Help with Ochem question

[LetsGetSocial]

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Hello! Can anyone help me with this reaction? I understand the second step that Br- ion is coming to attack by Sn2, but what is the pyridine doing here?

Thank you!

 

[Frank22]

Thionyl chloride, SOCl2, converts a hydroxyl group to a chloride group, which almost always acts as a leaving group in the next step. Pyridine is often added as a solvent with PBr3, PCl3, and SOCl2 when we wish to halogenate an alcohol because it is poorly nucleophilic but sufficiently basic to deprotonate an intermediate in the reaction and therefore prevent it from reverting to reactants (i.e., pyridine increases yields).

In the first step, a chloride group replaces the hydroxyl group in trans-4-tert-butylphenol. In the second step, the bromine ion in lithium bromide attacks and substitutes the chloride group, thereby generating 1-bromo-4-tert-butylbenzene.

 

[LetsGetSocial]

Thank you so much for the great explanation!

 

[rapidy]

Is there ever a case where the chloride won't function as the best leaving group?

 

[Frank22]

Is there ever a case where the chloride won't function as the best leaving group?

The bromine and iodine ions are superior leaving groups than the chlorine ion because their greater ionic radii allows them to distribute their negative charge across a greater surface area. This is also why fluorine is a poor leaving group, despite the fact that it's the most electronegative element thus far known.