How can you tell whether a carbonyl will act as a Nu- or an E+?

[alakait]

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Hey guys!

I was wondering if anyone can give me some clarification on how to predict the way a carbonyl will act in a mechanism. I know sometimes the first step of the mechanism is a Nucleophillic attack on the carbonyl carbon (electrophile). However, sometimes the first step is protonation of the carbonyl oxygen (nucleophile). How can you predict which way the carbonyl will act?

Thanks in advance!

 

[azor ahai]

it will pretty much always act as an electrophile. the carbonyl oxygen is only protonated with very strong acid, and even serves to only increase the electrophilicity of the carbonyl carbon making it more reactive.

 

[erythrocyte666]

Hey guys!

I was wondering if anyone can give me some clarification on how to predict the way a carbonyl will act in a mechanism. I know sometimes the first step of the mechanism is a Nucleophillic attack on the carbonyl carbon (electrophile). However, sometimes the first step is protonation of the carbonyl oxygen (nucleophile). How can you predict which way the carbonyl will act?

Thanks in advance!

depends on reaction conditions: acidic conditions you can protonate first followed by attack with neutral nucleophile, basic conditions you should have an anionic nucleophile