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EFFECTS OF LIGHT ON SIDE CHAIN CHLORINATION

    
Chlorination did not occur at room temperature or in the dark 
(heat or light is required to initiate the reaction)

The most effective initiation is blue light. 
A chlorine molecule is known to absorb light of that wavelength.

 (The light is activating the chlorine molecule)

The light-initiated reaction has a high quantum yield.
 i.e. one photon gives more than one molecule of product.
 (The reaction is probably a Chain process).


The Free Radical Chain Reaction

Free radical chain reactions comprise of three different steps:

Initiation

Propagation

Termination


 The initiation step This is where the reactive intermediate is generated).

Chlorine molecules absorb blue light, whereas toluene does not, 
so it is safe to presume it is the chlorine molecule that is being activated, , 
and doing the initiating.



Blue light contains the correct energy to split a chlorine molecule into , 
 two chlorine atoms (58kcal/mol).



This is the generation of a reactive intermediate: a short lived, highly reactive species 
that is never present in large concentrations due to its reactive nature.

Each chlorine atom has 7 valence electrons: 3 are paired, one is unpaired.

The unpaired electron is termed the odd electron or radical electron.

Species with unpaired electrons are called free radicals.

Normally radicals are electron deficient and strongly electrophilic.

 Propagation Steps 

The reactive chlorine atom collides with a methyl  molecule, and abstracts a hydrogen atom.
One of the C-H bond electrons stays on the C atom.

The first propagation step is the reaction of the reactive intermediate.

The radical reacts, and generates another radical. (Atom and radical ® atom and radical.

The reactive intermediate thus produced (methyl radical) reacts in the second propagation step.

In addition to the generation of the product , another chlorine radical is produced.

This can then react with another molecule of  toluene to give HCl and a methyl radical, 
which in turn reacts with another molecule of chlorine to give chloro toluene and a chlorine radical,
 which can react…

A chain reaction will continue is such a fashion until either all the  toluene is used up, 
or the radicals are consumed in another way (termination steps).

Termination steps
If a radical undergoes a reaction which does not generate another radical, then the chain reaction slows and stops. 

Therefore side reactions which consume radicals are detrimental to chain reactions. These side reactions are called Termination Reactions.

Possible Termination Steps 

 


Since the termination steps usually involve the combination of two free radicals (which are always in low concentration) these are less likely than the collisions of radicals and atoms (i.e. desired reactions).

 

 


  
Chlorination in the benzene nucelus, an ionic reaction,
    favoured by low temperatures (0-500) accompanied by high concentrations of
    dissolved chlorine, and facilitated by helogen carriers like iron and aluninium salts.
  
Substitution of hydrogen by chlorine in the side-chain,
    following a radical chain reaction mechanism. This will preferably occur at elevated
    temperatures (80-1800), 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 20th
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:

 


  
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 :