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    The alkyl substituted aromatic hydrocarbons, such as toluene and the xylene, behave exceptionally upon chlorination. This reactin can follow two different routes:

1. Chlorination in the benzene nucelus, an ionic reaction, favoured by low temperatures (0-50⁰C) accompanied by high concentrations of dissolved chlorine, and facilitated by helogen carriers like iron and aluninium salts.
2. Substitution of hydrogen by chlorine in the side-chain, following a radical chain reaction mechanism. This will preferably occur at elevated temperatures (80-180⁰C), and will be accelerated by actinic llight that causes photedissociation of chlorine molecules into chlorine radicals, and by the presence of phosphorus compounds or peroxides which enhance free radical formation. As a consequence, for side-chain chlorination the absence of catalysts that promote nuclear sustitution is essential, which thus excludes metals such as iron and aluninium as materials of construction ofr reactors.

The side-chain chlorinated products, benxylchloride, benzalchloride and benzotrichloride have originally been prepared from other sources that toluene: benzalchloride was prepared by Cahours (1848) from benzaldehyde by reaction with phosphorus pentachloride; Cannizzaro (1853) converted benzyl alcohol with hydrogen chloride to benzyl chloride; and Schischkoff and Rosing (1858) treated benzoyl chloride with phosphorus pentachloride which gave benzotrichloride.

Beilstein and Geitner (1866) carried out the first systematic study of chlorine reaction ontoluene and by 1900 many studies had been performed to isolated and identify the various chlorination products.

In the beginning of the 20^th century industrial production of chlorinated compounds from toluene was started by I.G. Farben in Europe and Heyden Chemical Corp. And Hcoker Electrochemical Cy. In the U.S.A.

The nuclear substituted products, mono-, di-and trichlorotoluenes, are used as such, or as chemical intermed ates in the manufacture of peroxides, pharmaceuticals, pesticides and dye-stuffs.

Nearly all of the side-chain chlorinated products manufactured are converted to other products or intermediates by reactions involving the chlorine substituents: benzylchloride is mainly converted to Butyl benzyl phthalate, a vinyl resin plasticizer (see phthalates) benzalchloride to benzaldehyde, and benzotrichloride to benzoylchloride (1,2); a discussionof the reverse reactions:

SIDE – CHAIN SUSNTITUTION:

1. Preparationof benzylchoride, benzalchloride and benzotrichloride :

Benzylchloride can be prepared by the actionof sulfurylchloride on toluene in the presence of peroxides, by the reaction of benzyl alcohol with hydrogen chloride and with zinc chloride as a catalyst, by the action of phosphorus trichloride or pentachloride on benzyl alcohol, or by chloromethylation of benzene. These methods have no commerical significance, however, since the starting materials are too expensive.

In commercial practice benzylchloride, benzalchloride and benzotrichloride are manufactured by the photochemical chlorination of toluene, batchwise or continuosly, at about 100-140⁰C.

An extensive kinetic study of the action of chlorine on toluene under various reaction conditionos has been carried out using a bench scale glass reactor (16), consisting of four concentirc tubes forming an annular reaction column, 10 cm outer diameter and 2 cm wide, surrounded by an inner and an outer cooling –water mantle. The apparatus was provided with a copper pipe still for continuous toluene feed, a reflux condenser and a hydrochloric acid asorber. Various thermocouple wells for control and measurement of the temperature were located in selected parts of the reactor.

As the source of actinic light standard 40 W fluorescent lamps could be inseted into the centre tube; from this type of lamp a series was available having different spectral energy distributions with maxima in the ultraviolet, blue, green, yellow, and red regions of the spectrum (compare Tab).

Analyses of the reaction product by titration or by distillation followed by measurement of the refractive index were only a limited success and gas-liquid chromatography was found to give the est results (see also (17). With this means the distribution of reaction products was determined in batch as well as in continuous chlorination experiments. In the continuous runs the chlorine and toluene input flows varied between 1 to 2 and 2.2 to 1 mole /mole.

The study revealed that the light-catalysed liquid-phase chlorination of toluene consists of two simultaneous reaction system:

(a) Side-chain chlorination as the main reaction system, in turn consisting of three pseudo first-order consecutive reactions:

Toluene  ------ k₁---->   Benzylchloride  --- k₂------>  Benzalchloride  --- k₃------> Benzotrichloride.

(b) Substitution of chlorine in the aromatic nucleus, accompanied by side-chain chlorinationof the nuclear chlorine compounds, as a simultaneous side-reaction system.

     Side-chain chlorination products :

Although the rate of chlorination proves to be markedly accelerated by the use of catalysts, such as actinic light, phosphorus trichloride or peroxides (18,19,20,21), experimental determination of the reaction product distributions for continuous or batch chlorination, under various reaction conditions, reveals that the reaction rate-constant ratios k₁/k₂ and k₂/k₃ – the so-called selectivity parameters (22)- are not affected by the type of catalyst. The product distribution apparently corresponds to a reaction kinetic equilibrium condition, fixed by the chlorine to toluene mole ratio, the so-called degree of chlorination and by the selectivity parameters of the consecutive reactions.

The catalysts essentially accelerate only the rate of formation of chlorine radicals and, thus, the velocity with which the fixed equilibrium condition is reached. The catalytic influence of light may clearly be shown by expressing it in multiples of the reaction –rate constants at 100 ⁰C in darkness having been determined in batch chlorination experiments: with blue light reaction –rate constants are 25 times those indarkness, with ultraviolet llight 15 times, with green light 8 times, and with yellow light 3 times those in darkness (see table VII .3).

Consecutive reaction-rate constants for the side-chain chlorination of toluene, with different light sources, at two temperature levels.

The conclusion that blue light gives higher maxium rates of chlorination than ultraviolet, although surprising at first sight, can be readily explained: blue light will effectively penetrate a larger depth of chlorine-containing solutions than ultraviolet. The following example, using Lambert-Beer’s law             I =I₀. 10 ^e.c.d.,     may illustrate this statement.

For absorption of 99% of the intensity I₀ of the incident light, either blue or ultraviolet, I/I₀ will be 0.01, and hence e.c.d = 2. From Int. Crit. Tables we know that e, the molal extincitioncoefficiency,

For ‘blue’ light (436 nm) =4

For ‘ultraviolet’ light (350 nm) = 125.

Then the ‘light path’ travelled over which 99% of the light is absorbed,

D = 2 e.c., can be calculated to be :

D (blue) =5 cm; d (ultraviolet) = 0.16 cm.

Since the overall rate of chlorination must be proportional to the effectively irradiated volume of reaction mixture, blue light will obviosly give a higher rate of chlorination.

The selectivity parameters :

    The rate-constant ratios – are on the other hand functions of the activation energies required for formation of the compounds concerned and thus are dependent of the temperature (cf. Table VII. 3): At 100 ⁰C these ratios are equal to about 6, at 40 ⁰C to about 8 which means that the selectivity of the rapid reactions is enhanced by lowering the temperature.

This effect, however, is counter-acted upon by the increase of nuclear chlorination which means a decrease in net yield of side-chain chlofinated products from a given amount of toluene.

The experimentally determined product distributions are in close agreement with theoretical curves of performance predicted by a mathematical analysis using the reaction-rate constants shown in Table VII.3).

The curves of fig. 7.2 show the relative quantities of toluene, benzylchloride, benzalchloride and benzotrichloride fromed as functions of the degree and the mode of chlorination. In batch the maximum concentration of benzylchloride is reached at a degree of chlorination of 1.07, the maximum concentration of benzalchloride at 2.07 moles of chlorine per mole of toluene.

In accordance with the theory, single-stage continuous chlorination of toluene results in a greater proportion of higher chlorinated products than batch reaction up to the same degree of chlorination. Multi-stage chlorination will give products approximating those of batch operation.

Product distribution from chlorination of toluene :