Question

Give a simple chemical test to distinguish between the following pair of compounds ethanal and propanal.

Answer 1

Ethanol and propanol:

1. Both ethanal and propanal are examples of simple aldehyde compounds.
2. They can be differentiated from the iodoform test
3. The iodoform test is used to see if an unknown chemical has carbonyl compounds with the structure $\mathrm{R}/\mathrm{H}-\mathrm{CO}-{\mathrm{CH}}_3$ or alcohols with the structure$\mathrm{R}-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_3$ i.e. should have one methyl group.
4. Ethanol reacts positively to the iodoform test because it possesses one methyl group linked to the carbonyl carbon atom, but propanal does not.

$\underset{\mathrm{Ethanal}}{{\mathrm{CH}}_3\mathrm{CHO}\left(\mathrm{l}\right)}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{4\mathrm{NaOH}\left(\mathrm{l}\right)}+\underset{\mathrm{Elemental}\mathrm{iodine}}{3{\mathrm{I}}_2\left(\mathrm{s}\right)}\to \underset{\mathrm{Iodoform}}{{\mathrm{CHI}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{formate}}{\mathrm{HCOONa}\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{Iodide}}{3\mathrm{NaI}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

$\underset{\mathrm{Propanol}}{{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CHO}\left(\mathrm{l}\right)}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{4\mathrm{NaOH}\left(\mathrm{l}\right)}+\underset{\mathrm{Elemental}\mathrm{Iodine}}{3{\mathrm{I}}_2\left(\mathrm{s}\right)}\to \mathrm{No}\mathrm{ppt}\mathrm{of}{\mathrm{CHI}}_3\mathrm{formed}.$

Therefore, the Iodoform test differentiates between ethanal and propanal.