Page 133 - Chemical and process design handbook

Speight_Part II_B 11/7/01 3:11 PM Page 2.74

BENZALDEHYDE

Benzaldehyde (C H CHO, melting point: –26°C, boiling point: 179°C, den-
6 5
sity: 1.046) is a colorless liquid that is sometimes referred to as artificial
almond oil or oil of bitter almonds because of the characteristic nutlike odor.
Benzaldehyde is slightly soluble in water but is miscible in all proportions
with alcohol or ether. On standing in air, benzaldehyde oxidizes readily to
benzoic acid.
Commercially, benzaldehyde may be produced (1) by heating benzal
chloride (C H CHCl ) with calcium hydroxide:
6 5 2
C H CHCl + Ca(OH) → C H CHO + CaO + 2HCl
6 5 2 2 6 5
(2) by heating calcium benzoate and calcium formate:

(C H COO) Ca + (HCOO) Ca → 2C H CHO +4 2CaCO
6 5 2 2 6 5 3
or (3) by boiling glucoside amygdalin of bitter almonds with a dilute acid.
Benzaldehyde is manufactured in two grades, technical and refined. The
technical grade is largely used as an intermediate in the synthesis of other
chemicals, such as benzyl benzoate, cinnamic aldehyde, and dyes.
Most of the technical grade is made by direct vapor-phase oxidation of
toluene, although some is made by chlorinating toluene to benzal chloride,
followed by alkaline or acid hydrolysis. For perfume and flavoring use, the
refined, chlorine-free grade is required, which is economically produced
o
by the direct vapor-phase oxidation of toluene with air at 500 C.
C H CH + [O] → C H CH=O
6 5 3 6 5
It is claimed that a catalyst mixture of 93% uranium oxide and 7% molyb-
denum oxide gives relatively high yields. The oxidation is sometimes car-
ried out in the liquid phase by using manganese dioxide/sulfuric acid at
o
40 C.
Benzaldehyde is used as a flavoring material, in the production of cin-
namic acid, in the manufacture of malachite green dye, as an ingredient in
pharmaceuticals, and as an intermediate in chemical syntheses.

2.74