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The above reaction looks very similar to a conventional sulfonation reaction but if the reaction pathway is analogous to sulfonation I do not see how the $\ce{SO_2Cl^+}$ would form. Surely $\ce{Cl^-}$ would leave rather than $\ce{H_2O}$ in the formation of the electrophile? Please help me find a mechanism.

Also, I would like to approach questions, such as this one, like I would a puzzle rather than calling upon my memory. Is there a series of steps that I should follow? (i.e: 1. Identify electro/nucleophile, 2. Identify acidic protons ...).


3 Answers 3


Rob, IMO this is a difficult problem, so don't feel bad about not "seeing" the solution right off.

Chlorosulfonic acid (or chlorosulfuric acid) is both difficult and hazardous to work with so it's reactions aren't discussed all that frequently. It can also give rise to different products at different temperatures. At higher temperatures it generates $\ce{SO3}$ which will behave as an electrophile and react with aromatic compounds to produce sulfonation products.

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At lower temperatures (your case), chlorosulfonic acid reacts by way of the following equilibrium (who knew this?)

$$\ce{3 ClSO2(OH) <=> SO2Cl^{+} + 2 SO3Cl^{-} + H3O^{+}}$$

The equilibrium generates $\ce{SO2Cl^{+}}$ which is the electrophile of interest in your reaction. $\ce{SO2Cl^{+}}$ will react with an aromatic nucleus to produce the corresponding sulfonyl chloride by way of a typical electrophilic aromatic substitution reaction.

If you'd like to read more about the complexities of reactions involving chlorosulfonic acid see here starting around page 11.

The difficult part here was identifying the source of the electrophile. Looking at the final product you get an idea of what the electrophile is, but how is it produced? Sometimes you have to do a little googling to sort things out.

Again, IMO this was a tough one.


The $\ce{OH}$-group is protonated by another molecule of chlorosulphuric acid, creating a cation. This cation can be attacked by benzene under the right conditions. $\ce{H2O}$ is not a better leaving group than $\ce{Cl-}$ (look at the charge, they mostly tell a lot about the ability of the leaving group, though not always). A famous example of this reaction is of course the famous old synthesis of saccharin (Clayden et al. p. 644 & p. 645) giving the widely used $\ce{TsCl}$ as byproduct:

Synthesis of saccharin - picture 1

Synthesis of saccharin - picture 2

  • $\begingroup$ Is the ortho product the major product? $\endgroup$
    – J_B892
    Commented Nov 12, 2019 at 5:26

The answers provided so far are actually incorrect.

First, the benzene reacts with chlorosulfonic acid to produce benzenesulfonic acid and HCl. The chloride acts as a leaving group, not the OH. Second, the benzenesulfonic acid reacts with a second equivalent of chlorosulfonic acid to produce benzenesulfonyl chloride and sulfuric acid. Thus, in net you need two equivalents of chlorosulfonic acid to convert benzene to benzenesulfonyl chloride.

Edit update: I should add that the arrow-pushing mechanism of sulfonation with SO3, provided above by Ron, is actually incorrect too. Modern research has shown that the traditional organic chemisty mechanism of electrophilic aromatic substitution, in this case, is actually not true. The intermediate (sometimes call the sigma complex or the Wheland intermediate) in which SO3 is bound to the arene leaving a positive charge on the arene, actually does not exist. Instead, modern computational research, as well as kinetics studies that are much older, indicate a termolecular pathway. Typically, three molecules are involved. In concentrated SO3 or oleum, two molecules of SO3 form a transition state with the arene... it is a concerted mechanism. In sulfuric acid, the termolecular complex involves the arene, sulfuric acid, and SO3. In fact, sulfonations with higher SO3 concentrations (e.g. oleum) actually don't produce the sulfonic acid product; they first produce sulfonic anhydrides which are later hydrolyzed during work up.

For whoever downvoted me, go look up the various papers by Cerfontain for the 2nd order in SO3 data, and look up various recent computation papers (Shi, Computational Theoretical Chemistry, 2017, 1112, 111; or, Schaefer, 2016, Acc. Chem Res., 49, 1191). You will see that the simplified world of organic chemistry's golden age is unfortunately not perfect or correct.

The mechanism provided for chlorosulfonation in which ClSO2+ is generated... this is very hand wavy. You should at least provide a reference justifying this sort of thing. It's hard to believe that equilibrium (if even real) out-competes the equilibrium of ClSO3H with SO3 and HCl.

Response to Ron: For the chlorosulfonic acid mechanism, the ref your provided is a review book which links to a journal from the Russian Journal of Organic Chem from 1975. I unfortunately can only access the abstract because that year does not appear to be electronically uploaded and I don't have access to the physical copy of the journal. Anyways, the abstract indicates kinetics that are third order in chlorosulfonic acid. This is great and pretty neat, but does not necessarily support the crazy ion-pair mechanism that is proposed. A lot of these crazy old mechanisms from the 'golden era' of organic chemistry (40s to 80s approx) are often wrong. Good data kinetic data was often obtained, but the mechanisms are often later found (often with computational support) to be imprecise.

One of the best examples is SO3, which you have listed above. The mechanism of sulfonation with sulfuric acid varies depending on conditions due to the wide variety of species present (see the ref you listed; the chlorosulfonic acid book). But, no one does sulfonations this way, because you need forcing conditions to drive the reaction to completion; people do them in highly concentrated sulfuric acid or with oleum. Under these conditions SO3 is often the sulfonating agent, or pyrosulfuric acid is, which is what you get when you react sulfuric acid with SO3 (e.g. oleum). Under these realistic conditions, the reaction is typically termolecular. Please read the refs I listed. The intermediate you listed above is actually too far uphill energetically, and modern academics have abandoned it. This is why I am extremely doubtful of your listed chlorosulfonation mechanism, and why the SO3 mechanism you listed is not currently supported.

  • $\begingroup$ Please start reading around p. 11 in the link provided in my answer. It provides data and references supporting the two mechanisms I described. $\endgroup$
    – ron
    Commented Apr 27, 2018 at 17:15
  • $\begingroup$ response was too long to fit in the edit box; please see response in original post above $\endgroup$
    – Bob
    Commented Apr 27, 2018 at 17:39

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