Chapter 8: Nucleophilic Substitution

Preparation and Reaction of Tosylates

Reaction 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.
• Used mostly for 1^o and 2^o ROH (SN2 reaction).
• The -OH reacts first as a nucleophile, attacking the electrophiliccenter of tosylate, displacing a Cl.
• Tosylates have a much better leaving group : the conjugate base of tosicacid, pKa = -2.8
• The advantage of this method is that the substitutions reactions are not under the strongly acidic conditions.
• Tosylates will react with nucleophiles in much the same way as alkyl halides.
• Alternatives to tosylates are mesylates (use CH₃SO₂Cl)and triflates (use 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 atomfrom the original alcohol is retained.
	In the reactions of tosylates, the displaced group is theresonance stabilized anion shown which is a good leaving group.