Summary of Organic Reaction Mechanisms : The Twelve Elementary Processes of Organic Chemistry

A-Level: Chemistry

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This is the fundamental process in Wagner-Meerwein rearrangments
and other 1,2-shift processes that do not involve a leaving group.
7. Bimolecular nucleophilic substitution (ANDN; SN2)
Nu C X Nu C + X
Nu C X Nu C + X
Nu C X Nu C + X
Nu C X Nu C + X
Concerted bimolecular substitution of a leaving group proceeds with
stereochemical inversion. The nucleophile may be neutral or anionic,
and the leaving group may depart as an anion or a neutral species.
Note that substitution reactions of this kind may occur at heteroatom centres
as well as at carbon.
8. Bimolecular elimination (DHDN; E2)
C C
H
X
B
C C
BH
X
C C
H
X
B
C C
BH
X
C C
H
X
B
C C
BH
X
C C
H
X
B
C C
BH
X
Concerted bimolecular elimination may also occur at heteroatom centres.
The base may be either neutral or anionic, while the leaving group may
depart as an anion or a neutral.
9. Trimolecular electrophilic addition to multiple bond (ANAH; reverse of pathway 8)
C C
H
X
B
C C
HB
X
C C
H
X
B
C C
HB
X
C C
H
X
B
C C
HB
X
C C
H
X
B
C C
HB
X
This is the reverse of the E2 elimination. Note that in this process, there is NO
intermediate cation - addition of the nucleophile and electrophile (proton) is
simultaneous. Because this reaction is trimolecular, it is statistically less
probable than the more-common multi-step addition process.
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