Download Alcohol Synthesis by Electrophilic Hydration

12-4 Alcohol Synthesis by Electrophilic Hydration:
Thermodynamic Control
When other nucleophiles are present, they may also attack the intermediate
carbocation.
Electrophilic hydration results when an alkene is exposed to an aqueous solution
of sulfuric acid (HSO4- is a poor nucleophile).
The addition of water by electrophilic hydration follows Markovnikov’s rule,
however carbocation rearrangements can occur because water is a poor
nucleophile.
The electrophilic hydration process is the reverse of the acid-induced elimination
of water (dehydration) of alcohols previously discussed.
Alkene hydration and alcohol dehydration are equilibrium processes.
All steps are reversible in the hydration of alkenes.
The proton serves as a catalyst only: it is
regenerated in the reaction.
In the absence of protons, alkenes are stable in water.
The position of the equilibrium in the hydration reaction can be changed by
adjusting the reaction conditions.
The reversibility of alkene protonation leads to alkene equilibration.
Protonation-deprotonation reactions may interconvert related alkenes and
produce an equilibrium mixture of isomers. Under these conditions, a reaction is
said to be under thermodynamic control.
This mechanism can convert less stable alkenes into their more stable isomers:
12-5 Electrophilic Addition of Halogens to Alkenes
Halogen molecules also act as electrophiles with alkenes giving vicinal dihalides.
The reaction with bromine results in a color change from red to colorless, which is
sometimes used as a test for unsaturation.
Halogenations are best carried out at or below room temperature and in inert
halogenated solvents (i.e. halomethanes)
12-5 Electrophilic Addition of Halogens to Alkenes
Bromination takes place through anti addition.
Consider the bromination of cyclohexene. No cis-1,2-dibromocyclohexane is
formed.
Only anti addition is observed. The product is racemic since the initial attack of
bromine can occur with equal probability at either face of the cyclohexene.
With acyclic alkenes the reaction is cleanly stereospecific:
Cyclic bromonium ions explain the stereochemistry.
The polarizability of the Br-Br bond allows heterolytic cleavage when attacked by a
nucleophile, forming a cyclic bromonium ion:
The bridging bromine atoms serves as the leaving group as the bromonium ion is
attacked from the bottom by a Br- ion.
In symmetric bromonium ions,
attack is equally probable at
either carbon atom leading to
racemic or meso products.
12-6 The Generality of Electrophilic Addition
The bromonium ion can be trapped by other nucleophiles.
Bromonation of cyclopentene using water as the solvent gives the vicinal
bromoalcohol (bromohydrin).
The water molecule is added anti to the bromine atom and the other product is
HBr.
Vicinal haloalcohols are useful synthetic intermediates.
Vicinal haloethers can be produced if an alcohol is used as the solvent, rather than
water.
Halonium ion opening can be regioselective.
Mixed additions to double bonds can be regioselective:
The nucleophile attacks the more highly substituted carbon of the bromonium
ion, because it is more positively polarized.
Electrophilic additions of unsymmetric reagents add in a Markovnikov-like fashion:
The electrophilic unit becomes attached to the less substituted carbon of the
double bond.
Mixtures of products are formed only when the two carbons are not sufficiently
differentiated.
Reagents of the type A-B, in which A acts as the electrophile, A+, and B the
nucleophile, B-, can undergo stereo- and regiospecific addition reactions to
alkenes: