vogel_practical_ochem_3.pdf

115] AROMATIC COMPOUNDS 689
AROMATIC ALDEHYDES
Aromatic aldehydes may be prepared :—
1. By side-chain chlorination of the hydrocarbon (Section IV,23), followed
by hydrolysis of the dichloro compound, say, with water at 95-100° in the
presence of iron as a catalyst, for example :
2C1, H,0
C6H6CH3 • C6H5CHC12 • C6H6CHO
Toluene Benzal chloride Benzaldehyde
p-Bromobenzaldehyde may be prepared by an analogous method :
CHBra
CHO
2Brt / \ H,0
(CaCO,)
Br Br Br
p-Bromotoluene p-Bromobenzal bromide p-Bromobenzaldehyde
2. By oxidation of the methyl derivative of an aromatic hydrocarbon with
a solution of chromic anhydride in acetic anhydride and acetic acid. The
aldehyde formed is immediately converted into the gera-diacetate, which is
stable to oxidation. The diacetate is collected and hydrolysed with sulphuric
acid, for example :
NO,
h
1
Y
CH,
p-Nitro toluene i
2(CH,C0),0
+ 2f 0] (CrO.)
N02
|
f \ —-
\ ) (H.S0.)
NO,
|
f J + 2CH,C00H
CH(OCOCH3)2
CHO
p-Nitrobenzal- p -Nitrobenzaldehyde
diacetate
Similarly, p- or o-bromotoluene —> p- or o-bromobenzaldehyde diacetate —•
p- or o-bromobenzaldehyde.
3. By passing a mixture of carbon monoxide and hydrogen chloride into the
aromatic hydrocarbon in the presence of a mixture of cuprous chloride and
aluminium chloride which acts as a catalyst (Gattermann - Koch reaction).
The mixture of gases probably reacts as the equivalent of the unisolated acid
chloride of formic acid (formyl chloride) :
CH8
|| + CO + HC1 (= HCOC1)
A1C1,
+ CuCl
CH3
V
CHO
luene
p-Tolualdehyde
The Gattermann-Koch formylation was found unsuited to the preparation of
aldehydes from phenols and phenol ethers : such aldehydes may be obtained by
Gattermann's aldehyde reaction.
4. By interaction of hydrogen cyanide and hydrogen chloride with an
aromatic compound (hydrocarbon, phenol or phenol ether) in the presence of
aluminium chloride (or zinc chloride). This is known as the Gattermann
f \