- Dehydration and RX formation from alcohols furnish another example of the
competition between nucleophilic substitution and elimination.
- The free energies of activation for these two reactions of carbocations are not
very different from one another.
- In conversion of alcohols to alkyl halides (substitution), very often, the reaction
is accompanied by the formation of some alkenes (elimination).
- Acid-catalyzed conversion of 1° alcohols and methanol to alkyl halides proceeds
through an SN 2 mechanism.
X C +
H
H
R
(a good leaving group)
C
H
H
R OH+ OH
H
X^ − +
(protonated 1o alcohol or methanol)
H
1) Although halide ions (particularly I– and Br–) are strong nucleophiles, they are
not strong enough to carry out substitution reactions with alcohols directly.
Br C +
H
H
C R
H
H
Br^ − + R OH X − OH
- Many reactions of alcohols, particularly those in which carbocations are formed,
are accompanied by rearrangements.
- Chloride ion is a weaker nucleophile than bromide and iodide ions ⇒ chloride
does not react with 1° or 2° alcohols unless zinc chloride or some Lewis acid is
added to the reaction.
- ZnCl 2 , a good Lewis acid, forms a complex with the alcohol through association
with an lone-pair electrons on the oxygen ⇒ provides a better leaving group for
the reaction than H 2 O.
R O + ZnCl 2 O −ZnCl 2
H
R
H
+
