Question

Which among the following pairs of molecules can be distinguished by haloform reaction?

• A. ${\mathrm{CH}}_3\mathrm{CHO}$ and ${\mathrm{CH}}_3{\mathrm{COCH}}_3$
• B. ${\mathrm{CH}}_3\mathrm{CHO}$ and ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$
• C. ${\mathrm{CH}}_3\mathrm{OH}$ and ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$
• D. $\mathrm{HCHO}$and ${\mathrm{CH}}_3\mathrm{CHO}$

Answer 1

Answer: (C), (D)

$\mathrm{HCHO}$and ${\mathrm{CH}}_3\mathrm{CHO}$

The explanation for correct options

(C) $\mathbf{}{\mathbf{CH}}_{\mathbf{3}}\mathbf{OH}$ and ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$

• Halaform reaction can be used to indicate the presence of methyl ketone.
• Alcohols that have the general formula ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OH}\right)-\mathrm{R}$ will also give a positive haloform test because they can undergo oxidation to give corresponding methyl ketone.
• Here ${\mathrm{CH}}_3\mathrm{OH}$ (methanol) cannot undergo oxidation to form a methyl ketone so it will not give haloform reaction.
• ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$ (ethanol) has the general formula ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OH}\right)-\mathrm{R}$ with $\mathrm{R}$ as $\mathrm{H}$, so can undergo oxidation to give a methyl ketone group. Thus will respond to the haloform reaction.
• Since one compound give a positive test for haloform reaction and the other is negative, we can distinguish both.
• Therefore the option C is correct

(D) $\mathbf{HCHO}$ and ${\mathbf{CH}}_{\mathbf{3}}\mathbf{CHO}$

• $\mathrm{HCHO}$ (formaldehyde) cannot give a positive haloform test since a methyl ketone group is absent.
• ${\mathrm{CH}}_3\mathrm{CHO}$ (acetaldehyde) can give a positive haloform test because it has a methyl keto group.
• Thus option D is correct.

The explanation for the incorrect options

(A) ${\mathrm{CH}}_3\mathrm{CHO}$ and ${\mathrm{CH}}_3{\mathrm{COCH}}_3$

• ${\mathrm{CH}}_3\mathrm{CHO}$ will give a positive haloform test.
• ${\mathrm{CH}}_3{\mathrm{COCH}}_3$ (acetone) will also give a positive haloform test since it contains methyl ketone.
• Both will give a positive test for haloform reaction. Thus these compounds cannot be distinguished by haloform reaction.
• Therefore option A is incorrect.

(B) ${\mathrm{CH}}_3\mathrm{CHO}$ and ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$

• Both ${\mathrm{CH}}_3\mathrm{CHO}$and ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$ will respond to haloform reaction.
• Since both are giving positive haloform tests we can't distinguish between both by this test.
• Thus option B is incorrect.

Therefore the correct options are (C) $\mathbf{}{\mathbf{CH}}_{\mathbf{3}}\mathbf{OH}$, ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$ and (D) $\mathbf{HCHO}$, ${\mathbf{CH}}_{\mathbf{3}}\mathbf{CHO}$.