Chemistry Reference
In-Depth Information
phenols
<
alcohols
<
aromatic amines
<
aliphatic amines. When Y
¼
CO
2
and
Z
H, acid chlorides are formed. In dehydration reactions, multivalent groups are
involved; Y
¼
a
Z is effectively an unsubstituted or substituted amidic group.
COCl plays the key role in phosgenation reactions that are of a stepwise na-
ture; the major part of these processes is COCl (chlorocarbonyl) transfer to R
R
x
Y
x
a
Y
a
H
generating chloroformates, carbamoyl chlorides, etc. R
COCl is of limited (low)
stability and this is the driving force behind its intermediacy in the synthesis of
chlorides and isocyanates under elimination conditions (eliminating CO
2
and/or
HCl), and also determines the character of a reactive substrate in further nucleo-
philic substitutions to form symmetrical and unsymmetrical substituted carbonic
acid derivatives (carbonates, carbamates, ureas)ordiaryl ketones. Commonly, chloro-
formylation and isocyanate formation are independent of the nature of R. Ob-
viously, the reactivity is very different due to the relative basic/nucleophilic ratio.
For example, Ar
a
Y
a
Cl cannot be prepared through a chloroformate intermediate nor
by direct phosgenation, but the reaction does work well in the aliphatic series.
Special reactions of COCl
2
with R
a
a
Y
a
Z are also known. When Y
¼
CO and
Z
¼
H (aldehyde), an a-chlorochloroformate R
a
CHCl
a
O
a
CO
a
Cl is formed in an
addition reaction to the aldehydic C
b
O double bond. When R
¼
NR
1
R
2
,Y
¼
CO,
and Z
¼
H, a Vilsmeier salt/reagent is formed.
4.2
Chloroformylation (Chlorocarbonylation)
4.2.1
Chloroformates (Chlorocarbonylation of Alcohols)
Phosgene is a source of the chlorocarbonyl group,
COCl, in reactions with many
and various nucleophilic species. Depending on the nature of the nucleophilic
reaction center (carbon, oxygen, nitrogen or sulfur), a wide variety of useful and, in
general, highly reactive products are formed. The reactivity of phosgene is typical
of that of acyl halides and the mechanism can be S
N
1 or tetrahedral. The scheme
below compares chlorocarbonyl group transfers to various nucleophilic reaction
centers (oxygen, carbon, and nitrogen, respectively).
Oxygen or sulfur nucleophiles:
a
O
Y
l
Y
R
H
R
Cl
Cl
O
O
Y
l
Cl
Cl
Cl
Y
O
Y = O, S
