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I have been asked to find the major product of the following reaction: enter image description here

Clearly, there are two possible sites for the lone pair of $\ce{NH_3}$ to attack, and thus two possible products.

enter image description here The carbon that is further from the $\ce{-Me}$ should be more electrophilic than the one that is closer to the methyl group (due to the positive inductive effect of the methyl group). With that logic, the product $(i)$ should be the major product. But, according to the book, product $(ii)$ will be the product formed.

Why is product $(ii)$ the major product?

How do you determine which one will be the major product in this kind of situations?

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1 Answer 1

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This would be more obvious if you had a strongly electron-withdrawing group (e.g. nitro or CF3) instead of the mildly electron-donating Me. In that case it would be clear that the carbonyl para to the EWG group would have electron density drawn away from the C, increasing the polarity of the C=O bond and making it more reactive to nucleophiles.

Instead we have a mild electron donation from the Me which makes the C=O bond slightly less polarised and reducing the electrophilicity. Thus the meta C=O, which sees less effect from the Me, is slightly more reactive and NH3 attack there will be the major product (though not by much).

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  • $\begingroup$ One thing I want to ask, when methyl is substituted at the benzene ring, it also shows hyperconjugative effect (basically resonance) , and that hyperconjugation won't operate at the meta position and only at para position increasing electron density there, though inductive effect is there, so how do we decide which effect is dominating? $\endgroup$
    – V.G
    Apr 21, 2021 at 13:01
  • $\begingroup$ You run an experiment, either in the lab to find the actual yields or on the computer to model the electron densities at each site. $\endgroup$
    – Waylander
    Apr 21, 2021 at 13:09
  • $\begingroup$ So is it experimental that hyperconjugation is negligible here? And we really cannot compare between inductive effect and hyperconjugation in such types of compounds using some arguments? $\endgroup$
    – V.G
    Apr 21, 2021 at 13:12
  • $\begingroup$ A mixture at best. $\endgroup$
    – user55119
    Apr 21, 2021 at 16:41

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