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My instructor mentioned a reaction for preparation of benzaldehyde by chlorination of toluene in presence of sunlight to form benzal dichloride, which on hydrolysis gives benzaldehyde.

However, I'm not sure why the chlorination of side chain should stop after two chlorine atoms are added? At what step does the side chain chlorination of toluene stop?

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It doesn't stop, it just gets a lot slower the more chlorine atoms you add. From the abstract of an article titled Photochemical side-chain chlorination of toluene[1]:

Where $k_1$, $k_2$, and $k_3$ are the rate consts. for the successive reactions: $\ce{PhMe}$ → $\ce{PhCH2Cl}$ → $\ce{PhCHCl2}$ → $\ce{PhCCl3}$, resp.; $r = \frac{k_1}{k_2}$ and $s = \frac{k_2}{k_3}$, values of $r$ and $s$, resp., were 6.0 and 5.7 at 100° and 8.4 and 8.5 at 40°, for batch operation, with light varying from ultraviolet to yellow.


[1] Haring, I. H. G.; Knol, H. W. Chemical and Process Engineering (London) 1964, 45 (10), 560–567.

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It will not stop at the dichloro compound, it is a free radical halogenation reaction.

The benzyl radical is very stable so it is easy for a chlorine atom to abstract a hydrogen from the methyl grup thus forming a benzyl radical. The benzyl radical then reacts with a chlorine molecule to form a chlorine atom and a molecule of benzyl chloride.

This process can be repeated again to form $\ce{C6H5CHCl2}$ and then $\ce{C6H5CCl3}$. It would be a good idea if you were making benzadehyde on the industrial scale to distill the chlorinated mixture to separate toluene, benzyl chloride, the dichloro compound (dichloromethylbenzene) and the trichloro compound (trichloromethylbenzene).

You can then react the dichloro compound with water to get the target molecule.

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