Process for the preparation of citral

Citral is prepared from prenol and prenal by condensation to form an acetal followed by cracking, both operations being effected in the presence of a lithium halide (e.g. lithium chloride) as catalyst.

The present invention relates to the preparation of citral from prenal and 
prenol. 
According to French Pat. No. FR 74/40,959 (2,246,529) it is known to 
prepare citral by cracking the diprenyl acetal of prenal, which is itself 
obtained by condensation of two moles of prenol with one mole of prenal. 
The condensation of prenol with prenal is carried out in the presence of a 
condensing agent which is preferably an ammonium salt such as ammonium 
nitrate, and of a dehydrating agent which may be calcium sulphate or a 
molecular sieve, and the pyrolysis of the diprenyl acetal thus obtained is 
carried out by heating to a temperature of between 120 and 230.degree. C., 
optionally in the presence of an acidic catalyst such as 
p-toluenesulphonic, o-nitrobenzoic acid or sodium hydrogen sulphate. 
It has now been found, and this is what forms the subject matter of the 
present invention, that the condensation and the pyrolysis may be carried 
out using a lithium halide, and more particularly lithium chloride, as a 
catalyst. 
The present invention accordingly provides a process for the preparation of 
citral which comprises condensing prenol with prenal in a molar ratio of 
at least 2:1 and in the presence of a lithium halide as catalyst to 
produce the diprenyl acetal of prenal, and then cracking the said acetal 
also in the presence of a lithium halide as catalyst. 
The acetalization is carried out using at least two moles of prenol per 
mole of prenal and generally by operating in an inert organic solvent such 
as an aromatic hydrocarbon e.g. toluene, at a temperature of 70.degree. to 
100.degree. C. so that the water formed is entrained by azeotropic 
distillation. It may be advantageous to operate under reduced pressure, 
generally between 20 and 100 mm Hg (2.7 to 13.3 kPa), depending on the 
nature of the solvent employed. 
The cracking of the diprenyl acetal of prenal is generally carried out by 
heating to a temperature of between 120.degree. and 150.degree. C. in the 
presence of the lithium halide, preferably lithium chloride, the operation 
being carried out in a suitable organic solvent such as 
1,2-dichlorobenzene. It may be advantageous to operate under reduced 
pressure, generally close to 100 mm Hg (13.3 kPa). 
In the process of the invention, it is unnecessary to isolate the diprenyl 
acetal of prenal before performing the cracking. In this case, it is not 
necessary to add more lithium halide to perform the cracking, since the 
lithium halide present in the crude product of the acetalization stage is 
used. 
It may be particularly advantageous to operate the acetalization and 
cracking steps in the presence of a polymerization inhibitor such as 
hydroquinone. 
In the process of the invention, from 0.01 to 0.1 mole of lithium halide is 
generally employed per mole of prenal or of the diprenyl acetal of prenal. 
The process of the invention makes it possible to obtain citral selectively 
in yields which are generally higher than 60% relative to the prenal 
converted, the degree of conversion of prenal being generally higher than 
75%.

The following Example illustrates the invention. 
EXAMPLE 
Prenal of 97.2% purity (35.1 g, 405.58 mmol), pure prenol (87 g, 1010.22 
mmol), toluene (11.5 cc) and hydroquinone (0.1 g) are introduced into a 
round-bottom flask fitted with a distillation column. Lithium chloride 
(0.34 g, 8.02 mmol) is added and the mixture is then heated for 2 hours 30 
minutes under reduced pressure (80 mm Hg; 10.7 kPa) to a temperature of 
between 75.degree. and 79.degree. C. while the water formed (3 cc) is 
entrained azeotropically. Lithium chloride (0.34 g) is added again and 
heating is continued for 2 hours under reduced pressure (80 mm Hg; 10.7 
kPa) at a temperature of between 82.degree. and 86.degree. C. while the 
water formed (5.2 cc in all) is entrained azeotropically. After removal of 
the toluene by distillation, the round-bottom flask contains a pale yellow 
oil (117.68 g) containing the diprenyl acetal of prenal. 
Analysis of the reaction mixture shows that the degree of conversion of 
prenal is 73% and that the yield of the diprenyl acetal of prenal is 93% 
based on the prenal converted. 
To the crude diprenyl acetal of prenal (116.8 g) obtained previously, which 
analysed 55.8% by titration, i.e. 271.74 mmol, 1,2-dichlorobenzene (10 g) 
is added. After the excess prenol and unreacted prenal have been removed 
by distillation under reduced pressure (15 mm Hg; 2 kPa), the mixture is 
heated under reduced pressure (90 mm of mercury; 12 kPa) for 3 hours 30 
minutes at a temperature of between 135.degree. and 142.degree. C. A 
distillate (80 cc, 70.9 g) is collected. The residue (54.2 g) in the flask 
is an orange-yellow oil consisting essentially of citral. 
Analysis of the reaction mixture shows that the degree of conversion of the 
diprenyl acetal of prenal is 70% and that the yield of citral is 96%, 
based on the acetal converted.