[laurag]

I'm looking at the differnet mechanisms by which base catalysed carbamate hydrolysis takes place. I know there are two mechanisms, the E1cB and the BAC2 and that the E1cB goes via an anionic intermediate and that the BAC2 mechanism goes via a tetrahedral intermediate and tends to only occur for carbamates without an N-H.
I've read that the E1cB mechanism is much faster than the BAC2 mechanism but I'm just not sure why. Why is the anionic intermediate favoured over the tetrahedral intermediate?

Thanks

[pacifyer]

Primary carbamates (R1 or R2 = H) undergo hydrolysis via an E1cB type mechanism much faster than secondary carbamates, which hydrolyze via a BBAC2 mechanism.
The consequences are pretty different rates for both types...

Take a look at this:
http://web.viu.ca/krogh/chem331/HYDROLYSIS%202006.pdf

Hope it helps...

[laurag]

Thanks.
I just don't understand why the E1cB mechanism is faster? Is it just that its easier to remove the H than it is for the -OH to attack the carbonyl?