[addle]

I was given a problem of intra vs intermolecular aldol rxn and which is faster. I looked up some sites online and texts that point out that intramolecular ones are very fast because entropy is favored. Don't really understand the entropy reason (is it because intermolecular brings two seperate things together making less entropy and so slower than intra??). What I'm thinking is the intermolecular is between other molecules in which it's surrounded in so shouldn't that be faster? Anyone have and idea? Please explain

-Thanks

[movies]

Yeah, you've got the idea.  In one case you start with one molecule and end up with one molecule.  In the other you start with two molecules and end up with one, so entropy decreases. 

You can also think of this problem in terms of kinetics: in the intermolecular case you have two bring two distinct things close together so that they can react.  In the intramolecular reaction they are already relatively close together because the two reacting ends are tethered and can't get very far away from one another.