Organic chemistry revision: Comparison of mechanisms
Alkanes
Electron-rich
Alkenes (e- rich)
Gener
-al
formu
la
CnH2n+2 (non cyclic)
CnH2n (non cyclic)
Reacti
vity
Rxn
type
under
gone
Saturated; non-polar C–C &
C–H bond only
Free Radical Substitution
C=C double bond is electron rich
thus attracts electrophile
Electrophilic addition
Delocalised  electrons
attracts electrophile.
Electrophilic substitution
Delta positive Carbon bonded to X
Draw mechanism of reaction
between
Draw mechanism of reaction
between propene and HCl(g).
Draw mechanism of reaction
between benzene & Cl2 in
anhydrous AlCl3, heat.
Generation of electrophile, E+
(eg. Cl+)
Draw mechanism of reaction between
bromoethane and NaOH(aq), heat.
Mechanism
(Main
reacti
on
type)
C
Aldehydes or ketones
+
X
Nu:
-
C
Nu
R
CH3
H
C
Step1
(SLOW):
CH3
CH3
and limited Br2, uv
H
H
Initiation (arrows required in A level)
Br
C
H
2Br
Br
Propagation (arrows not needed in A
level)
Br
H
C
CH3
CH3
C
CH3
+H
Br
Br
+
Br
CH3
C CH3
CH3
Termination (Any 2 radicals combine)
(arrows not needed in A level)
Br
Br
Br-Br
CH3
Br
C CH3
CH3
d-
H
C
+ 
-
CH3
Br
C CH3
CH3
Note
For FRS use half-arrow
Cl2 + AlCl3
H
+
+ Cl
CH3
-
⇌AlCl4 - +
Cl +
Cl+
Use AlCl3 to form
, need
stronger electrophile with Full positive
charge as benzene is less reactive
than alkene
Step 1: (SLOW)
Cl
Step 2 (FAST):
H
CH3
Br
Cl
d+
C
H
slow
+ Cl +
H
H
C
H
C
H
+
CH3
Cl
- fast
H
C
H
H
C
Cl
electrophile
CH3
+
+
C
+
fast
+ AlCl3 + HCl
Generate back catalyst AlCl3
Similar mechanism for other
electrophile
Eg. RCl, FeCl3, heat
FeCl4  +
1
Step 1: Slow
+
C
-
Br
R'
SN1 (2 step)
Rate = k[RX] (1 stands for 1st order kinetics)
favoured by 3o halogenoalkanes RX
-If starting RX is optically active, product of SN1 is
R+
Br
CH3
slow
CH3
+
C
CH 2CH 2CH 3
CH 2CH 3
–
CH 2CH 3
R
H
CH 2CH 2CH 3
(catalyst regen)
RCl + FeCl3
HO
H
C
AlCl4-
Br
H

d++ d-
H
CH3

H
Cl
H
Cl
-
R
R'
+
C
Generate nucleophile
NaCN  Na+ + CN-
optically inactive (racemic mixture)
Reason: Trigonal planar abt C+, equal probability
arenium ion of attack of Nu: fr top or bottom side of C+produces
racemic mixture.
Step 2: (Fast)
Note:
Markovonikov rule: delta +ve
atom (electrophile) adds to
alkene C bonded directly to
more H, to form more stable
carbocation
Stability of carbocation:
3o > 2o > 1o carbocation
Rate = k[RX][OH-] (2 stands for 2nd order kinetics)
-favoured by 1o halogenoalkanes RX
-Inversion of configuration
-If starting RX is optically active, product of SN2 is
optically active
HO :
C
-
O
Draw mechanism of reaction
between ketone and HCN, trace
NaCN, cold
SN2 (1 step)
CH3
+ +X:
-
Delta positive Carbon (bonded to O)
attracts Nucleophile
Nucleophilic addition
(Halogen: Cl, Br or I) attracts Nucleophile
Nucleophilic substitution
Br
CH3
CH3
C CH3
CH3
H
slow H
Note: Electrophile is + H of HCl
CH3
CH3
H
C
OR
O -

Benzene ring
uv light
Draw
Mec
hani
sm
Has delta positive C
HalogenoALKANES
Arenes (e- rich)
+ Br
+ C O
slow
O-
R'
NC:
tetrahedral intermediate
Step 2:
fast
R
R
NC C
R
NC C
O-
+ H
R'
tetrahedral intermediate
CN
NC C
R'
OH
+
:CN-
Organic chemistry revision: Comparison of mechanisms
Alkanes
shows movement of 1eFRS involves free radicals
(species with unpaired
electron)
Electron-rich
Alkenes (e- rich)
Arenes (e- rich)
(Note: Why electrophilic
substitution and not addition?
Delocalised  e- cloud results in
stability, loss of aromatic
character is  unfavourable.
Has delta positive C
HalogenoALKANES
Aldehydes or ketones
OH
C*
CH3
fast
CH3
+
C
–
HO :
CH 2CH 3
mirror
CH 2CH 2CH 3
CH 2CH 3
fast
CH 2CH 3
CH3
C
*
OH
2
50%
CH 2CH 2CH 3
CH 2CH 2CH 3
50%
Organic chemistry revision: Comparison of mechanisms
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