Hydroboration–oxidation of alkynes: preparation of aldehydes and
ketones Hydroboration–oxidation of terminal alkynes gives syn addition
of water across the triple bond. The reaction is regioselective and follows
anti-Markovnikov addition. Terminal alkynes are converted to
aldehydes, and all other alkynes are converted to ketones. A sterically
hindered dialkylborane must be used to prevent the addition of two
borane molecules. A vinyl borane is produced with anti-Markovnikov
orientation, which is oxidized by basic hydrogen peroxide to an enol.
This enol tautomerizes readily to the more stable keto form.
B(CH 3 ) 2
H
H
R
RC CH
R
O
H
H
OH
R
H
KOH
Aldehyde
anti-Markovnikov addition
(CH 3 ) 2 BH
H 2 O 2
Mechanism.
RC CH
B(CH 3 ) 2
H
C CH
H
R
B(CH 3 ) 2
R
O
H
H
OH
R
H
+
δ +
δ −
THF
− :OH
H 2 O 2
..
anti-Markovnikov addition
Addition of sulphuric acid to alkenes: preparation of alcohols
Addition of concentrated H 2 SO 4 to alkenes yields acid-soluble alkyl
hydrogen sulphates. The addition follows Markovnikov’s rule. The sulphate
is hydrolysed to obtain the alcohol. The net result is Markovnikov addition
of acid-catalysed hydration to an alkene. The reaction mechanism of H 2 SO 4
addition is similar to that of acid-catalysed hydration.
C
H 2
CHCH 3
OH
CH 3 CHCH 3
OSO 3 H
CH 3 CHCH 3
Propene
H 2 O
+ H 2 SO 4
Conc H 2 SO 4
Propanol
Heat
Isopropyl hyrogen
sulphate
No heat
Addition of alcohols to alkenes: acid catalysed. Preparation of ethers
Alcohols react with alkenes in the same way as water does. The addition of
alcohols in the presence of an acid catalyst, most commonly aqueous
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CH5 ORGANIC REACTIONS