Reactions of Alcohols

POCl3 for Dehydration of Alcohols

Dehydration of alcohols using phosphorus oxychloride (POCl₃) and pyridine (an amine base) in place of H₂SO₄ or TsOH is a good alternative for converting alcohols to alkenes when working with compounds that decompose in the presence of strong acids:

Let’s compare the mechanisms of acid-catalyzed dehydration and the elimination using POCl₃. In both reactions, the OH needs to be converted into a good leaving group, and POCl₃ helps in this by converting it into ^–OPOCl_2, much like the strong acid does in acid-catalyzed dehydration:

Once the hydroxyl is converted into a good leaving group, pyridine removes a β-proton, which provides the electrons for making the C=C π bond.

The POCl₃ elimination works for primary, secondary, and tertiary alcohols.

Notice that the elimination involves the base and the substrate, and all the bonds are being broken and formed at the same time.

You may recognize that this is an E2 mechanism, which is another advantage of using POCl₃ over acid-catalyzed dehydration.

What do you think this advantage might be?

Remember, bimolecular reactions such as E2 and S_N2 are always preferred since they do not form carbocations, which means no rearrangements can occur.

For example, using sulfuric acid for the dehydration of the following alcohol results in a rearrangement forming an alkene that might not have been the desired product:

However, POCl₃ prevents this, and the major product of the reaction is the alkene that is expected according to the Zaitsev’s rule.

The elimination of alcohols can also be achieved by converting the alcohol into an alkyl halide using either a hydrohalic acid (HCl, HBr, and HI), or SOCl₂ or PBr₃, and reacting it with a strong non-hindered base:

One advantage of the POCl₃ compared to these methods is the saving of time since the transformation is achieved in one step.

If the Hofmann product is the desired alkene, the alcohol can first be converted into a mesylate or tosylate, then treated with a strong, bulky base such as potassium tert-butoxide (tBuOK) to drive the elimination.

In summary, POCl₃ works for most alcohols, allows to carry out the reaction in mild conditions, and no rearrangements occur since the reaction goes by the E2 mechanism according to Zaitsev’s rule. A great technique!

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Alcohols Quiz – Naming, Preparation, and Reactions

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Practice

The following practice problems start with alcohol conversion reactions. The second part involves alkene reactions, so feel free to try those as well if you’ve already covered alkenes in class.

Predict the product(s) that are formed after each step for reactions A-G. In each case, consider the formation of any chiral center(s) and draw all expected stereoisomers.