1. AQA organic reaction mechanisms
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Free Radical Substitution
Electrophilic Addition
Nucleophilic Substitution
Elimination of HX from haloalkanes A2
Electrophilic Substitution
Friedel-Crafts Alkylation
Acylation
Nucleophilic Addition
Nucleophilic Addition Elimination

2. 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

3. Free radical substitution mechanism
ultra-violet
Cl2
Cl + Cl
initiation step
two propagation steps
CH4 + Cl
CH3 + HCl
CH3 + Cl2
CH3Cl + Cl
termination step
CH3 + Cl
CH3Cl
CH CH3
CH3CH3
minor termination step

4. 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

5. 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

6. Electrophilic addition mechanism
bromine with propene
reaction equation
CH3 H C
CH3 H C Br +
carbocation + - Br Br -
CH3 H C Br Br
1,2-dibromopropane

7. hydrogen bromide with trans but-2-ene
Electrophilic addition mechanism
hydrogen bromide with trans but-2-ene
reaction equation
CH3 H C
CH3 H C +
carbocation - + Br H
CH3 H C Br Br -
2-bromobutane

8. concentrated H2SO4 with cis but-2-ene
Electrophilic addition mechanism
concentrated H2SO4 with cis but-2-ene
reaction equation
CH3 H C
CH3 H C +
carbocation
OSO3H -
OSO3H H + -
CH3 H C
OSO3H
2-butylhydrogensulphate

9. 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

10. hydroxide ion with bromoethane
Nucleophilic substitution mechanism
hydroxide ion with bromoethane
CH3 H Br C
CH3 H OH C + - Br - - OH
ethanol
reaction equation

11. cyanide ion with iodoethane
Nucleophilic substitution mechanism
cyanide ion with iodoethane
CH3 H Br C
CH3 H CN C + - Br - CN -
propanenitrile
reaction equation

12. ammonia with bromoethane
Nucleophilic substitution mechanism
ammonia with bromoethane
CH3 H Br C H
CH3
NH2 C + Br - + -
NH3
NH3
CH3 H
NH2 C
H NH3+Br -
aminoethane
reaction equation

13. Elimination of HX from haloalkanes
Elimination of HBr from 2-bromopropane
CH3CHBrCH3
+ OH-
CH3CH=CH2 + H2O + Br-
(in ethanol)
CH3 H C Br
CH3 H C
propene Br - OH -
H OH
acting as a base

14. Haloalkanes and hydroxide ions
alcohol
nucleophilic substitution
RCH3CH2OH + Br-
+ OH-
(aqueous)
hydroxide acts as a nucleophile
RCH2CH2X
+ OH-
(ethanol)
hydroxide acts as a base
elimination
RCH=CH2 + H2O + X-
alkene

15. Electrophilic Substitution
Nitration of benzene
C6H6
+ HNO3
C6H5NO2
+ H2O
Conditions / Reagents
concentrated HNO3
and concentrated H2SO4
50oC
mechanism

16. electrophilic substitution mechanism (nitration)
1. Formation of NO2 +
the nitronium ion
HNO3
+ 2H2SO4
NO2 +
+ 2HSO4-
+ H3O+
2. Electrophilic attack on benzene
NO2 + +
NO2 H
O SO3H-
NO2
3. Forming the product
and re-forming the catalyst
H O SO3H
reaction equation

17. 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

18. - Alkylation example With chloroethane overall reaction equation
C6H6 + CH3CH2Cl
C6H5CH2CH3
+ HCl
Three steps in electrophilic substitution mechanism
1. Formation of the electrophile (a carbocation) -
Cl AlCl3
AlCl3
CH3CH2 +
CH3CH2 Cl

19. Alkylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
CH3CH2 + + H
CH3CH2 -
Cl AlCl3
3. Forming the product
and re-forming the catalyst
CH3CH2
AlCl3
H Cl
ethylbenzene

20. 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

21. - Acylation example With ethanoyl chloride overall reaction equation
C6H6 + CH3COCl
C6H5COCH3
+ HCl
Three steps in electrophilic substitution mechanism
1. Formation of the electrophile (an acylium ion)
CH3C Cl O +
CH3C O -
Cl AlCl3
AlCl3

22. Acylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene + H
CH3C O +
CH3C O -
Cl AlCl3
3. Forming the product
CH3C O
H Cl
and re-forming the catalyst
AlCl3
phenylethanone

23. Nucleophilic Addition
Reduction of carbonyls
primary alcohol
RCHO
+ 2[H]
RCH2OH
secondary alcohol
RCOR
+ 2[H]
RCH(OH)R
Conditions / Reagents
NaBH4
and H2SO4(aq)
Room temperature and pressure

24. Nucleophilic Addition Mechanism
RCOR
+ 2[H]
RCH(OH)R
alcohol
reduction of propanone
NaBH4 is a source of hydride ions H H+
from H2SO4 (aq) + -
CH3 C O H+
CH3 C O H
CH3 C O H H
propan-2-ol

25. Nucleophilic Addition
addition of hydrogen cyanide to carbonyls to form hydroxynitriles
RCOR
+ HCN
RC(OH)(CN)R
RCHO
+ HCN
RCH(OH)CN
Conditions / Reagents
NaCN (aq)
and H2SO4(aq)
supplies H+
supplies the CN- nucleophile
Room temperature and pressure

26. Nucleophilic Addition Mechanism
hydrogen cyanide with propanone
CH3COCH3
+ HCN
CH3C(OH)(CN)CH3
NaCN (aq) is a source of cyanide ions
C N + - H+
from H2SO4 (aq)
CH3 C O H+
CH3 C O CN
CH3 C O CN H CN
2-hydroxy-2-methylpropanenitrile

27. Nucleophilic Addition Elimination
Acylation of water to give carboxylic acids
RCOCl
+ H2O
RCOOH
+ HCl
carboxylic acid
R C O OH
Conditions
room temperature and pressure

28. Formation of ethanoic acid
ethanoyl chloride
CH3COCl
+ H2O
CH3COOH
+ HCl
ethanoic acid
CH3 C O OH
mechanism

29. Nucleophilic Addition Elimination Mechanism+ -
CH3 Cl C O OH H +
nucleophilic addition
CH3 Cl C O OH H
elimination
CH3 C O OH H +
CH3 C O OH Cl Cl H
reaction equation

30. Acylation of primary amines to N-alkyl amides
RCOCl
+ 2 R’NH2
RCONHR’
+ R’NH3+Cl-
N-alkylamide
R C O
NHR’
Conditions
room temperature and pressure
CH3 C O
N-propylethanamide
NHCH2CH2CH3

31. Formation of N-propyl ethanamide
from
ethanoyl chloride
1-aminopropane
CH3COCl
+ CH3CH2CH2NH2 2
CH3CONHCH2CH2CH3
+ CH3CH2CH2NH3+Cl-
N-propylethanamide
mechanism

32. Nucleophilic Addition Elimination Mechanism+ -
nucleophilic addition
CH3 Cl C O
NHCH2CH2CH3 H +
CH3 Cl C O
NHCH2CH2CH3 H
elimination
CH3 C O
NHCH2CH2CH3 H +
CH3 C O
NHCH2CH2CH3 Cl
NH2CH2CH2CH3 H +
NH2CH2CH2CH3 Cl
reaction equation

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34. References
Steve Lewis for the Royal Society of Chemistry