Organic! Benzaldehyde is less reactive than propanal towards nucleophilic addition reaction.

Question

Plz help! Organic chemistry. Benzaldehyde is less reactive than propanal towards nucleophilic substitution reaction. Ppl say its because of the resonance effect on benzaldehyde.. but why cant we put on the fact that phenyl in benzaldehyde is an electron withdrawing group and hence decreases the negative charge on oxygen atom making it less negative and less prone to nucleophilic addition reaction.  Can i have this fact too..?? Or is it wrong?

Answer 1

Try drawing out the resonance structure of benzaldehyde to illustrate why what you are saying is not correct. Push the electrons from the aromatic ring towards oxygen and the resonance structure you get will put more negative charge on oxygen.

 

You kind of want to think about this problem in the opposite way. The aldehyde is an electron withdrawing group. It pulls electron density from the aromatic ring and makes the aldehyde more electron rich. A good electrophile should be less electron rich. Since propanal does not have this resonance effect where negative charge is donated into the carbonyl, it is more electrophilic and more reactive towards nucleophilic addition.

Answer 2

Look at it from the perspective of stereoelectronics. 

 

Disha - you are accusing a person who is asking a question of being incorrect. That is no way to answer a question! That's the thrust of the question...he's asking if he is incorrect. But then you keep going and are actually incorrect.

 

You are incorrect in saying that the R- effect increases the dipole moment.  The formyl group on benzene has a constant amount of R- and constant I- (it is not a dynamic process in the way undergraduates tend to like to think it). Those are terms (EDG and EWG groups) used when, as you said there are changes in the way that additional groups are added but not to the carbonyl carbon! To the ring itself, a different reaction entirely. You are using one phenomenon relevant to one reaction SeAr (electrophilic aromatic substitution) and applying it to this question about nucleophilic addition at the carbonyl. 

 

If anything, the benzene ring distributes the constant (unchanging) amount of inductive and resonance withdrawal of oxygen. (Weakens it relative to an aliphatic aldehyde). This is true of even propenal (the α,β-unsaturated version of propanal). The positive charge on carbon is distributed to the β carbon, the larger coefficient of the LUMO is at the Michael position depending on the nature of the HOMO of the nucleophile.   

 

The stereoelectronic effect is exactly as the question-asker stated, except that he used the wrong term (electron withdrawing group). Substitute his term "electron-withdrawing-group" with "resonance" or "extended conjugation" and he is not incorrect. 

Compare the molecular orbital coefficient on the LUMO of the carbonyl carbon of benzaldehyde (bit.ly/1MfbXHR) compared to its counterpart on propanal and you will realize that 1) Risha you are mistaken about charge distribution and its relevance and 2) the reason which explains this observation is:

 

Benzaldehyde is less reactive because it decreases the amount of conjugation in the pi-system overall. The perturbation of molecular orbitals in the presence of the formyl group is counter to what you are saying.

 

But it is the point the question-asker, Nihal, it sounds to me, was making in the first place. 

  

Nihal, you are correct to think in those terms. 

Answer 3

Hello Nihal,

Your statement " phenyl in benzaldehyde is an electron withdrawing group" is incorrect.

 

Electron withdrawing and donating groups are the substituent groups of benzene (the phenyl ring). Groups with -C=O will pull electrons from the ring towards itself (-R effect) and the overall dipole moment will be towards O. Hence this group C=O will be more electron rich (less electrophilic)  than C=O in an aliphatic aldehyde, with less Nu reaction in the former.