Question

Hello everyone! I am sorry in advance for so many questions.

There is experimental procedure for benzocaine synthesis from p-aminobenzoic acid: 12 ml of ethanol and 0.9 g of p-aminobenzoic acid was added and stirred until dissolving after 1 ml of conc. H₂SO₄ was added. Then it was heated under reflux for 40 minutes and cooled, next the mixture was transferred into 250 ml flask with 30 g of ice and 30 ml of water and 10 ml of 10% sodium bicarbonate solution was added and pH of 8 was reached by adding small amounts of sodium bicarbonate. Precipitate was collected by vacuum filtration and after washed with 3x10 ml of cold distilled water and then product was dried by vacuum filtration then yield and melting point were recorded.

Yield that I recorded - 59 %

Melting point that I recorded - 79-81

Literature value of melting point of benzocaine - 88-90

So my questions are:

1) How does my own melting point compare to the literature values. What does it tell about the identity and purity of synthesized compound? Please describe a way that could be used for further purification of compound.

2) The yield of the experiment is 59% Why it is not 100%?

Answer 1

1) As per your procedure there is very little chance to have impurity since staring material at PH of 8 and ethanol and H2SO4 all are water soluble and product is precipitated from water. The melting point difference is may be your product is not completely dried. The compound may have traces water. So dried completely. If you have TLC please check for purity. If any impurities are there then recrystalize compound in water.

2) All most majority of reactions 100% Yield is not possible. How ever this reaction can get more than 90% yield. 59% yield is due to the reaction is not completed. The un reacted starting material is in the water layer at basic PH it is water soluble. And second reson is you are not distil the ethanol from reaction mixture. Some amount of product went into water along ethanol.