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In this reaction in a Khan academy video, cyclohexanone reacts with Brady's reagent to form hydrazone product.

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My question is why doea Brady's reagent + cyclohexanone not also form an enamine product where the central, secondary amine group attacks the carbonyl?

My two guesses would be sterics and/or electron withdrawing effects of the ring, but is there another reason? Would this product exist, but minimally?

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Perhaps the nitrogen next to the aromatic ring could attack the carbonyl group. However:

  • The bulky phenyl group is closer to the reaction site, and thus causes more steric interference, than would be the case with the primary nitrogen attack.

  • The nitrogen next to the phenyl ring conjugates its electron pair to the ring just as in a typical aromatic amine, so it is electronically deactivated as a nucleophile. This is especially the case when the ring has strongly electron-withdrawing ligands at other position.

Arylhydrazines are thus much more reactive nucleophiles at the nitrogen that is farther from the aromatic ring.

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