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  1. Phenyl ethanal or ethanal

I thought phenyl methanal is less reactive towards nucleophilic attack due to presence of bulky group. But answer in my book suggest ethanal is less reactive.

  1. Para-methoxybenzaldehyde vs para-methylbenzaldehyde.

in this case due to the prescence of $\ce{OCH3}$ $\ce{O}$ will act as an electron withdrawing group as well as electron donating (resonace) whereas $\ce{CH3}$ group will only donate it's electron (resonance) thus para-methoxybenzaldehyde should be more reactive. Again, the answer in my book states para-methylbenzaldehyde is more reactive.

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  • $\begingroup$ The resonance effect of the methoxy group (donation of electrons) is more important than its inductive effect and the resonance effect of the methoxy group overwhelms the donation effect of the methyl group. Thus, your book is correct. [BTW: By phenyl methanal do you mean benzaldehyde? Ethanal (acetaldehyde) should be more reactive toward nucleophiles than benzaldehyde owing to resonance contribution from the phenyl ring.] $\endgroup$
    – user55119
    Jun 21, 2019 at 3:16
  • $\begingroup$ i meant phenyl ethanl benzene with ch2-cho $\endgroup$
    – Bhavay
    Jun 21, 2019 at 5:02
  • $\begingroup$ I think you mean phenylacetaldehyde. As to my previous comments, you might look at: chemistry.stackexchange.com/questions/110641/… $\endgroup$
    – user55119
    Jun 22, 2019 at 3:19

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