you tell two differences between the products formed by the

twodifferent types of halogenation?

Back

AnswerAlkenes give dihalogenated products while alkanes usually give

polysubstituted products. Another difference is the position of the

attachment of the halogen atom. For alkenes, the halogen atom is

fixed to the carbon atom of the carbon=carbon double bond. In the

substitution reaction of alkanes, the position of the halogen atom

varies.


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(a) What chemical tests would you use to distinguish between two unlabelled bottles containing hexane and hex-1-ene respectively? Answer

(a) We can perform either one of the following tests:

Hex-1-ene can decolourize bromine water or chlorine water in

the dark while hexane cannot.

Hex-1-ene can decolourize acidified potassium manganate(VII)

solution while hexane cannot.


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(b) What is the major product of each of the following reactions?

(i)

(ii)

Answer


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(b) (i)

(ii)


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(c) Give the products for the following reactions:

(i) CH3CH = CH2 + H2

(ii) CH3CH = CHCH3

(iii) CH3CH = CHCH3 + Br2

Ni

conc. H2SO4


Answer


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(c) (i) CH3CH2CH3

(ii)

(iii)

Back


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143


(a) Arrange the following carbocations in increasing order of stability. Explain your answer briefly.

Answer


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(a) The increasing order of the stability of carbocations is:

Tertiary carbocations are the most stable because the three alkyl

groups release electrons to the positive carbon atom and thereby

disperse its charge. Primary carbocations are the least stable as

there is only one alkyl group releasing electrons to the positive

carbon atom.


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(b) Based on your answer in (a), arrange the following molecules in the order of increasing rates of reaction with hydrogen chloride.

Answer


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(b) The reaction of these compounds with hydrogen chloride involves

the formation of carbocations. Therefore, the order of reaction

rates follows the order of the ease of the formation of

carbocations, i.e. the stability of carbocations:

Therefore, the rates of reactions of the three compounds with

hydrogen chloride increase in the order:

Back

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