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Chapter 6 : Reactions of Alkenes : Addition Reaction |
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Electrophilic
Addition reactions
Electrophilic addition reactions
are an important class of reactions that allow the interconversion of C=C
and C≡C
into a range of important functional groups including alkyl halides and alcohols.
Conceptually, addition is the reverse of elimination (see
Chapter 5) which can be used to prepare alkenes.
What does the term "electrophilic addition"
imply ?
A electrophile, E+,
is an electron poor species that will react with an electron rich species
(the C=C)
An addition implies that two systems combine to a single entity.
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Depending on the relative timing of
these events, slightly different mechanisms are possible:
- Reaction of the C=C with
E+
to give a carbocation (or another cationic intermediate)
that then reacts with a Nu-,
or,
- Simultaneous formation of the
two new σ bonds
The following pointers may aid your
understanding of these reactions:
- Recognise the electrophile
present in the reagent combination
- The electrophile adds first
to the alkene, and dictates the regioselectivity.
- If the reaction proceeds via a
planar carbocation, the reaction is not stereoselective
- If the two new σ bonds form at
the same time from the same species, then syn addition is observed
- If the two new σ bonds form at different times from different species, then anti addition is observed
