What are tosylates used for?

1 Answer
anor277
Oct 30, 2017

Just to retire this question, #"tosylates"# are the leaving group par excellence.

Explanation:

Substitution of an alcoholic group is especially difficult....

#RCH_2OH + X^(-) rarr RCH_2X + HO^-#

.....and thus owes to the strength of the #C-O# bond....and the basicity of the hydroxide group....

On the other hand, with #"p-tosyl choride"#, #"p-toluenesulfonyl chloride"#, #H_3C-C_6H_4SO_2Cl# you can form an inorganic ester with alcohols according to the following reaction....

#p-C_7H_7SO_2Cl+ROHstackrel("pyridine")rarrp-C_7H_7SO_2R+C_6H_5N*HCl#

The reaction is not very convenient in that you need fairly dry solvents, and tosyl chloride has a sour, buttery smell that your lab neighbours WILL complain of bitterly (and of course both pyridines and tertiary amines smell pretty rank), but once you have connected it to your organic radical you have a leaving group that can be facilely substituted by nucleophiles....say here azides...

#p-C_7H_7SO_2R+N_3^(-) rarr R-N_3 + p-C_7H_7SO_2^(-)#

And even moderate nucleophiles can displace the tosyl group....i.e. iodides and bromides.....

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