Part 6: MECHANISMS
Note that no other reagents are needed in order to complete
any of these sequences, you should only be using what is there.
A.
This is an acid
catalysed hydration of an alkene. The first step is protonation of
the
alkene with the H+ to give the more
stable carbocation, in this case a 2o
carbocation. Then the
lone pairs on the O act as the nucleophile attacking the carbocation.
Loss
of
the proton gives the product. It is important to remember
that
in an acidic solution (we have H+ defined) that the amount of hydroxide
ion (i.e. HO-)
is minimal.... think of the pKa which
for this dissociation would be about 16. So the nucleophile will be the
water itself.
B.
This is a slight
variation of the formation
of a halohydrin, the difference here is that the nucleophile is an
alcohol and the reaction is intramolecular. So the alkene reacts with
the bromine to form the cyclic bromonium ion (n.b. alcohols don't react
with bromine), this cationic intermdiate is attacked by the
lone pairs on the O acting as the nucleophile in an SN1 like reaction
as the nucleophile attacks the more substituted end of the bromonium
ion due the greater +ve character. Loss
of
the proton gives the product. The base could be the bromide
ion is no other bases are added)
C.
This is the first part
of the acid catalysed polymerisation of an alkene. The first step is protonation of
the
alkene with the H+ to give the more
stable carbocation, in this case a 3o
carbocation. Then the pi electrons in the second molecule of the alkene
act as the nucleophile attacking the carbocation. Loss
of
the proton gives the product. The base could be another molecule of
alkene of the conjugate base of the original acid.
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