Preparation and Reaction of TosylatesReaction type: Nucleophilic Substitution (usually SN2)
Summary Alcohols can be converted into tosylates using tosyl chloride and a base to "mop-up" the HCl by-product.
Tosylates are good substrates for substitution reactions, reacting with nucleophiles in much the same way as alkyl halides.
The advantage of this method is that the substitutions reactions are not under the strongly acidic conditions.
Used mostly for 1o and 2o ROH (hence SN2 reactions).
The -OH reacts first as a nucleophile, attacking the electrophilic center of tosylate, displacing a chloride ion, Cl-.
Tosylates have a much better leaving group than the original alcohol : the conjugate base of tosic acid, pKa = -2.8 compared to hydroxide, the conjugate base of water, pKa = 15.7.
Alternatives to tosylates are mesylates (using CH3SO2Cl) and triflates (using CF3SO2Cl)
This is the reagent used to prepare the tosylate ester. It maybe referred to by any of the terms shown.
The tosylate ester is shown. Note that the oxygen atom from the original alcohol is retained.
In the reactions of tosylates, the displaced group is the resonance stabilised anion shown, which is a good leaving group.
Note that the preparation of the tosylate is similar to the reaction of an alcohol with thionyl chloride, SOCl2.
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