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I was taught that, in general, aldehydes are more reactive than ketones because of two reasons: 1) less steric hindrance, and 2) alkyl groups on both sides of carbonyl group in ketones have a stronger +I effect and reduce the partial positive charge on the carbonyl carbon.

Here, even though the second compound is a ketone, it has small alkyl groups which would provide less hindrance than a bulky phenyl group in benzaldehyde. Also, there will be a resonance effect in benzaldehyde reducing the carbonyl carbon's positive charge more(?) than it would be reduced in butanone.

But my teacher said benzaldehyde would be more reactive, because "aldehydes > ketones in reactivity", and he also said that in butanone, electron donating and hindering groups are present on both sides of the carbonyl.

I would like a stronger argument (if there is one) for why the reactivity of benzaldehyde is more; comparing hindrance and electron donating behaviour like this doesn't seem concrete to me.

Any help is appreciated!

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  • $\begingroup$ "there will be a resonance effect in benzaldehyde reducing the carbonyl carbon's positive charge more(?) than it would be reduced in butanone." That's what you got wrong. $\endgroup$
    – Mithoron
    Commented Dec 2, 2023 at 15:24
  • $\begingroup$ @Mithoron so 2 alkyl groups have stronger electron donating effect (+I) than one phenyl ring's +R? $\endgroup$
    – AVS
    Commented Dec 2, 2023 at 15:34
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    $\begingroup$ Phenyl has withdrawing effect. If you drew relevant structure, you'd see it only donates mesomerically to oxygen, not carbon. $\endgroup$
    – Mithoron
    Commented Dec 2, 2023 at 15:58

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