[AlphaScent]

Why does DMAP (4-dimethylaminopyridine) help so much in the acetylation of a secondary alcohol?

I do the acetylation of alcohols usually neat. 

1.5 eq. pyridne, the alcohol, cool to 0 deg and add the acetic anhydride dropwise.  After about 2 hours the reaction is done.  This is the case for primary alcohols.

Today I am doing a secondary alcohol and know that a catalyst, like DMAP, is usually used.  I have looked at both mechanisms and am wondering what it is actually helping with?  Is it a thermo issue?  Sterics?

[discodermolide]

DMAP reacts with the acylating agent activating it towards attack by perhaps sterically challenged nucleophiles.
So you get an acyl-DMAP+ species which is the active acylating agent.

[AlphaScent]

@Disco,

Yea I see the mechanism, I guess it is just "activated".  More apt to react toward sterically hindered nucleophiles.  I guess I was over thinking it a bit. 

Cheers