Question

1. You use an ice bath when adding nitric acid to sulfuric acid. Why?

2. You use an ice bath when adding sulfuric acid to methyl benzoate. Why?

Exothermic is NOT the correct answer for 1 or 2.

Answer 1

1.

The process of mixing nitric acid and sulfuric acid is very exothermic, and the heat given off could evaporate nitric acid (commercially available nitric acid has a concentration of 68% in water) which is volatile. The use of an ice bath is to cool down the mixture, and thus to avoid the evaporation of nitric acid.

2.

Methyl benzoate is an aromatic compound and the aromatic rings, being electron rich, reacts with electrophiles. While performing nitration reaction of methyl benzoate, we use sulfuric acid and nitic acid. Aromatic nitrations easily result in "over-nitration" to form dinitrated products and/or decomposition products if the reaction gets out of hand. This problem is lessened if the reaction is done at lower temperature. Nitration reactions are also exothermic reactions, so in the absence of a cold water bath, the increase in temperature could cause the reaction to get out of control.

It may not be required to chill the methylbezoate when adding only H2SO4, but if some impurities are present that could react exothermically, doing the mixing at low temperature would help prevent possible sudden overheating and/or dangerous splattering of hot acid. In fact, whenever concentrated sulfuric acid is used, it is always a good idea to do all mixing in a cold condition and adding the acid dropwise with continuous stirring.