Anions with the general formula RSO2O− are called sulfonates. They are the conjugate bases of sulfonic acids with formula RSO2OH. As sulfonic acids tend to be strong acids, the corresponding sulfonates are weak bases. Due to the stability of sulfonate anions, the cations of sulfonate salts such as scandium triflate have application as Lewis acids.
A classic organic reaction for the preparation of sulfonates is that of alkyl halides with sulfites such as sodium sulfite, first described by Adolph Strecker in 1868 (Strecker sulfite alkylation). The general reaction is:
- RX + M2SO3 → RSO3M + MX
Esters with the general formula R1SO2OR2 are as a category called sulfonic esters, with individual members of the category being named analogously to how ordinary carboxyl esters are named. For example, if the R2 group is a methyl group and the R1 group is a trifluoromethyl group, the resulting compound is methyl trifluoromethanesulfonate.
Sulfonic esters are used as reagents in organic synthesis, chiefly because the RSO2O- group is a good leaving group in Sn1, Sn2, E1 and E2 reactions. Methyl triflate, for example, is a strong methylating reagent. They are commonly used to lend water solubility to protein crosslinkers such as N-hydroxysulfosuccinimide (Sulfo-NHS), BS3, Sulfo-SMCC, etc.
- Mesylate (methanesulfonate), CH3SO2O-
- Triflate (trifluoromethanesulfonate), CF3SO2O-
- Esylate (ethanesulfonate), C2H5SO2O-
- Besylate (benzenesulfonate), C6H5SO2O-
- Tosylate (p-toluenesulfonate), CH3C6H4SO2O-