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Answer 1 · 2016-09-25T18:26:07

Some esters are good leaving groups.

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

$"Nu"^"-" + "R-OCOR" → "Nu-R" + stackrel("-")("O")"COR"$

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

Some common strong acids are

trifluoromethanesulfonic acid ( $"CF"_3"SO"_3"H", "TfOH"$ ), $color(white)(l)"p"K_a "= -14"$
p-toluenesulfonic acid ( $"CH"_3"C"_6"H"_4"SO"_3"H", "TsOH"$ ), $color(white)(ll)"p"K_a "= -2.8"$
methanesulfonic acid ( $"CH"_3"SO"_3"H", "MsOH"$ ), $color(white)(mmml)"p"K_a "= -2.6"$

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 $"OH"$ group into a better leaving group, convert it to a "triflate", "tosylate", or "mesylate".

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