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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 conversion of C=C
and C≡C
into a large 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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In general terms, depending on the relative timing of
the bonding 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 via a concerted reaction
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 this dictates the regioselectivity.
- If the reaction proceeds via a
planar intermediate (e.g. carbocation or radical), the reaction is not stereoselective
- If the two new σ bonds form at
the same time from the same species, then they tend to be to the same face of the alkene and then a syn addition is observed.
- If the two new σ bonds form at different times from different species, then they tend to be to the opposite face of the alkene and then an anti addition is observed.
