Question

8.2 Rank the following solvents in order of increasing polarity: dichloromethane, acetic acid, methanol, and diethyl ether. (Hint: refer to Appendices 1 and 3). 8.3 Rank the following compounds in the order in which they would elute off an alumina column: benzoic acid, benzene, benzaldehyde, and benzyl alcohol. S

Answer 1

8.2.

Acetic acid is most polar.

Methanol is more polar than diethyl ether .

Diethyl ether is more polar than dichloromethane.

Hence, increasing order of polarity

Dichoroethane < diethyl ether < methanol < acetic acid.

8.3

Alumina is polar, hence polar substance adsorbed more strongly on the stationary phase and elute slowly. Alumina is basic so it retained the benzoic acid most strongly .

Polarity order is

Benzene < Benzaldehyde < Benzyl alcohol < Benzoic acid.

Therefore Benzene elutes first and Benzoic acid elutes last.

So, time taken to elute off in alumina column is

Benzene < Benzaldehyde < Benzyl alcohol < Benzoic acid .