# Question cbdcf

Sep 25, 2016

Some esters are good leaving groups.

#### Explanation:

If a good nucleophile is a strong base, a good leaving group is a weak base.

Another way of saying it is, A good leaving group is the conjugate base of a strong acid.

Consider the reaction

$\text{Nu"^"-" + "R-OCOR" → "Nu-R" + stackrel("-")("O")"COR}$

The ester of a carboxylic acid is not a good leaving group because the carboxylate ion $\text{RCOO"^"-}$ is the conjugate base of a weak acid.

Some common strong acids are

• trifluoromethanesulfonic acid ($\text{CF"_3"SO"_3"H", "TfOH}$), $\textcolor{w h i t e}{l} \text{p"K_a "= -14}$

• p-toluenesulfonic acid ($\text{CH"_3"C"_6"H"_4"SO"_3"H", "TsOH}$), $\textcolor{w h i t e}{l l} \text{p"K_a "= -2.8}$

• methanesulfonic acid ($\text{CH"_3"SO"_3"H", "MsOH}$), $\textcolor{w h i t e}{m m m l} \text{p"K_a "= -2.6}$

Their esters make good leaving groups.

They also have the advantage that their esters are usually solids and therefore easier to handle.

If you want to convert an $\text{OH}$ group into a better leaving group, convert it to a "triflate", "tosylate", or "mesylate".

"Nu"^"-" + "R-OTs" → "Nu-R" + underbrace(stackrel("-")("O")"Ts")_color(red)("excellent leaving group")#