1. Edexcel organic reaction mechanisms
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Homolytic
Free Radical Substitution
Free Radical Addition
Heterolytic
Electrophilic Addition
Nucleophilic Substitution
SN2
SN1
Electrophilic Substitution
Nitration
Br2
Alkylation
Acylation
Nucleophilic Addition

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 Cl
Cl Cl
initiation step
H3C H Cl
H3C
H Cl
two propagation steps
H3C
Cl Cl
H3C Cl Cl
H3C Cl
H3C Cl
termination step
minor termination step
H3C
H3C CH3
CH3

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. Free radical addition addition polymerisation of ethene
i.e. homolytic breaking of covalent bonds
Overall reaction equation
n H2C=CH2
[ CH2CH2 ]n
ethene
polyethene
Conditions
free radical source
(a species that generates free radicals that allow the polymerisation of ethene molecules)

6. Free radical addition mechanism
R R R
initiation step
H2C CH2 R
H2C CH R
H2C CH2
H2C CH2R
H2C CH CH2CH2R
chain propagation steps
Addition of H2C=CH2 repeats the same way until:
R(CH2)nCH2
H2C(CH2)mR
R(CH2)nCH2 CH2(CH2)mR
termination step
polyethene

7. 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

8. 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

9. 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

10. Nucleophilic substitution
hydroxide ion with bromoethane
mechanism
CH3CH2Br
+ OH-
(aqueous)
CH3CH2OH + Br-
ethanol
hydroxide ion with 2-bromo,2-methylpropane
mechanism
(CH3)3CBr
+ OH-
(aqueous)
(CH3)3COH + Br-
2-methylpropan-2-ol

11. hydroxide ion with bromoethane (SN2)
Nucleophilic substitution mechanism
hydroxide ion with bromoethane (SN2)
CH3 H Br C
CH3 H OH C + - Br - - OH
ethanol
SN2
reaction equation
S (substitution)
2(species reacting in the slowest step)
N(nucleophilic)

12. OH- ion with 2-bromo,2-methylpropane (SN1)
Nucleophilic substitution mechanism
OH- ion with 2-bromo,2-methylpropane (SN1) Br - Br -
CH3 Br C
CH3 C +
CH3 OH C + - - OH
2-methylpropan-2-ol
SN1
reaction equation
S (substitution)
1(species reacting in the slowest step)
N(nucleophilic)

13. Nucleophilic substitution
cyanide ion with iodoethane
mechanism
CH3CH2I (ethanol)
+ CN-(aq)
CH3CH2CN + I-
propanenitrile
cyanide ion with 2-bromo,2-methylpropane
mechanism
(CH3)3CBr
(ethanol) + CN-
(aqueous)
(CH3)3CCN + Br-
2,2-dimethylpropanenitrile

14. cyanide ion with iodoethane (SN2)
Nucleophilic substitution mechanism
cyanide ion with iodoethane (SN2)
CH3 H I C
CH3 H CN C + - I - CN -
propanenitrile
SN2
S (substitution)
2(species reacting in the slowest step)
N(nucleophilic)
reaction equation

15. CN- ion with 2-bromo,2-methylpropane (SN1)
Nucleophilic substitution mechanism
CN- ion with 2-bromo,2-methylpropane (SN1) Br - Br -
CH3 Br C
CH3 C +
CH3 CN C + -
SN1 CN -
2,2-dimethyl propanenitrile
S (substitution)
1(species reacting in the slowest step)
N(nucleophilic)
reaction equation

16. Electrophilic Substitution
Nitration of benzene
Where an H atom attached to an aromatic ring is replaced by an NO2 group of atoms
C6H6
+ HNO3
C6H5NO2
+ H2O
Conditions / Reagents
concentrated HNO3
and concentrated H2SO4
50oC
mechanism

17. 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

18. Bromination of benzene
Where an H atom attached to an aromatic ring is replaced by a Br atom
electrophilic substitution
C6H6
+ Br2
C6H5Br
+ HBr
R = alkyl group
Conditions / Reagents
Br2
and anhydrous AlBr3
25oC

19. Electrophilic substitution mechanism
1. Formation of the electrophile
Br AlBr3 - Br +
AlBr3
Br Br
2. Electrophilic attack on benzene Br +
Br AlBr3 - + H Br Br
3. Forming the products
H Br
and re-forming the catalyst
AlBr3
bromobenzene

20. Alkylation of benzene
Where an H atom attached to an aromatic ring is replaced by a C atom
electrophilic substitution
C6H6
+ RCl
C6H5R
+ HCl
R = alkyl group
Conditions / Reagents
RCl (haloakane)
and anhydrous AlCl3
0 - 25oC
to prevent further substitution

21. - 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

22. 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

23. Acylation of benzene
An H atom attached to an aromatic ring is replaced by a C atom where C is part of C=O
electrophilic substitution
C6H6
+ RCOCl
C6H5COR
+ HCl
Conditions / Reagents
RCOCl (acyl chloride)
and anhydrous AlCl3
50 oC

24. - 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

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

26. 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

27. 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

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29. References
Steve Lewis for the Royal Society of Chemistry