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AQA organic reaction mechanisms. Click a box below to go to the mechanism. Click here for advice. AS. Free Radical Substitution. Electrophilic Addition. Nucleophilic Substitution. (nitrile hydrolysis reaction). Elimination of HX from haloalkanes. Elimination of water from alcohols.

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aqa organic reaction mechanisms
AQA organic reaction mechanisms

Click a box below to go to the mechanism

Click here for advice

AS

Free Radical Substitution

Electrophilic Addition

Nucleophilic Substitution

(nitrile hydrolysis reaction)

Elimination of HX from haloalkanes

Elimination of water from alcohols

Friedel-Crafts Alkylation

Acylation

A2

Electrophilic Substitution

Nucleophilic Addition

Nucleophilic Addition Elimination

free radical substitution
Free radical substitution

chlorination of methane

i.e. homolytic breaking of covalent bonds

Overall reaction equation

CH4 + Cl2

CH3Cl + HCl

Conditions

ultra violet light

excess methane

to reduce further substitution

free radical substitution mechanism
Free radical substitution mechanism

Cl + Cl

CH4 + Cl

CH3 + HCl

CH3 + Cl2

CH3Cl + Cl

CH3 + Cl

CH3 + CH3

ultra-violet

Cl2

initiation step

two propagation steps

termination step

CH3Cl

minor termination step

CH3CH3

further free radical substitutions
Further free radical substitutions

Overall reaction equations

CH3Cl + Cl2

CH2Cl2 + HCl

CH2Cl2 + Cl2

CHCl3 + HCl

CHCl3 + Cl2

CCl4 + HCl

Conditions

ultra-violet light

excess chlorine

electrophilic addition
Electrophilic addition

bromine with propene

mechanism

CH3CH=CH2

+ Br2

CH3CHBrCH2Br

1,2-dibromopropane

hydrogen bromide with but-2-ene

mechanism

CH3CH=CHCH3

+ HBr

CH3CH2CHBrCH3

2-bromobutane

concentrated sulphuric acid with but-2-ene

mech

CH3CH=CHCH3

+ HOSO3H

CH3CH2CH(OSO3H)CH3

2-butylhydrogensulphate

bromine with propene
bromine with propene

H

H

C

C

H

H

CH3

H

C

C

H

CH3

+

Br

-

Br

Br

Br

+

Br

Br

H

H

-

C

C

H

CH3

Br

Br

Electrophilic addition mechanism

reaction equation

carbocation

1,2-dibromopropane

hydrogen bromide with trans but 2 ene
hydrogen bromide with trans but-2-ene

CH3

H

C

C

H

H

CH3

H

CH3

C

C

CH3

+

+

H

H

-

Br

Br

H

H

-

CH3

C

C

CH3

Br

H

Electrophilic addition mechanism

reaction equation

carbocation

2-bromobutane

concentrated h 2 so 4 with cis but 2 ene
concentrated H2SO4 with cis but-2-ene

H

H

C

C

H

H

CH3

CH3

C

C

CH3

CH3

+

+

H

H

-

OSO3H

OSO3H

H

H

-

CH3

C

C

CH3

H

OSO3H

Electrophilic addition mechanism

reaction equation

carbocation

2-butylhydrogensulphate

nucleophilic substitution
Nucleophilic substitution

hydroxide ion with bromoethane

mechanism

CH3CH2Br

+ OH-

(aqueous)

CH3CH2OH + Br-

ethanol

cyanide ion with iodoethane

mechanism

CH3CH2I (ethanol)

+ CN-(aq)

CH3CH2CN + I-

propanenitrile

ammonia with bromoethane

mechanism

CH3CH2Br

+ NH3

2

CH3CH2NH2

+ NH4+Br-

aminoethane

hydroxide ion with bromoethane
hydroxide ion with bromoethane

H

-

+

OH

C

CH3

H

H

-

Br

Br

C

CH3

-

H

OH

Nucleophilic substitution mechanism

ethanol

reaction equation

cyanide ion with iodoethane
cyanide ion with iodoethane

H

-

+

CN

C

CH3

H

H

-

Br

Br

C

CH3

H

-

CN

Nucleophilic substitution mechanism

propanenitrile

reaction equation

ammonia with bromoethane
ammonia with bromoethane

-

Br

-

+

H

NH3

+

H

H

NH3

NH2

Br

C

C

CH3

CH3

H

H

H

-

H NH3+Br

NH2

C

CH3

H

Nucleophilic substitution mechanism

aminoethane

reaction equation

nitrile hydrolysis
Nitrile hydrolysis

Acid hydrolysis of nitriles:

CH3CH2CN

+ H2O + H+

2

CH3CH2COOH

+ NH4+

propanoic acid

Reflux with strong acid

eg HCl (aq)

elimination of hx from haloalkanes
Elimination of HX from haloalkanes

H

H

-

-

C

C

OH

Br

CH3

H

H

H

H OH

C

C

H

CH3

Br

H

Elimination of HBr from 2-bromopropane

+ OH-

CH3CH=CH2 + H2O + Br-

CH3CHBrCH3

(in ethanol)

propene

acting as a base

haloalkanes and hydroxide ions
Haloalkanes and hydroxide ions

alcohol

nucleophilic substitution

RCH3CH2OH + Br-

(aqueous)

+ OH-

hydroxide acts as a nucleophile

RCH2CH2X

hydroxide acts as a base

(ethanol)

+ OH-

elimination

RCH=CH2 + H2O + X-

alkene

elimination of water from alcohols
Elimination of water from alcohols

Elimination of H2O from propan-1-ol

CH3CH2CH2OH

CH3CH=CH2 + H2O

Heat with concentrated H2SO4

elimination mechanism water from propan 1 ol
Elimination mechanism – water from propan-1-ol

+

+

H

H

H

H

H

H

H

C

C

OH

CH3

C

C

OH

CH3

+

H

H

H

H

H OH

H OH

H

H

H

H

C

C

+

C

C

CH3

CH3

H

H

H

protonated alcohol

propene

carbocation

reaction equation

electrophilic substitution
Electrophilic Substitution

Nitration of benzene

C6H6

+ HNO3

C6H5NO2

+ H2O

Conditions / Reagents

concentrated HNO3

and concentrated H2SO4

50oC

mechanism

electrophilic substitution mechanism nitration
electrophilic substitution mechanism (nitration)

NO2

+

1. Formation of NO2

H

+

+

NO2

O SO3H-

+

NO2

NO2

H O SO3H

the nitronium ion

HNO3

+ 2H2SO4

+ H3O+

+ 2HSO4-

2. Electrophilic attack on benzene

3. Forming the product

and re-forming the catalyst

reaction equation

friedel crafts alkylation
Friedel-Crafts alkylation

Where an H atom attached to an aromatic ring is replaced by a C atom

Alkylation of benzene

electrophilic substitution

C6H6

+ RCl

C6H5R

+ HCl

R = alkyl group

Conditions / Reagents

RCl (haloakane)

and anhydrous AlCl3

0 - 25oC

to prevent further substitution

alkylation example
Alkylation example

-

Cl AlCl3

+

CH3CH2 Cl

CH3CH2

With chloroethane

overall reaction equation

C6H6 + CH3CH2Cl

C6H5CH2CH3

+ HCl

Three steps in electrophilic substitution mechanism

1. Formation of the electrophile (a carbocation)

AlCl3

alkylation electrophilic substitution mechanism 2
Alkylation electrophilic substitution mechanism 2

-

Cl AlCl3

+

CH3CH2

H Cl

H

+

CH3CH2

CH3CH2

2. Electrophilic attack on benzene

3. Forming the product

and re-forming the catalyst

AlCl3

ethylbenzene

friedel crafts acylation
Friedel-Crafts acylation

An H atom attached to an aromatic ring is replaced by a C atom where C is part of C=O

Acylation of benzene

electrophilic substitution

C6H6

+ RCOCl

C6H5COR

+ HCl

Conditions / Reagents

RCOCl (acyl chloride)

and anhydrous AlCl3

50 oC

acylation example
Acylation example

-

Cl AlCl3

O

+

CH3C

O

CH3C

Cl

With ethanoyl chloride

overall reaction equation

C6H6 + CH3COCl

C6H5COCH3

+ HCl

Three steps in electrophilic substitution mechanism

1. Formation of the electrophile (an acylium ion)

AlCl3

acylation electrophilic substitution mechanism 2
Acylation electrophilic substitution mechanism 2

-

Cl AlCl3

O

CH3C

H Cl

O

+

CH3C

O

H

CH3C

+

2. Electrophilic attack on benzene

3. Forming the product

and re-forming the catalyst

AlCl3

phenylethanone

nucleophilic addition
Nucleophilic Addition

Reduction of carbonyls

primary alcohol

RCH2OH

+ 2[H]

RCHO

secondary alcohol

+ 2[H]

RCOR

RCH(OH)R

Conditions / Reagents

NaBH4

andH2SO4(aq)

Room temperature and pressure

nucleophilic addition mechanism
Nucleophilic Addition Mechanism

+ 2[H]

RCOR

RCH(OH)R

H

-

O

+

O

C

CH3

O

H

C

H

CH3

CH3

C

H

CH3

CH3

CH3

H

alcohol

reduction of propanone

NaBH4 is a source of hydride ions

fromH2SO4 (aq)

H+

H+

propan-2-ol

nucleophilic addition28
Nucleophilic Addition

addition of hydrogen cyanide to carbonyls to form hydroxynitriles

+ HCN

RCOR

RC(OH)(CN)R

RCH(OH)CN

+ HCN

RCHO

Conditions / Reagents

NaCN (aq)

andH2SO4(aq)

supplies H+

supplies the CN- nucleophile

Room temperature and pressure

nucleophilic addition mechanism29
Nucleophilic Addition Mechanism

C N

-

O

+

O

H

O

C

CH3

C

CN

CH3

CH3

C

CN

CH3

CH3

CH3

CN

hydrogen cyanide with propanone

+ HCN

CH3COCH3

CH3C(OH)(CN)CH3

NaCN (aq) is a source of cyanide ions

fromH2SO4 (aq)

H+

H+

2-hydroxy-2-methylpropanenitrile

nucleophilic addition elimination
Nucleophilic Addition Elimination

O

R C

Acylation of water to give carboxylic acids

RCOOH

+ HCl

+ H2O

RCOCl

carboxylic acid

OH

Conditions

room temperature and pressure

formation of ethanoic acid
Formation of ethanoic acid

O

CH3 C

ethanoyl chloride

CH3COCl

+ H2O

CH3COOH

+ HCl

ethanoic acid

OH

mechanism

nucleophilic addition elimination mechanism
Nucleophilic Addition Elimination Mechanism

O

+

C

OH

CH3

H

Cl

OH

-

O

O

H

O

+

C

CH3

+

C

CH3

C

OH

CH3

Cl

OH

H

H

Cl

Cl

nucleophilic addition

elimination

reaction equation

acylation of primary amines to n alkyl amides
Acylation of primary amines to N-alkyl amides

O

R C

O

CH3 C

+ 2 R’NH2

RCONHR’

+ R’NH3+Cl-

RCOCl

N-alkylamide

NHR’

Conditions

room temperature and pressure

N-propylethanamide

NHCH2CH2CH3

formation of n propyl ethanamide
Formation of N-propyl ethanamide

from

1-aminopropane

ethanoyl chloride

CH3COCl

+ CH3CH2CH2NH2

2

CH3CONHCH2CH2CH3

+ CH3CH2CH2NH3+Cl-

N-propylethanamide

mechanism

nucleophilic addition elimination mechanism35
Nucleophilic Addition Elimination Mechanism

O

+

C

NHCH2CH2CH3

CH3

H

NHCH2CH2CH3

Cl

H

-

O

O

O

+

C

CH3

+

C

CH3

C

NHCH2CH2CH3

CH3

Cl

NHCH2CH2CH3

H

NH2CH2CH2CH3

H

Cl

Cl

+

NH2CH2CH2CH3

nucleophilic addition

elimination

reaction equation

advice
Advice

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references
References

Steve Lewis for the Royal Society of Chemistry