Chapter 8: Nucleophilic Substitution

Preparation and Reaction of Tosylates

Reaction type: Nucleophilic Substitution (usually S_N2)

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 1^o and 2^o ROH (hence S_N2 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, pK_a = -2.8 compared to hydroxide, the conjugate base of water, pK_a = 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.

© Dr. Ian Hunt, Department of Chemistry