Question

Which of the following aldehydes does not form iodoform on heating with $\mathrm{I_2}$ and alkali?

• A. $\mathrm{CH_3-C-CHO}$
• B. $\mathrm{I-CH_2-CHO}$
• C. $\mathrm{CH_3CH_2CHO}$
• D. $\mathrm{CH_3-CH-CHO}$

Hint:

Key points to remember:

• Iodoform test detects methyl ketones ($\mathrm{CH_3-CO-R}$)
• Also positive for ethanol and $\mathrm{CH_3-CH(OH)-R}$
• Propanal and simple aldehydes give negative result
• Yellow $\mathrm{CHI_3}$ precipitate is positive indicator

Answer 1

The Correct Option is C

Step 1: Understand iodoform test
The iodoform test identifies compounds containing: \[ \mathrm{CH_3-C=O} \quad \text{or} \quad \mathrm{CH_3-CH-OH} \] groups that can be oxidized to these structures.
Step 2: Analyze each option
(a) $\mathrm{CH_3-CO-CHO}$ (methyl ketone) - Positive (has $\mathrm{CH_3-CO-}$)
(b) $\mathrm{I-CH_2-CHO}$ - Can form $\mathrm{CHI_3}$ through halogenation
(c) $\mathrm{CH_3CH_2CHO}$ (propanal) - Negative (lacks $\mathrm{CH_3-CO-}$)
(d) $\mathrm{CH_3-CH(OH)-CHO}$ - Can oxidize to $\mathrm{CH_3-CO-}$
Step 3: Reaction mechanism
The test requires: \[ \mathrm{R-CO-CH_3} + 3\mathrm{I_2} + 4\mathrm{OH^-} \rightarrow \mathrm{R-COO^-} + \mathrm{CHI_3} + 3\mathrm{I^-} + 3\mathrm{H_2O} \]
Step 4: Conclusion
Only propanal (option c) lacks the necessary $\mathrm{CH_3-CO-}$ group or oxidizable structure to give iodoform.