Question

Organic II lab: Fischer Esterification: Preparation of Benzocaine

The protocol for this week’s experiment calls for the adjustment of the pH with sodium carbonate prior extracting the aqueous mixture with diethyl ether.

(a) Why is it necessary to adjust the pH to 8?

(b) Why would it be unwise to use strong bases such as NaOH or KOH in the pH adjustment? Illustrate your thoughts by a chemical reaction

Answer 1

The reaction involved in the preparation of Benzocaine is

The mechanism of the reaction is as follows

At this stage of the reaction we have a charged species which is not organic soluble and the reaction is yet to be completed. This is completed by pulling out a H+ from the final species which is accomplished by a base. So we use a base like sodium carbonate to increae the pH beyond neutralization point of 7 (pH to 8) so that all the species are completely deprotonated.

Here the synthetic methodology says to use sodium carbonate becasue using a strong base like NaOH or KOH can carry out saponification of the ester, wherein the ester will be converted back to the carboxylic acid and will exist as its sodium (or Potassium) salt. so THE Ph IS ALSO ADJUSTED TO 8 AND NOT AQLLOWED TO GO TO higher basic values.

The reaction of saponification is shown as