Organic Chemistry
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Leaving Group Conversions - Using HX
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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.
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Why do we need to convert Alcohol into a good leaving group?
We’re aiming to turn alcohols into alkyl halides or sulfonate esters. Let’s explore the different ways to accomplish this.
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).
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Using HX acids via SN1 reaction.
This is the predominant mechanism for strong halohydric acids with 2° and 3° alcohols.
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Using HX acids via SN2 reaction.
This is the predominant mechanism for strong halohydric acids with 1° alcohols.
Complications: Strong HX acids conversions come with three major complications.
- Regiochemistry of products:Carbocation intermediates can rearrange.
- Stereochemistry of products:Racemic products are non-stereo specific.
- Competition of Elimination mechanisms:Some product won’t even be an alkyl halide.
These complications render this method mostly useless. Sorry not sorry.