Question

Why does p-tolualdehyde show a positive test with the iodoform test? I already know the specific aldehydes that show positive tests with iodoform but p-tolualdehyde shouldn't show

Answer 1

Describe iodoform test. What type of compounds will give a positive iodoform test?

Any compounds containing the CH3C=O group or the CH3CH(OH) group give a positive result with the iodoform test. When I2 and NaOH is added to a compound containing one of these groups, a pale yellow precipitate of iodoform (triiodomethane) is formed.

The iodoform test can therefore be used to identify aldehydes and ketones; is the compound is an aldehyde then it must be ethanal (this is the only aldehyde with the CH3C=O group). This occurs as three I atoms replace the H atoms of CH3C=OR, and the C-C bond breaks due to the electron withdrawing effect of the three I atoms (as I is more electronegative than C) forming CHI3 and the salt anion of a carboxylic acid (depending on the R group of the original compound, which influences the length of the carbon chain of the anion RCOO- that is formed).

This test can also be used to identify alcohols; if the alcohol is a tertiary alcohol then it gives no result as it cannot be be oxidised. If the alcohol is a primary alcohol then it must be ethanol (as this is oxidised to ethanal, which is the only aldehyde that gives a positive result with the iodoform test). All secondary alcohols give a positive result, as they are oxidised to ketones.