For the mechanisms below, why is it that the base reaction of H2O in number 70 r
ID: 478031 • Letter: F
Question
For the mechanisms below, why is it that the base reaction of H2O in number 70 removes a hydrogen from the molecule while the base reaction of -Br in number 71 reacts by attaching itself to the molecule? They are both weak bases so shouldnt they react in a similar fashion?
70. Propose a mechanism for the following reaction: CH CH HOH H SO CH CHs 70, CH OH IPHOH H20 CH1 CH H H20 71. What product would be formed if the four-membered ring alcohol in Problem 70 were heated with an equivalent amount of HBr rather than with a catalytic amount of H2SO4? CH3 CH, CH3 CHy TCH H OH CHOH CH, CH3 Br BrExplanation / Answer
The answer is that water acts as a base in acidic solution thus abstracts a proton. Whereas, Br- which is a conjugate base of HBr is a strong nucleophile hence, will donate its lone pair to the electrophilic carbon to form a covalent bond.
It is important to note that the conjugate base is always a better nucleophile, for example if we had HO- (conjugate base of water) , which is a better nucleophile instead of H2O, the reaction number 70 would have the similar fate as 71.