Question

What is esterifications? How is an ester obtained from alcohol or phenol?

Answer 1

1. Alcohols and phenols form esters by reaction with carboxylic acid, acid halides and acid anhydrides. The reaction between alcohol or phenol with a carboxylic acid to form an ester is called esterification. This is reversible reaction and the formation of an ester is favoured using excess of alcohol in the presence of conc.
   

   \[\begin{array}{cc}
   \phantom{......}\ce{O}\phantom{..................}\ce{O}\\
   \phantom{......}||\phantom{...................}||\\
   \ce{\underset{(alcohol)}{R - OH} + \underset{(acid)}{HO - C} - R' <=>[H⊕] \underset{(ester)}{R - O - C - R'} + H2O}
   \end{array}\]

   \[\begin{array}{cc}
   \phantom{......}\ce{O}\phantom{..................}\ce{O}\\
   \phantom{......}||\phantom{...................}||\\
   \ce{\underset{(phenol)}{Ar - OH} + \underset{(acid)}{HO - C}- R' <=>[H⊕] \underset{(ester)}{Ar - O - C - R'} + H2O}
   \end{array}\]

2. Alcohols and phenols react with acid anhydrides in presence of acid catalyst to form ester.
   \[\begin{array}{cc}
   \ce{O}\phantom{........}\ce{O}\phantom{.............}\ce{O}\phantom{......}\\
   \phantom{}||\phantom{........}||\phantom{..............}||\phantom{......}\\
   \ce{\underset{(alcohol)}{R - OH + R'} \underset{(anhydride)}{- C - O - C - R} <=>[H⊕] \underset{(ester)}{R - C - OR'} + R' - \underset{(acid)}{COOH}}
   \end{array}\]
   \[\begin{array}{cc}
   \phantom{}\ce{O}\phantom{........}\ce{O}\phantom{...................}\ce{O}\phantom{...}\\
   ||\phantom{........}||\phantom{....................}||\phantom{...}\\
   \ce{\underset{(phenol)}{Ar - OH +}R' - \underset{(anhydride)}{C - O - C - }R' <=>[H⊕] \underset{(ester)}{Ar - O - C} - R' + R' - \underset{(acid)}{COOH}}
   \end{array}\]
3. The reaction of alcohol and phenols with acid chloride is carried out in the presence of pyridine (base), which neutralizes HCl.
   \[\begin{array}{cc}
   \phantom{....}\ce{O}\phantom{...................}\ce{O}\phantom{}\\
   \phantom{....}||\phantom{...................}||\\
   \ce{\underset{(alcohol)}{R - OH} + \underset{(acid chloride)}{Cl - C - R ->[pyridine]} R' \underset{(ester)}{- C - OR} + HCl}
   \end{array}\]