Alcohol Reaction With POCl3


Alcohol Reaction With POCl3


Alcohol Reaction With POCl3 Definition

Alcohols are used as an important intermediate in the organic synthesis, because they are easily prepared and easily transformed into other compounds. Phosphoryl chloride is also known as phosphorous oxychloride. When it hydrolyses in moist air, it releases the fumes of hydrogen chloride and phosphoric acid. When it is added to alcohol it forms a O−P bond and cleave the P−Cl which is called as chlorophosphate ester.

Overview of Alcohol Reaction With Pocl3
When alcohols are treated with phosphorous oxychloride, they get converted into alkene. Hydroxide is a poor leaving group. For alcohols to participate in the elimination and substitution reaction, it is necessary to alter the oxygen in such a way that it would be able to stabilize the negative charge which is generated when the C−O bond cleave. It is also possible to convert an alcohol into an alkylphosphate, but OH groups present on the phosphate are acidic and it could interfere with the main reagents which is used for elimination. Therefore, it is compromised to use the reagent phosphoryl chloride, which is a derivative of phosphoric acid. When this reagent is added to alcohol, it forms a new O−P bond and cleave the P−Cl which is called as chlorophosphate ester. This acts as a good leaving group. In the presence of base such as pyridine, this leaving can be easily eliminated and form an alkene.
The alcohol dehydration using phosphorous oxychloride and pyridine in the place of TsO Hand sulfuric acid is a best alternative to conversion of alcohols into alkenes. Consider the mechanism for the conversion of alcohol into alkene using phosphoryl chloride. In this case, the OH should be converted into the good leaving group and phosphoryl chloride which helps in this case by converting it into −OPOCl2 similar to what acid does in the acid catalyzed dehydration. Once the hydroxyl group is converted into the good leaving group, the base (pyridine) eliminates the beta proton which gives the electrons for producing the  bond. This elimination of phosphoryl oxychloride also works for primary, secondary and tertiary alcohols.
Reactions of alcohols with Phosphoryl Chloride:

The E elimination of the tertiary alcohols under the non acidic conditions can be accomplished by the treatment of the phosphoryl chloride in pyridine. This protocol also works with the hindered secondary alcohols, but for the unhindered primary alcohols an SN2 chloride ion substitution of the intermediate called as chlorophosphate competes with the elimination. Most of the non-Zaitsev product was presumed because of the steric hindrance of the methylene group hydrogen atom that interferes with the approach of the base at that site. In the case of secondary alcohols, the use of the reagent phosphoryl oxychloride works well, since  substitution which is retarded by the steric hindrance. Dehydrogenation of alcohols is an elimination reaction. The rates of the reaction differ for primary, secondary and tertiary alcohols. Such variation of the rate can be explained with the stability of the carbocation formed. The carbocation will be more stable in the case of alcohols and hence the rate of dehydration is greater for the tertiary alcohols compared to primary and secondary alcohol. This dehydrogenation process is a highly endothermic reaction, and as such, there is a restricted reaction to the equilibrium. The dehydration of alcohols follows E1 or E2 mechanism. The elimination reaction in the primary alcohols follows E mechanism whereas the elimination in secondary and tertiary alcohols follows E1 mechanism.

Reaction of phosphoryl oxychloride with primary alcohol:

Primary alcohols on reaction with phosphoryl oxychloride in the presence of pyridine gives alkene.

Reactions of phosphoryl oxychloride with secondary alcohol:

When secondary alcohols react with phosphoryl oxychloride, it is converted into alkene. In this case, Zaitsev’s rule is applied. Trans-alkenes are preferred over cis-alkenes.

Reactions of Phosphoryl oxychloride with tertiary alcohols:

The tertiary alcohols can be converted into alkenes when it reacts with phosphoryl oxychloride in the presence of pyridine.

Advantage of using Phosphoryl oxychloride reagent:

The main advantage of using Phosphoryl oxychloride is the E2 mechanism. The bimolecular reactions includes E2 and SN were always preferred because they don’t generate the carbocations which means no rearrangement occurs. For example, the usage of the sulfuric acid for the dehydration of the alcohols leads to the rearrangement producing an alkene which might not have a desired product. However the phosphoryl oxychloride prevents such rearrangement and the major product of the reaction is the alkene which is expected according to Zaitsev’s rule. The elimination of alcohols can also be achieved by the conversion of alcohol into the alkyl halide by utilizing sulphonyl chloride, phosphorous tribromide, and hydrogen halide acid and reacting it with a strong hindered base. The advantage of the Phosphoryl oxychloride compared to the other methods is that it is timesaving, because transformation can be achieved in only one step. When Hoffman’s product is the desired alkene, then the alcohol elimination may be achieved by the conversion of alcohol to an alkyl sulfonate such as tosylates or mesylate followed by the treatment with strong base. The reagent phosphoryl oxychloride works for most of the alcohols which allows to perform the reaction in mild condition and no rearrangement occurs because the reaction goes by E2 which is a great technique.

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