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Unformatted text preview: Conversion of Alcohols into Mesylates and Tosylates The hydroxyl group, -OH, is a poor leaving group in nucleophilic substitution reactions because the hydroxide ion is a relatively unstable anion. Nu:- + R-OH slow Nu-R + HO- Sulfonate Esters: Better Leaving Groups It is relatively easy to convert the hydroxyl group into a sulfonate ester that is a much better leaving group. The alcohol is reacted with derivatives of alkyl or aryl sulfonic acids called sulfonyl chlorides. CH 3-S-OH O = O = O = methanesulfonic acid pK a = -7 CH 3 S-OH O = p-toluenesulfonic acid pK a -7 ~ ~ CH 3-S-Cl O = O = methanesulfonyl chloride O = CH 3 S-Cl O = p-toluenesulfonyl chloride The Synthesis of Sulfonate Esters from Alcohols Alcohols react readily with sulfonyl chlorides in the presence of amine bases producing sulfonate esters. R-O-H : : + R'-S-Cl O = O = base (-HCl) R-O- :: S-R' O = O = alkyl sulfonate ester Examples CH 3 CH 2 OH + CH 3-S-Cl O = O = ethanol base (-HCl) CH 3 CH 2 O- S-CH 3 O = O = ethyl methanesulfonate...
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