Edexcel organic reaction mechanisms

1. Edexcel organic reaction mechanisms

Click a box below to go to the mechanism
Homolytic
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Free Radical Substitution
Free Radical Addition
Heterolytic
Electrophilic Addition
Nucleophilic Substitution
Electrophilic Substitution
SN2
Nitration
S N1
Br2
Alkylation
Acylation
Nucleophilic Addition
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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
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3. Free radical substitution mechanism

Cl
H3C
H3C
H3C
H3C
Cl
ultra-violet
H
Cl
Cl
Cl
initiation step
Cl
H3C
H
Cl
Cl
H3C
Cl
Cl
Cl
H3C
Cl
CH3
H3C
CH3
two
propagation
steps
termination step
minor
termination step
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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
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5. Free radical addition

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

R
H2C
H2C
CH2
CH2
R
R
R
R
H2C
H2C
initiation step
CH2
R
H2C
CH2R
CH2
CH2CH2R
chain propagation steps
Addition of H2C=CH2 repeats the same way until:
R(CH2)nCH2
H2C(CH2)mR
termination step
R(CH2)nCH2
CH2(CH2)mR
polyethene
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7. Electrophilic addition

bromine with propene
CH3CH=CH2 + Br2
mechanism
CH3CHBrCH2Br
1,2-dibromopropane
hydrogen bromide with but-2-ene
CH3CH=CHCH3+ HBr
mechanism
CH3CH2CHBrCH3
2-bromobutane
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8. bromine with propene

Electrophilic addition mechanism
bromine with propene
H
reaction equation
H
C C
CH3
H
+
Br
Br
-
H
H
carbocation
CH3 C
+
C
H
Br
H
H
Br
CH3 C
C
Br
Br
1,2-dibromopropane
Br-
H
Br
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9. hydrogen bromide with trans but-2-ene

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

hydroxide ion with bromoethane
CH3CH2Br + OH- (aqueous)
mechanism
CH3CH2OH + Brethanol
hydroxide ion with 2-bromo,2-methylpropane
mechanism
(CH3)3CBr+ OH- (aqueous)
(CH3)3COH + Br2-methylpropan-2-ol
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11. hydroxide ion with bromoethane (SN2)

Nucleophilic substitution mechanism
hydroxide ion with bromoethane
H
+
CH3 C
H
Br
CH3 C
H
OH-
(SN2)
OH
H
S N2
Br
ethanol
reaction equation
S (substitution) N(nucleophilic) 2(species reacting
in the slowest step)
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12. OH- ion with 2-bromo,2-methylpropane (SN1)

Nucleophilic substitution mechanism
OH- ion with 2-bromo,2-methylpropane
(SN1)
-
CH3
+
CH3 C Br
CH3
Br
CH3 C+
CH3 C
CH3
CH3
S N1
CH3
Br
OH
CH3
OH-
2-methylpropan-2-ol
reaction equation
S (substitution) N(nucleophilic) 1(species reacting
in the slowest step)
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13. Nucleophilic substitution

cyanide ion with iodoethane
CH3CH2I (ethanol) + CN-(aq)
mechanism
CH3CH2CN + Ipropanenitrile
cyanide ion with 2-bromo,2-methylpropane
mechanism
(CH3)3CBr (ethanol) + CN- (aqueous)
(CH3)3CCN + Br2,2-dimethylpropanenitrile
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14. cyanide ion with iodoethane (SN2)

Nucleophilic substitution mechanism
cyanide ion with iodoethane (SN2)
H
+
CH3 C
I
H
-
CH3 C
H
CN-
CN
H
I
-
propanenitrile
SN2
S (substitution) N(nucleophilic) 2(species reacting
in the slowest step)
reaction equation
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15. CN- ion with 2-bromo,2-methylpropane (SN1)

Nucleophilic substitution mechanism
CN- ion with 2-bromo,2-methylpropane
(SN1)
-
CH3
+
CH3 C Br
CH3
CH3
Br
CH3 C+
CH3 C
CH3
S N1
CH3
Br
CN
CH3
CN-
2,2-dimethyl
propanenitrile
S (substitution) N(nucleophilic) 1(species reacting
in the slowest step)
reaction equation
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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
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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
3. Forming the product
and re-forming the catalyst
reaction equation
NO2
H O SO3H
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18. Bromination of benzene

Where an H atom attached to an aromatic ring
is replaced by a Br atom
electrophilic substitution
C 6 H6
+ Br2
C6H5Br
+ HBr
R = alkyl group
Conditions / Reagents
Br2
and anhydrous AlBr3
25oC
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19. Electrophilic substitution mechanism

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

Where an H atom attached to an aromatic ring
is replaced by a C atom
electrophilic substitution
C 6 H6
+ RCl
C6H5R
+ HCl
R = alkyl group
Conditions / Reagents
RCl (haloakane)
and anhydrous AlCl3
0 - 25oC to prevent further substitution
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21. Alkylation example

With chloroethane
C6H6 + CH3CH2Cl
overall reaction equation
C6H5CH2CH3 + HCl
Three steps in electrophilic substitution mechanism
1. Formation of the electrophile (a carbocation)
CH3CH2
Cl
AlCl3
+
CH3CH2
Cl
-
AlCl3
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22. Alkylation electrophilic substitution mechanism 2

2. Electrophilic attack on benzene
+
CH3CH2
CH3CH2
H
+
-
Cl
AlCl3
3. Forming the product
and re-forming the catalyst
CH3CH2
H
AlCl3
Cl
ethylbenzene
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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
C 6 H6
+ RCOCl
C6H5COR + HCl
Conditions / Reagents
RCOCl (acyl chloride) and anhydrous AlCl3
50 oC
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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)
O
CH3C
Cl
+
CH3C O
AlCl3
Cl
-
AlCl3
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25. Acylation electrophilic substitution mechanism 2

2. Electrophilic attack on benzene
O
+
CH3C O
CH3C
+
H
Cl
3. Forming the products
and re-forming the catalyst
O
CH3C
-
AlCl3
H
Cl
AlCl3
phenylethanone
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26. Nucleophilic Addition

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

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

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29. References

Steve Lewis for the Royal Society of Chemistry
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