Process for the preparation of benzal chlorides

Process for the preparation of benzal chlorides from benzotrichlorides, according to which benzotrichlorides are reacted with thiols in the presence of metal salts at elevated temperatures.

The present invention relates to a process for the preparation of benzal 
chlorides from benzotrichlorides. 
Benzal chlorides are important intermediates which, for example, can be 
converted into the corresponding benzaldehydes by hydrolysis. 
Benzaldehydes are used, for example, for the preparation of herbicides 
(see U.S. Pat. No. 4,212,998 and U.S. Pat. No. 4,212,999). 
It is already known that benzotrichlorides can be converted into benzal 
chlorides. For example, benzotrichloride can be reacted with trivalent 
phosphorus compounds to give benzal chloride. Good results are obtained in 
this reaction only in particular cases, for example if phosphorous acid 
tri-(di-N-ethyl)-amide in ether is used as the trivalent phosphorus 
compound and the reaction is carried out in the presence of ethanol (see 
I. M. Downie and J. B. Lee, Tetrahedron Letters 1968, 4951). 
CCl.sub.3 groups bonded to aliphatics can be reacted with nickel 
tetracarbonyl in tetrahydrofuran to give CHCl.sub.2 groups. However, this 
process cannot be applied to CCl.sub.3 groups bonded to aromatics, since 
no benzal chlorides are formed and dehalogenating dimerization takes 
place, diphenylethane derivatives being formed (see T. Kunieda et al., 
J.C.S.Chem.Comm. 1972, 885). 
Another process for converting CCl.sub.3 groups bonded to aliphatics into 
CHCl.sub.2 groups uses thiols as reducing agents and is carried out in the 
presence of iron carbonyls or iron chlorides (see R. G. Petrova and R. Kh. 
Freidlina, Izvest.Akad.Nauk. SSSR 1970, 1483 (English)). It is not known 
whether this process can be applied to CCl.sub.3 groups bonded to 
aromatics. In this case also, it must be expected that if CCl.sub.3 groups 
bonded to aromatics are used, diphenylethane derivatives are formed and/or 
Friedel-Crafts reactions take place. 
A process has now been found for the preparation of benzal chlorides from 
benzotrichlorides, which is characterized in that benzotrichlorides are 
reacted with thiols in the presence of metal salts at elevated 
temperatures. 
The most diverse benzotrichlorides are suitable for use in the process 
according to the invention. The starting material can be unsubstituted 
benzotrichloride (C.sub.6 H.sub.5 CCl.sub.3), or benzotrichlorides which 
are mono- or poly-substituted on the aromatic nucleus. 
Examples of suitable benzotrichlorides are those which correspond to the 
formula 
##STR1## 
in which R.sub.1 to R.sub.5 independently of one another represent 
hydrogen, nitro, halogen, alkyl, fluoroalkyl, aryl, substituted aryl, 
O-alkyl, O-fluoroalkyl, phenoxy, substituted phenoxy and/or cyanide. 
Halogen radicals here are preferably fluorine and chlorine. Alkyl, 
fluoroalkyl, O-alkyl and O-fluoroalkyl radicals here preferably contain 1 
to 12 C atoms, particularly preferably 1 to 4 C atoms. Aryl, substituted 
aryl, phenoxy and substituted phenoxy radicals here preferably contain 6 
to 10 C atoms, particularly preferably 6 to 8 C atoms. Substituted aryl 
and substituted phenoxy are preferably alkyl- or nitro-substituted aryl 
and alkyl- or nitro-substituted phenoxy whereby methyl is the particularly 
preferred alkyl substituent. 
Unsubstituted benzotrichloride or benzotrichlorides which are mono-, di- or 
tri-substituted on the aromatic nucleus are preferably used in the process 
according to the invention. Examples of preferred benzotrichlorides are 
those which correspond to the formula 
##STR2## 
in which 
R.sub.6 to R.sub.8 independently of one another represent hydrogen, nitro, 
fluorine, chlorine, cyanide, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 
-fluoroalkyl and/or O-C.sub.1 -C.sub.4 -fluoroalkyl. 
Benzotrichlorides of the formula (II) in which R.sub.6 to R.sub.8 
independently of one another represent hydrogen, nitro, fluorine, 
chlorine, cyanide, methyl, trifluoromethyl and/or o-trifluoromethyl are 
particularly preferably used in the process according to the invention. 
Benzotrichloride, o-chlorobenzotrichloride, p-chlorobenzotrichloride, 
o,p-dichlorobenzotrichloride, m-fluorobenzotrichloride, 
p-fluorobenzotrichloride, o-fluoro-o'-chlorobenzotrichloride, 
o,o'-difluorobenzotrichloride, m-methylbenzotrichloride, 
o-trifluoromethylbenzotrichloride, m-trifluoromethylbenzotrichloride, 
p-trifluoromethylbenzotrichloride, m-trifluormethoxybenzotrichloride, 
o-chloro-m-trifluoromethylbenzotrichloride, 
m-nitro-m'-trifluoromethyl-p-chlorobenzotrichloride, 
o-cyanobenzotrichloride, m-cyanobenzotrichloride and 
p-cyanobenzotrichloride. 
The most diverse thiols are suitable for use in the process according to 
the invention. Thiols which are liquid under the reaction conditions are 
preferably employed. Other thiols must be used in dissolved form. 
Aliphatic thiols with 4 or more C atoms and aromatic thiols with 6 or more 
C atoms are preferably used, for example aliphatic thiols with 4 to 10 C 
atoms or aromatic thiols with 6 to 8 C atoms. The use of butanethiol and 
thiophenol is particularly preferred. The use of thiophenol is very 
particularly preferred. 
2 moles of thiol are required for converting 1 mole of a benzotrichloride 
into the corresponding benzal chloride. 2 moles of a thiol per mole of a 
benzotrichloride are therefore preferably used in the process according to 
the invention, since virtually no unreacted benzotrichloride or unreacted 
thiol then has to be separated off from the reaction mixture. However, the 
process according to the invention can also be carried out with molar 
ratios of benzotrichlorides to thiols of less than or more than 1:2. 
The process according to the invention is carried out in the presence of 
metal salts. The metal salts can be of the most diverse type, for example 
they can also be metal complexes. The reaction is preferably carried out 
in the presence of salts of sub-group elements. Copper salts and iron 
salts, for example copper halides and iron halides, are particularly 
preferred. Of the copper salts, those in which copper is in the +1 
oxidation stage are preferred. The use of copper-I bromide is very 
particularly preferred. 
It is sufficient for small amounts of the metal salts to be present. 
Examples of suitable amounts are those from 0.01 to 10% by weight, those 
from 0.1 to 5% by weight are preferred and those from 0.5 to 2% by weight 
are particularly preferred, in each case based on the benzotrichloride 
employed. 
The process according to the invention is in general carried out in the 
absence of solvents. However, it can also be carried out in the presence 
of solvents. The presence of solvents is necessary, for example, if thiols 
which are not liquid under the reaction conditions are to be used. 
Examples of suitable solvents are those which have a melting point below 
80.degree. C. and a boiling point above 100.degree. C., are sufficiently 
inert and have an adequate dissolving power for the particular 
benzotrichloride and/or thiol employed. In general, toluene, xylene, 
mesitylene and methylcyclohexane are particularly suitable solvents. 
Hydrogen chloride is split off during the reaction according to the 
invention. The lower temperature limit for the process according to the 
invention is therefore the temperature at which the evolution of hydrogen 
chloride starts in the particular case. In general, this temperature is 
about 80.degree. C., but in particular cases it can also be higher or 
lower, for example in the range from 50.degree. to 120.degree. C. The 
upper temperature limit for the process according to the invention is less 
critical. From practical considerations, the maximum temperature at which 
the reaction is carried out is in general the temperature at which the 
reaction mixture boils under reflux, it being possible for this 
temperature to be influenced, for example by appropriate choice of the 
thiol employed, a solvent and/or the pressure in the reaction vessel. The 
process according to the invention is preferably carried out at 
temperatures in the range from 50.degree. to 150.degree. C., particularly 
preferably at temperatures in the range from 80.degree. to 120.degree. C. 
For example, a procedure may be followed in which the reaction mixture is 
first heated to 80.degree. C. and the temperature is then slowly increased 
to 120.degree. C. and kept at 120.degree. C. until the evolution of 
hydrogen chloride has ended. 
The process according to the invention is in general carried out under 
normal pressure. It can also be carried out under increased or reduced 
pressure, for example in the pressure range from 0.5 to 20 bar. It may be 
advantageous to carry out the reaction under a reduced pressure if 
splitting off and removal of the hydrogen chloride is to be facilitated. 
Increased pressure may be advantageous if the reaction is to be carried 
out at temperatures above the boiling point of the component of the 
reaction mixture which has the lowest boiling point under normal pressure. 
The reaction mixture is in general kept at the reaction temperature until 
the evolution of hydrogen chloride has ended. This time can be, for 
example, in the range from 2 to 15 hours, and is usually in the range from 
5 to 10 hours. 
When the reaction has ended, the reaction mixture can be worked up, for 
example, by distillation. A procedure can be followed here, in which, if a 
solvent is present, this is separated off and the resulting benzal 
chloride is removed from the reaction mixture by distillation under 
reduced pressure. Examples of suitable pressures here are those in the 
range from 0.1 to 50 mbar. As a rule, the benzal chlorides are thereby 
already obtained in a purity sufficient for further reactions. If 
appropriate, the benzal chlorides thus separated off can be distilled 
again, for further purification. This is particularly advantageous if the 
benzal chlorides initially separated off are contaminated by troublesome 
amounts of unreacted benzotrichlorides and/or disulphides formed from the 
thiols, which may be the case if the boiling points of the 
benzotrichloride employed, the benzal chloride formed and/or the 
disulphide formed are close to one another. 
The disulphides, which in general remain in the distillation residue, can 
be converted back into the corresponding thiols in a known manner by 
catalytic hydrogenation, and these can then be used again in the reaction 
according to the invention. 
The process according to the invention permits the preparation of the most 
diverse benzal chlorides from the corresponding benzotrichlorides in a 
simple manner and with good yields. It is decidedly surprising that if the 
formation of diphenylethane derivatives occurs at all, it is only to a 
very minor degree. Traces of diphenylethane derivatives may be formed, the 
maximum being 2%. The advantages of the process according to the invention 
are most pronounced if thiophenol is employed as the thiol and copper-I 
bromide is employed as the metal salt.