Alkenes from alcohols [POCl3]

Alkenes from alcohols [POCl3] Definition:


When treated with phosphorus oxychloride (POCl3) alcohols are converted to alkenes via an elimination reaction.



Alkenes from alcohols [POCl3] Explained:


As we know, hydroxide (OH‾) is a very poor leaving group. In order for alcohols to participate in substitution and elimination reactions, it is the best to modify the oxygen to give more stable form, thereby stabilize the negative charge generated when the C-O bond breaks. For this purpose, we used protons to give water which is a good leaving group. But it does not always have to be a proton. Instead of this, we can use the reagent phosphorus oxychloride (POCl3), a derivate of phosphoric acid. This is a useful reagent for direct “dehydration” (elimination) of alcohols to alkenes under non-acidic conditions. When POCl3 is added to an alcohol, we form a new O-P bond which is very strong and break a P-Cl bond to form a chlorophosphate ester. To do all this, we need a good base such as pyridine which even uses as the solvent (excess of pyridine). It works for primary, secondary and tertiary alcohols.


STEP 1. Attack of alcohol on phosphorus oxychloride to form intermediate which is an excellent leaving group (chlorophosphite)



STEP 2. Elimination of chloride



STEP 3. Deprotonation by pyridine



STEP 4. Elimination by pyridine which performs an elimination reaction to give the alkene