# Why is acylation giving the major product compared to alkylation in the reaction between benzene and chloroacetyl chloride?

Why are we getting this products as major and the other as minor ?

According to my knowledge, the carbocation formed on the $$\alpha$$-carbon of the carbonyl chloride is more stable than the carbocation formed on the doubly bonded carbon.

Shouldn't the major and minor product be reversed ?

• Then your knowledge is incorrect - acyl cations are way more stabilised then alkyl. Jun 3 at 16:00
• @Mithoron how ?? the carbocation is formed on doubly bonded carbon. so how is it stable ? Jun 4 at 8:39
• Did you bother you read Safdar's answer? Not the "not an answer" you marked as accepted, which missed the point entirely... Jun 4 at 12:45
• @Mithoron i never saw the edit. now it is better and does explain the concept Jun 5 at 5:28

## 2 Answers

This can be explained by comparing the stability of the carbocations formed in Friedel-Crafts acylation and Friedel-Crafts alkylation using chloroacetylchloride. This is because the rate determining step in both reactions is the formation of the carbocation.

As you can see, the acyl carbocation is resonance stabilized, whereas the carbocation formed in the alkylation process is very weakly resonance stabilized.[1]

According to supporting information to J. Chem. Educ. 2012, 89 (1), 147–149[2]

This o-toluidine selective attack at the acyl carbon atom of the bifunctional reagent is illustrative of the significantly greater reactivity of nucleophiles toward acylation, compared to alkylation

Therefore the acylation gives you the major product compared to the Friedel-Crafts alkylation product.

I was able to find a reference to your a similar reaction in J Flow Chem 2019, 9 (1), 35–42 with difluorobenzene instead of benzene.

A series of experiments were performed reacting a solution of 1,3-difluorobenzene (6) and aluminum trichloride ($$\ce{AlCl3}$$) in nitromethane [...] however increasing the temperature and time increased conversion to $$22–74\%$$ (entries 2–6). At $$\pu{90 °C}$$ (entry 5) the product was accompanied by the 2-acylated regioisomer. To further improve the reaction, the amounts of chloroacetyl chloride (7) and $$\ce{AlCl3}$$ were increased.These modifications gave the highest conversion, $$79\%$$ conversion [...]

$$\begin{array}{c|ccc} \hline \bf{Entry} & \bf{Residence\,time\,(min)}& \bf{Temp\,(^\circ C)} & \bf{Conversion\,(\%)} \\ \hline 1 & 5 & 50 & 0 \\ 2 & 5 & 60 & 22 \\ 3 & 10 & 60 & 45 \\ 4 & 15 & 60 & 46 \\ 5 & 15 & 90 & 77 \\ 6 & 25 & 70 & 74 \\ 7 & 25 & 80 & 77 \\ 8 & 25 & 80 & 79 \\ \hline \end{array}$$

Another paper that talks about the acetylation of benzene and toluene is J. Am. Chem. Soc. 1971, 93 (25), 6964–6967 which includes multiple reagents including chloroacetyl chloride.

### Reference:

1. Creary, X. Stabilization of α-keto cations by carbonyl conjugation. J. Am. Chem. Soc. 1981, 103 (9), 2463–2465. DOI: 10.1021/ja00399a078.
2. Demare, P.; Regla, I. Synthesis of Two Local Anesthetics from Toluene: An Organic Multistep Synthesis in a Project-Oriented Laboratory Course. J. Chem. Educ. 2012, 89 (1), 147–149. DOI: 10.1021/ed100838a.

No the products shouldn’t be reversed as, to put simply, acylation is preferred over alkylation.

This is a conclusion based on various experimental results and there are many reasons that can be given for the same.

As said in the concluding lines of this article from Chemistry Libretexts,

Friedel-Crafts Acylations offer several synthetic advantages over Friedel-Crafts Alkylation. These advantages provide greater control over the production of reaction products

I looked up online but couldn’t find any relevant reason particular to the acylation of chloroacetyl chloride, as asked in your question. But I did find some explanations to why acylation is preferred over alkylation:

1. In friedel crafts aklylation there is a chance of polyalkyation. Because the alkyl group increases benzene ring electron density and ring becomes activated to another elelctrophilic substitution.... thus polyalkyation occurs….But in Fridel crafts acylation there is no chance of polyacylation because the acyl group (RC=O) decreases benzene ring electron density and henceforth the second acylation does not take place.

2. During acylation reaction the carbon chain of the acyl halide does not rearrange and hence there's no possibility of formation of Isomeric ketone compound and thus 100% product yield is obtained in acylation reaction.

This is an answer to a question asked on Quora.

Again, these are all just explanations given to explain the experimental results and might not be conclusive or convincing enough.

• Sorry for a misunderstanding. "This answer doesn't explain why acylation takes place over alkylation but rather that the product of acylation is preferred compared to the products generated by alkylation when each reaction takes place in isolation."No the products shouldn’t be reversed as, to put simply, acylation is preferred over alkylation", what is the proof for this statement? Jun 3 at 8:15