Summary of Reaction Mechanisms Required for A2 Chemistry
Module 3 Introduction to Organic Chemistry
•
Electrophilic addition
•
Nucleophilic substitution – primary haloalkanes with OH–, CN– and NH3.
•
Formation of alkenes from alcohols (elimination reaction in terms of the loss of a proton from a
carbonium ion generated from the protonated alcohol).
•
Concurrent displacement and elimination – 2° haloalkanes with potassium hydroxide
•
Free radical substitution – methane + chlorine (initiation, propagation and termination steps).
– symmetrical alkenes with HBr, Br2 and H2SO4.
Module 4 Further Physical and Organic Chemistry
•
Electrophilic addition
•
Electrophilic substitution – nitration, alkylation and acylation of benzene.
•
Nucleophilic addition-elimination – acyl chlorides with water, alcohols, ammonia and 1º amines.
•
Nucleophilic substitution – haloalkanes with ammonia and amines to form 1°, 2° and 3° amines
and 4° ammonium salts.
•
Elimination reactions
•
Generation of the nitronium ion, NO2+ (below):
– asymmetrical alkenes with HBr and H2SO4.
– alkenes from haloalkanes (and alcohols – see CHM03).
HNO3 + H2SO4 H2NO3+ + HSO4–
(formation of protonated nitric acid)
H2NO3+ NO2+ + H2O
(decomposition to form nitronium ion)
H2O + H2SO4 H3O+ + HSO4–
Overall: HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4–
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Copyright © 2003 Nigel Saunders N-ch4-04
Electrophilic addition
e.g. of H–Br, Br–Br, H2SO4 (view as H–OSO2OH)
X
δ
X
δ
Y
+
C
C
+
X
–
C
C
+
+
:Y–
C
C
Y
Nucleophilic addition
i.e. of carbonyl compounds with HCN to make hydroxynitriles
δ+
C
δ–
O
H+
+
–
–
O
..
C
CN
+
H+
C
CN
OH
CN
Nucleophilic substitution
e.g. of haloalkanes with OH– and CN–
δ+
C
δ–
X +
Y–
C
Y
+
X–
Elimination in alcohols to form alkenes
H
R
H
H
C
C
H
H
..
O
..
R
H
H+
H
R
C
C
+
O
H
H
H
H
H
C
C
H
H
+ H2O
Elimination in haloalkanes to form alkenes
(favoured by using alcoholic solutions of concentrated alkali)
X
H
C
C
H
R
H
R
C
+ H2O + X–
C
H
H
H
H
–
OH
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Copyright © 2003 Nigel Saunders N-ch4-04
Nucleophilic addition-elimination
e.g. acyl chlorides with water, alcohols, ammonia and amines
R
δ
C
δ
+
–
O
+
R
H
..
R'OH
..
O+
C
R'
Cl
R
O–
H
Cl
δ+
C
δ–
O
O+
+ Cl–
R'
R
C
O
+
HCl
+
H+
OR'
Electrophilic substitution
(nitration of benzene)
Remember the reactions involved in production of the nitronium ion (see first page).
H
+
NO2
NO2
NO2
+
Electrophilic substitution
e.g. Friedel–Crafts alkylation of benzene
Generation of the electrophile, e.g. as a carbocation complex:
RCl + AlCl3 = R+ AlCl4–
then
R
H
R
R+AlCl4–
+
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+ AlCl4–
+ AlCl3
+ HCl
Copyright © 2003 Nigel Saunders N-ch4-04
Electrophilic substitution
e.g. Friedel-Crafts acylation of benzene
Generation of the electrophile, e.g. as an acyl cation complex:
RCOCl + AlCl3 = RC+O AlCl4–
then
COR
H
COR
RC+O AlCl4–
+
+ AlCl4–
+ AlCl3
+ HCl
Chlorination of methane (free radical substitution)
Initiation
Homolysis of the Cl–Cl bond using energy from absorbed light or heat supplied:
Cl2 → 2Cl
Note: Homolysis of the C–H bond is also possible, but less energy is needed to break the Cl–Cl
bond (bond energy terms are 435 kJ mol–1 and 242 kJ mol–1 respectively).
Propagation
Cl + CH4 → HCl + CH3
CH3 + Cl2 → CH3Cl + Cl
Termination
Cl + Cl → Cl2
CH3 + CH3 → C2H6
Cl + CH3 → CH3Cl
Other reactions are possible.
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Copyright © 2003 Nigel Saunders N-ch4-04