Nomeclature and functional groups, #5

Hey Alex,

Just wondering if you could explain to me why the alkene isn't a site
of electrophilic attack.  I know it has a cloud rich in electron
density, but couldn't the extra electrons move to the neighboring bond
to bump off the Br?  So basically why the answer isn't D.

---------------

This question reads:

Which of the functional groups on the following molecule are susceptible to nucleophilic attack?

Looking first at (a) - the Br attached to the secondary carbon is electron-withdrawing, so it renders the carbon at a electrophilic (thus, suscpetible to nucleophilic attack).  For the carbonyl carbon at (c), there is similar logic since the oxygen is electron-withdrawing.  As a side note, carbonyl carbons are the most common electrophile on the MCAT.

For (b) - the main problem with the logic above is that there is no driving force for this sort of reaction to occur.  All reactions in organic chemistry need to have reason to run.  Here, an incoming nucleophile would find it very difficult to be attracted to the reaction site; it has a large electron-rich cloud that will strongly repel any negatively-charged (or even neutral) nucleophile coming in.  Even if the nucleophile were attracted, there is an energy barrier to overcome here.  This reaction would require breaking the double bond, and then undergoing some E2-like reaction mechanism.  Still, any incoming nucleophile would be very strongly repelled from the sp2-hybridized carbons at (b).

Thus, a and c are the reaction sites susceptilbe to nucleophilic attack.