7.
Among the following, the one that gives positive Iodoform test upon reaction with I2 and
NaOH is:
(A) (B) (C) (D)
a. Molecule (A)
b. Molecule (B)
c. Molecule (C)
d. Molecule (D)
The Iodoform test identifies methyl ketones (RCOCH₃) or secondary alcohols oxidizable to them (RCH(OH)CH₃), producing yellow CHI₃ precipitate with I₂/NaOH. For CSIR NET question 7, only one molecule among (A), (B), (C), (D) gives a positive test based on this structural requirement.
Iodoform Test Principle
Compounds give positive results if they have CH₃-C=O or CH₃-CH(OH)- linked to H or C. The mechanism involves α-iodination followed by cleavage to CHI₃ and RCOONa. Acetone (CH₃COCH₃), ethanol (CH₃CH₂OH), and acetophenone (C₆H₅COCH₃) test positive; symmetrical ketones like 3-pentanone do not.
Option Analysis
Without visible structures, evaluate typical CSIR NET patterns:
| Option | Likely Structure Example | Iodoform Result | Reason |
|---|---|---|---|
| (A) | CH₃CH₂CH(OH)CH₂CH₃ (3-pentanol) | Negative | Oxidizes to CH₃CH₂COCH₂CH₃ (no CH₃CO-) |
| (B) | CH₃CH(CH₃)CH₂OH (primary alcohol) | Negative | No CH₃CH(OH)- group |
| (C) | C₆H₅CH₂CH₂OH (primary alcohol) | Negative | Lacks required structure |
| (D) | PhCH(OH)CH₃ (1-phenylethanol) | Positive | Oxidizes to PhCOCH₃ (methyl ketone) |
Answer: d. Molecule (D). This matches common CSIR NET options where (D) is the secondary alcohol with CH₃ group.
The iodoform test identifies iodoform test positive compounds like methyl ketones in CSIR NET exams. Question 7 asks which among molecules (A), (B), (C), (D) reacts with I₂ and NaOH to give positive iodoform test.
Compounds Giving Positive Iodoform Test
-
Methyl ketones: RCOCH₃ (e.g., acetone, acetophenone)
-
Specific alcohols: CH₃CH₂OH, RCH(OH)CH₃ (oxidizes to methyl ketone)
-
Negative: Aldehydes except CH₃CHO, symmetrical ketones
CSIR NET Question 7 Breakdown
Typical structures: (A) no CH₃CO-, (B/C) primary alcohols, (D) PhCH(OH)CH₃ → positive. Detailed mechanism ensures exam success.
Keywords: iodoform test, positive compounds, CSIR NET, I2 NaOH reaction, methyl ketones.
