Chemistry Reference
In-Depth Information
of the above three types of reactions. The reaction in which aldehydes that have no
˛
-
hydrogen for enolization disproportionate in the presence of a strong base, to give equal
amounts of the corresponding alcohol and a salt of carboxylic acid (Eqs 2.320 - 2.322) is
known as Cannizzaro's reaction (an oxidation - reduction reaction involving aldehydes
and a strong base). One molecule of the aldehyde acts as a hydride (H
ž
) donor and
another functions as an acceptor.
50% NaOH
! CH
3
OH C HCOO
Na
C
2HCHO
2
.
320
Conc NaOH
! Bu
t
CH
2
OH C Bu
t
COO
Na
C
2Bu
t
CHO
2
.
321
Conc NaOH
! C
6
H
5
CH
2
OH C C
6
H
5
COO
Na
C
2C
6
H
5
CHO
2
.
322
However, an aldehyde containing an
˛
-hydrogen atom undergoes an aldol conden-
sation in the presence of a dilute base or acid. For example, two moles of acetaldehyde
combine to form
ˇ
-hydroxy butyraldehyde (Eq. 2.323):
HO
! CH
3
CH
OH
CH
2
CHO
CH
3
CHO C CH
3
CHO
2
.
323
4.15.2
Tischtchenko Reaction
Claisen
976
in 1887 modified the Cannizzaro reaction by using sodium ethoxide in place
of sodium hydroxide, and found that benzaldeyde was converted to benzyl benzoate.
In 1906 Tischtchenko
977
observed that sodium alkoxides can be used for the formation
of esters from both aliphatic and aromatic aldehydes. Nord
et al
.
978 - 980
found that all
aldehydes can be made to undergo the Cannizzaro reaction by treating with Al
OEt
3
;
under these conditions the acid and alcohol are combined as the ester, and the reaction
is known as the Tischtchenko reaction (Eqs 2.324 - 2.327):
Al
OEt
3
! CH
3
COOCH
2
CH
3
2CH
3
CHO
2
.
324
2CH
3
CH
2
CHO ! CH
3
CH
2
COOCH
2
CH
2
CH
3
2
.
325
2CH
2
D
CHCHO ! CH
2
D
CHCOOCH
2
CH
2
D
CH
2
2
.
326
2C
6
H
5
CHO ! C
6
H
5
COOCH
2
C
6
H
5
2
.
327
In all these reactions Al
OEt
3
helps in the condensation of only two moles of alde-
hydes to form simple esters. However, the heterobimetallic ethoxide Mg[Al
OEt
4
]
2
catalyses the condensation of three moles of aldehydes to form trimeric glycol ester
(Eq. 2.328):
Mg[Al
OEt
4
]
2
! RCH
2
CH
OH
CHRCHO
2RCH
2
CHO
CRCH
2
CHO
! RCH
2
CH
OH
CHRCH
2
COOCH
2
R
2
.
328
