Leaving Group Conversions

Alcohols are terrible leaving groups. So we’re going to learn an entire class of reagents whose sole job is to convert alcohol into better leaving groups. 

We’re aiming to turn alcohols into alkyl halides or sulfonate esters. Let’s explore the different ways to accomplish this. 

Using HX Acids

The simplest way to theoretically convert alcohols into alkyl halides is just to react them with a strong halohydric acid (HX). There are some complications associated with this conversion however.

Can you predict what they are? (Three major complications given below).  

This is the predominant mechanism for strong halohydric acids with 2° and 3° alcohols. 

This is the predominant mechanism for strong halohydric acids with 1° alcohols.

Complications: Strong HX acids conversions come with three major complications.

  1. Regiochemistry of products: Carbocation intermediates can rearrange.
  2. Stereochemistry of products: Racemic products are non-stereo specific.
  3. Competition of Elimination mechanisms: Some product won’t even be an alkyl halide.   

These complications render this method mostly useless. Sorry not sorry.  

Using SOCl2 or PBr3

Don’t you wish there a method to convert alcohols into alkyl halides without all these complications? Ah, looks like we’ve got just the reagents for you!

These reagents proceed through SN2 mechanisms, so they will only work on 1° and 2° alcohols. That said, we expect inversion of configuration from this conversion. 

Example: Predict the mechanism of PBr3, and draw the final product.   

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Using Sulfonyl Chlorides

Now that we’ve covered alkyl halides, sulfonyl chlorides can be used to convert alcohols into another great leaving group, sulfonate esters.

This conversion proceeds without the use of SN2, meaning that we expect retention of configuration.

Summary of Alcohol Conversions

Here is a chart with the most important info you need to know for each conversion. 

Concept: Comparing and contrasting the Alcohol Conversions.  

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