Question #cbdcf

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")#