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My textbook mentions that chlorination of alkanes by $\ce{SO2Cl2}$ is preferred over $\ce{SOCl2}$, but I didn't understand why.

Could someone explain which of the two reagents I should use in what situations, and briefly tell me why?

There is a similar question here: How does SO2Cl2 react with alcohols?

However the answers there did not completely clarify my doubts.

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Mahathi Vempati
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2 Answers2

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The reactivity patterns of $\ce{SO2Cl2}$ and $\ce{SOCl2}$ are quite different. $\ce{SOCl2}$ is a good electrophile, and can be thought of as a source of $\ce{Cl-}$ ions. These ions can go on to react in their typical nucleophilic fashion. $\ce{SO2Cl2}$ however is often a $\ce{Cl2}$ source, as it readily decomposes giving off sulfur dioxide. Usually much easier/safer to use this than measuring out (and getting into solution) chlorine gas. The chlorination of simple alkanes by $\ce{Cl2}$ gas (or something that makes it in solution) happens by a radical mechanism i.e. $\ce{Cl.}$ not $\ce{Cl-}$.

Martin - マーチン
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StevieD
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    A solid answer. – Lighthart Mar 22 '16 at 05:17
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  • Thanks a lot for the precise answer. So the major difference is socl2 reacts by nucleophilic substitution, but so2cl2 reacts by free radical mechanism, right? – Mahathi Vempati Mar 22 '16 at 07:49
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    I would say that the major difference is the oxidation state of the Cl formed. As stated in the answer, SOCl2 is a source of chloride ions whereas SO2Cl2 is a source of chlorine. That is a consequence of the different reactivity of the two species (SOCl2 is an electrophile and SO2Cl2 decomposes into SO2 + Cl2) I'm just restating the answer... my goodness. – orthocresol Mar 22 '16 at 09:35
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Chlorination of alkanes by $\ce{SO2Cl2}$ is preferred over $\ce{SOCl2}$. Just pay little more attention on the word alkanes. So, the statement is absolutely correct. Just go through it (link)! Hence, For chlorination in alkanes, if you use $\ce{SO2Cl2}$, It undergoes a free radical substitution reaction and will get the product.

If we use $\ce{SOCl2}$, it undergoes a nucleophilic substitution reaction. Here, this reaction mechanism is explained.

So, You can see here that $\ce{SOCl2}$ doesn't produce a chlorine free radical. Hence, It is not possible to chlorinate alkanes using $\ce{SOCl2}$, which $\ce{SO2Cl2}$ does.

M.A.R.
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Arkendu Roy
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