Question

9. Carboxylic acids often produce a streak rather than a spot when analyzed on silica gel TLC plates. Adding a drop or two of acetic acid to the eluant reduces streaking. Explain why carboxylic acids may streak on silica gel TLC plates and why adding acetic acid to the eluent reduces streaking. Amines also often streak on silica gel TLC. What might you add to the elu- ant to reduce such streaking? a. b.

Answer 1

a. Carboxylic acids are higly polar compounds and so is silica. The acids have C=O and silica has OH groups which can lead to strong hydrogen bonding. These interactions drag the acid moleule. Adding acetic acid which is also a polar carboxylic acid, will minimize the interactions of silica with the actual carboxylic acid, and hence reduces streaking.

b. Silica gel is mildly acidic and hence can interact with amines by protonating them. Adding a a small amount of amine like triethylamine or ammonia to the eluent will reduce such streaking. As the triethylamine or ammonia added would complete to interact with silica, there by leaving the amine free.