[suzanka]

While drawing resonance structures of benzaldehyde , I started wondering - as the carbonyl group causes positive charges at ortho and para position, why don't nucleophiles attack there? What's the reason? Is it the 6π electron delocalisation that will be disturbed?

An additional question - a the pheny, group delocalises the positive charge of the carbonyl atom, do I write the electron effect towards the carbon or towards the benzene ring? Sorry, didn't explain it properly - it's about a problem where I have to compare the reactivity of aldehydes and ketones - I may take a picture of the sheet if it might help.

Thank you

[Dan]

While drawing resonance structures of benzaldehyde , I started wondering - as the carbonyl group causes positive charges at ortho and para position, why don't nucleophiles attack there? What's the reason? Is it the 6π electron delocalisation that will be disturbed?

The energy penalty for breaking aromaticity and the electron richness of the π-system is the reason why it is difficult for for nucleophiles to attack. It can happen though, and is quite common with strongly electron withdrawing groups such as nitro. Look up nucleophilic aromatic substitution (S_NAr).

In the case of benzaldehyde, which carbon do you think is the most electrophilic?

[suzanka]

So, for benzaldehyde, the most electrophilic carbon has to be the carbonyl one as the oxygen has negative inductive effect... but the phenyl group has resonance with this carbon... so it makes the positive charge on the carbonyl C weaker? And thus - less reactive than acetaldehyde?

[Dan]

So, for benzaldehyde, the most electrophilic carbon has to be the carbonyl one as the oxygen has negative inductive effect... but the phenyl group has resonance with this carbon... so it makes the positive charge on the carbonyl C weaker? And thus - less reactive than acetaldehyde?

That's right, and nucleophiles reacting with benzaldehyde will react at the carbonyl carbon more readily than the electrophilic positions on the bnzene ring.

If the carbonyl group is blocked, it is possible to add nucleophiles to the the carbons on the benzene rings of benzaldehyde. This was shown, for example, by Hisashi Yamamoto in 1995 where the use of an exceptionally bulky aluminium Lewis acid catalyst blocked attack at the carbonyl carbon: J. Am. Chem. Soc. 1995, 117, 9091 -9092

[suzanka]

Thank you so much Did my homework now

If the carbonyl group is blocked, it is possible to add nucleophiles to the the carbons on the benzene rings of benzaldehyde. This was shown, for example, by Hisashi Yamamoto in 1995 where the use of an exceptionally bulky aluminium Lewis acid catalyst blocked attack at the carbonyl carbon: J. Am. Chem. Soc. 1995, 117, 9091 -9092

Got the idea about the strength of the electrophilic centres. The article just confirms what our tutor says - in chemistry everything is possible