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question

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?

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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.


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

EAS mechanism

The rate-determining step is presumably the first step, where a relatively unstable arenium ion is formed.


The second one is the carbonyl alpha-substitution reaction:

CASR mechanism

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


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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  • $\begingroup$ You should have the Lewis acid $\ce{FeBr3}$ coordinate to the carbonyl oxygen before tautormerising. And the amide group is meta-directing, not ortho/para afaik. Apart from that, this is practically spot-on. $\endgroup$ – Jan Jan 23 '17 at 16:52
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    $\begingroup$ @Jan AFAIK, the amide is meta-directing when the carbon in the amide is attached to the benzene, and ortho*/*para-directing when the nitrogen is attached to the benzene instead... $\endgroup$ – DHMO Jan 23 '17 at 16:55
  • $\begingroup$ @Jan is it alright now? The coordination is amazing. $\endgroup$ – DHMO Jan 23 '17 at 17:20
  • $\begingroup$ Curly arrows in second step are wrong. $\endgroup$ – orthocresol Jan 23 '17 at 17:45
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    $\begingroup$ @orthocresol I have fixed it now. $\endgroup$ – DHMO Jan 24 '17 at 10:18

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