Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

BENZYL CHLOKIDE FKOM TOLUENE 101
Uses : for benzyl cyanide (p. 137), ethyl benzylmalonate (p. 255),
the Grignard reaction.
The theoretically simplest method of replacing hydrogen united to
carbon by halogen consists in acting on saturated hydrocarbons with
free halogen. The process, like the reaction between chlorine and hydrogen,
is catalytically accelerated by light and when applied to methane
and chlorine leads to the conversion of this hydrocarbon into mono-,
di-, tri-, and tetrachloromethane. The higher paraffins are also chlorinated
in this way, but, as a preparative method, the process is inconvenient
and has the disadvantage that various reaction products are
produced simultaneously and are difficult to separate. As a general
rule, the chlorine first attacks that carbon atom which is poorest in
hydrogen. In the aliphatic series the alcohols, which are more easily
obtainable in the pure state than the hydrocarbons, constitute the sole
starting materials for the preparation of the halogen compounds
(reactions 1 and 2). Substitution by chlorine is much simpler in the case
of toluene and the homologous methylbenzenes (xylenes, etc.). Here
we have two quite distinct processes.
1. Substitution occurs exclusively in the ring when typical halogencarriers
such as iron filings or iodine are used. From toluene the oand
y-derivatives are produced together.
2. In the absence of such a carrier the benzene ring is not attacked
in the slightest, even at the boiling point. The rate of substitution in
the methyl group (side chain), which is unmeasurably small in the cold,
increases to an extent sufficient for preparative purposes, in accordance
with the general law that each rise of 10° in the temperature produces a
two to threefold increase in the velocity of the reaction. This reaction
is sensitive to light, like all direct replacement of hydrogen by chlorine.
The statement that addition of phosphorus pentachloride also accelerates
the reaction is incorrect. The reaction between toluene and
chlorine presents a very fine example of the specific influence of various
catalysts on a reacting system.
For preparative purposes it is of great importance that the entrance
of the second chlorine atom into the side chain proceeds at a much
lower rate than the first phase of the reaction. Hence almost all the
chlorine is used up by the toluene present before any significant further
chlorination of the benzyl chloride occurs. The proximity of the
benzene ring makes the chlorine less firmly attached to the side chain,
i.e. more mobile than in the true paraffins. In explanation it is suggested
that the benzene ring uses up more of the combining energy of the
methane carbon atom than does alkyl or hydrogen, and that therefore
less is available for the chlorine. (Thiele and Werner's theory of the
distribution of affinity.) From this example we learn that although
the connecting lines in our formulae give formal expression to the
quadrivalency of carbon, they tell us nothing about the energy relation-