[Cooper]

Hi,

I know that substitution reactions can't occur at sp2 hybridized carbons, but my textbook says to make esters like this: (see pic. below)

It seems like it'd be an sn2 reaction; the base would deprotonate the alcohol and it would attack the sp2 carbon. But if substitution isn't possible at sp2 carbons, what is the mechanism for this reaction? My book talks about this in the biosynthesis section, do you need enzymes to catalyze the reaction?

Thanks

[Calicum]

I notice that a lot of people seem to think like that at first, it looks like that overall, but it is really a two step process. The carboxylic acid
functions as a nucleophile and attacks thionyl chloride, a chloride ion from the thionyl is expelled as a leaving group, and then a chloride ion
functions as a nucleophile and attacks the carbonyl group. Lastly to reform that carbonyl group a leaving group is expelled which then degrades to give
SO2 gas and a chloride ion.

That second reaction is pretty much the same concept, I guess they use pyridine to stop the formation of HCl since you're probably going to have some chloride ion floating around.

[Cooper]

I notice that a lot of people seem to think like that at first, it looks like that overall, but it is really a two step process. The carboxylic acid
functions as a nucleophile and attacks thionyl chloride, a chloride ion from the thionyl is expelled as a leaving group, and then a chloride ion
functions as a nucleophile and attacks the carbonyl group. Lastly to reform that carbonyl group a leaving group is expelled which then degrades to give
SO2 gas and a chloride ion.

That second reaction is pretty much the same concept, I guess they use pyridine to stop the formation of HCl since you're probably going to have some chloride ion floating around.

Isn't a nucleophilic attack a substitution reaction though?

[fledarmus]

This is usually referred to as an addition-elimination reaction rather than a substitution.