Question

Give a simple chemical test to distinguish between the following pair of compounds:

Benzaldehyde and Acetophenone

Answer 1

Tollen’s Test: Aldehydes respond to Tollen’s test. Benzaldehyde, being an aldehyde, reduces Tollen’s reagent to give a red-brown precipitate of Cu₂O, but acetophenone, being a ketone, does not.

\[\ce{\underset{acetophenone}{C6H5COCH3} + \underset{Hypoiodite}{\underset{Sodium}{3NaOI}}->\underset{Benzoate}{\underset{Sodium}{C6H5COONa}} + \underset{(Yellow ppt)}{\underset{Iodoform}{CHI3}} + 2NaOH}\]

Iodoform test: Acetophenone, being a methyl ketone, undergoes oxidation by sodium hypoiodite (NaOI) to give a yellow ppt. of iodoform. However, benzaldehyde does not respond to this test.

\[\ce{\underset{Acetophenone}{C6H5COCH3} + \underset{hypoiodite}{\underset{Sodium}{3NaOI}}->\underset{benzoate}{\underset{Sodium}{C6H5COONa}} + \underset{(yellow ppt)}{\underset{iodoform}{CHI3}} + 2NaOH}\]

\[\ce{\underset{Benzaldehyde}{C6H5CHO} + NaOI -> No yellow ppt of CHI3}\]