Question

An organic compound exhibits the [M]+, [M+2]+, and [M+4]+ peaks in the intensity ratio 1:2:1 in the mass spectrum, and shows a singlet at \( \delta \) 7.49 in the \( ^1H \) NMR spectrum in CDCl\(_3\). The compound is:

• A. 1,4-dichlorobenzene
• B. 1,4-dibromobenzene
• C. 1,2-dibromobenzene
• D. 1,2-dichlorobenzene

Hint:

In mass spectrometry, the 1:2:1 intensity ratio is characteristic of bromine due to its two isotopes \( ^{79} \text{Br} \) and \( ^{81} \text{Br} \).

Answer 1

The Correct Option is B

Step 1: Analyze the Mass Spectrum.
The given intensity ratio 1:2:1 for the [M]+, [M+2]+, and [M+4]+ peaks suggests that the compound contains a halogen, which leads to the isotopic pattern due to the presence of isotopes. The 2:1 ratio is characteristic of a bromine atom, as bromine has two naturally occurring isotopes: \( ^{79} \text{Br} \) and \( ^{81} \text{Br} \), which give this ratio in the mass spectrum.

Step 2: Analyze the NMR Spectrum.
A singlet at \( \delta = 7.49 \) in the \( ^1H \) NMR spectrum suggests a symmetrical environment for the protons, which is consistent with the 1,4-substitution pattern in the benzene ring.

Step 3: Conclusion.
Based on the mass spectrum and the NMR singlet, the compound is most likely 1,4-dibromobenzene.

Final Answer: \[ \boxed{\text{1,4-dibromobenzene}} \]