Chemistry Reference
In-Depth Information
Cyanohydrin formation is a useful synthetic reaction,
in that it utilizes a simple reagent, cyanide, to create a
new C-C bond. The cyano (nitrile) group may easily
be modified to other functions, e.g. carboxylic acids
via hydrolysis (see Box 7.9) or amines by reduction.
hydrolysis
CO
2
H
useful reactions
of nitrile groups
C
N
CH
2
NH
2
reduction
Box 7.7
Natural cyanohydrins and cyanogenic glycosides
Natural cyanohydrins feature as toxic constituents in a number of plants, e.g. laurel, bitter almonds, and
cassava. In the plant, the
cyanohydrin
is bound through an acetal linkage to a sugar, usually glucose, to
produce what is termed a
cyanogenic glycoside
. Cyanogenic means cyanide-producing, because, upon hydrolysis,
the glycoside breaks down to the sugar, the carbonyl compound, and HCN. When a plant tissue containing
a cyanogenic glycoside is crushed, hydrolytic enzymes also in the plant, but usually located in different
cells, are brought into contact with the glycoside and begin to hydrolyse it. Alternatively, hydrolysis may
be brought about by ingesting the plant material. Either way, it leads to the production of HCN, which is
extremely toxic to humans. The glycoside itself is not especially toxic, and toxicity depends on the hydrolysis
reaction.
hydrolysis shown
as acid catalysed
acetal
CH
2
OH
CH
2
OH
Ph
Ph
H
Ph
O
O
Ph
HO
HO
H
+
HO
O
H
HO
O
H
HO
H
HO
CN
OH
OH
H
CN
CN
CN
prunasin
benzaldehyde
cyanohydrin
(mandelonitrile)
Ph
CH
2
OH
CH
2
OH
HCN
O
O
O
HO
HO
H
2
O
H
HO
OH
HO
OH
benzaldehyde
OH
glucose
The main cyanogenic glycoside in laurel is
prunasin
,the
β
-
D
-glucoside of benzaldehyde cyanohydrin. The
enzymic hydrolysis of prunasin may be visualized as an acid-catalysed process, first of all hydrolysing the acetal
linkage to produce glucose and the cyanohydrin. Further hydrolysis results in reversal of cyanohydrin formation,
giving HCN and benzaldehyde.
CH
2
OH
O
HO
HO
O
OH
CH
2
OH
O
Ph
O
CH
3
HO
HO
HO
O
H
HO
O
CH
3
OH
CN
OH
CN
amygdalin
linamarin
