Hemiacetal and Hemiketal formation

[sbook2]

I was just wondering if the hemiacetal and hemiketal formations are Sn2 reactions.

I feel unsure because an alcohol acts as a nucleophile and attacks the carbonyl of the ketone or aldehyde in what reminds me of the back side attack process, but there isn't a leaving group so I am not sure. If it's not an Sn2 reaction (I know - or at least think - it's not an Sn1 reaction) then what kind of reaction is it?

Thanks!

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[aldol16]

It's neither an SN1 nor SN2 reaction. SN1 and SN2 require population of a sigma antibonding orbital so that you break the sigma bond. That's why the leaving group can leave in the first place. In hemiacetal formation, the nucleophile is populating a pi antibonding orbital to break the pi bond. That's why the sigma is still intact and the carbonyl oxygen becomes an alcohol instead of just floating off.

 

[victorias]

It is a nucleophilic addition reaction (you are getting rid of a double bond and a new sigma bond is formed) and OH from alcohol acts as a nucleophile attacking the carbocation.

 

[aldol16]

It is a nucleophilic addition reaction (you are getting rid of a double bond and a new sigma bond is formed) and OH from alcohol acts as a nucleophile attacking the carbocation.

There's no carbocation.

 

[victorias]

In an acid catalyzed reaction, there is resonance stabilization and carbocation form does exist.

 

[aldol16]

In an acid catalyzed reaction, there is resonance stabilization and carbocation form does exist.

That's assuming it's in acid and ignoring the other half of these reactions that occur in base. Based on what OP wrote, there's no reason to suspect that carbocation formation is a certainty - all we know is that a pi bond is broken (via population of a pi antibonding orbital) and a sigma bond forms.

 

[7331poas]

Hemiacetal formation is through a 1-2 addition reaction. The carbonyl reformation is through an E1cb mechanism.

I can go through more detail of necessary.


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