Predicting the product of bromination of acetanilide

[![question](https://i.stack.imgur.com/U8jtJ.jpg)](https://i.stack.imgur.com/U8jtJ.jpg)


I figured the answer would be option (B) as the group attached to the benzene ring is *ortho*- and *para*- directing.


I thought $\ce{FeBr3}$ would be used in electrophilic aromatic substitution.


However, the model answer to the question is (D).


Is this correct? If so, what makes it possible? 




This is a classic $\alpha$-bromination of a carbonyl. You can achieve this with an electrophilic source of bromine like in the reaction conditions provided or something like *N*-bromosuccinimide.


The reason you're not getting the EAS product is because that's a much higher energy pathway since you need to temporarily break aromaticity. Given more time, heat, and bromine, you can probably get the ortho- and para- products, but you will still $\alpha$-brominate on the amide *first*.


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There are two competing mechanisms.




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The first one is the electrophilic aromatic substitution which you expected:


[![EAS mechanism](https://i.stack.imgur.com/djoNd.png)](https://i.stack.imgur.com/djoNd.png)


The rate-determining step is presumably the first step, where a relatively unstable [arenium ion](https://en.wikipedia.org/wiki/Arenium_ion) is formed.




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The second one is the [carbonyl alpha-substitution reaction](https://en.wikipedia.org/wiki/Carbonyl_alpha-substitution_reactions):


[![CASR mechanism](https://i.stack.imgur.com/9aBOP.png)](https://i.stack.imgur.com/9aBOP.png)


I failed to identify the rate determining step because of the coordination of the Lewis acid $\ce{FeBr3}$.




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Therefore, the second mechanism occurs at a higher rate compared to the first mechanism, so more 2-bromo-*N*-phenylacetamide is produced per unit time, which makes it the **major product**.


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