Question

The compound that does not answer iodoform test is

• A. Ethanal
• B. Acetone
• C. Ethanoic acid
• D. Acetophenone

Hint:

To quickly check for a positive iodoform test, look for the CH\(_3\)CO- group or a CH\(_3\)CH(OH)- group. Remember that carboxylic acids, esters, and amides containing a CH\(_3\)CO- moiety do not give this test.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The iodoform test ($I_{2}/NaOH$) is a specific qualitative test that yields a yellow precipitate of iodoform ($CHI_3$) for compounds containing the methyl ketone group or secondary alcohols oxidizable to it.
Step 2: Key Formula or Approach:
Examine the structure of each compound to identify the presence or absence of the methyl ketone ($-COCH_3$) or methyl carbinol ($-CH(OH)CH_3$) group.
Step 3: Detailed Explanation:
(A) Ethanal ($CH_{3}CHO$): Contains the $CH_{3}-CO-$ group. Gives a positive test.
(B) Acetone ($CH_{3}COCH_{3}$): Contains the $CH_{3}-CO-$ group. Gives a positive test.
(C) Ethanoic acid ($CH_{3}COOH$): Although it has a methyl group adjacent to a carbonyl group, it is a carboxylic acid. The acidic $-OH$ group undergoes rapid acid-base neutralization with the base ($NaOH$) to form a resonance-stabilized carboxylate ion. The delocalization prevents the enolization and halogenation of the adjacent methyl group. Carboxylic acids do not give the iodoform test.
(D) Acetophenone ($C_{6}H_{5}COCH_{3}$): Contains the $CH_{3}-CO-$ group attached to a phenyl ring. Gives a positive test.
Step 4: Final Answer:
Ethanoic acid does not give a positive iodoform test.