Preparing Alkyl Halides from Alcohols : Reacting Alcohols with Phosphorus Halides
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26-7-2019
1670
Reacting Alcohols with Phosphorus Halides
Alcohols react with liquid phosphorus(III) chloride (also called phosphorus trichloride) to yield chloroalkanes.
3CH3CH2CH2OH+PCl3→3CH3CH2CH2Cl+H3PO3
Alcohols also violently react with solid phosphorus(V) chloride (phosphorus pentachloride) at room temperature, producing clouds of hydrogen chloride gas. While it is not a good approach to make chloroalkanes, it is a good test for the presence of -OH groups. To show that a substance was an alcohol, you would first have to eliminate all the other groups that also react with phosphorus(V) chloride. For example, carboxylic acids (containing the -COOH group) also react with it (because of the -OH in -COOH) as does water (H-OH).
If you have a neutral liquid not contaminated with water, and clouds of hydrogen chloride are produced when you add phosphorus(V) chloride, then you have an alcohol group present.
CH3CH2CH2OH+PCl5→CH3CH2CH2Cl+POCl3+HCl
There are also side reactions involving the POCl3 reacting with the alcohol.
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