Choose the correct compound for the given IR spectrum. % Transmittance 100 80 60 40 20 0 4000 3500 3000 2500 2000 A) NH
B) Wavenumber (cm
) 1500 1000
The Correct Answer and Explanation is:
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Correct Answer: A
Explanation:
To identify the correct compound, we must analyze the key absorption bands in the provided Infrared (IR) spectrum and match them to the functional groups present in the candidate molecules.
- Analysis of the IR Spectrum:
- N-H Stretching Region (3300-3500 cm⁻¹): The most prominent feature in this spectrum is a pair of sharp, medium-intensity peaks (a doublet) located at approximately 3300 cm⁻¹ and 3400 cm⁻¹. This two-peak pattern is the characteristic signature of the symmetric and asymmetric N-H stretching vibrations of a primary amine (-NH₂). A secondary amine (R₂NH) would show a single peak, and a tertiary amine (R₃N) would show no peaks in this region.
- C-H Stretching Region (2850-3100 cm⁻¹): There are strong, sharp absorption bands just below 3000 cm⁻¹, specifically in the 2850-2960 cm⁻¹ range. These are characteristic of C(sp³)-H stretching vibrations, indicating the presence of a saturated alkyl chain. Importantly, there are no significant peaks above 3000 cm⁻¹, which rules out the presence of C(sp²)-H (alkene or aromatic) or C(sp)-H (alkyne) bonds.
- Bending Region (1500-1650 cm⁻¹): A medium-intensity peak is visible at approximately 1600 cm⁻¹. This corresponds to the N-H bending (scissoring) vibration, which is also characteristic of a primary amine.
- Evaluation of the Candidate Compounds:
- Compound A (Butylamine): This molecule has a primary amine group (-NH₂) and a butyl group (a saturated alkyl chain).
- The primary amine group (-NH₂) would produce the characteristic N-H stretching doublet around 3300-3400 cm⁻¹ and the N-H bending peak around 1600 cm⁻¹.
- The butyl group consists entirely of sp³-hybridized carbons and would produce the C(sp³)-H stretching peaks just below 3000 cm⁻¹.
- All the major features of the spectrum are perfectly consistent with the structure of butylamine.
- Compound B (1-Pentene): This molecule is an alkene.
- It lacks any N-H bonds, so it cannot be responsible for the crucial doublet observed at 3300-3400 cm⁻¹.
- As an alkene, it should exhibit a C(sp²)-H stretching peak above 3000 cm⁻¹, which is absent from the spectrum.
- While it would have a C=C stretch around 1640 cm⁻¹, the overwhelming evidence for a primary amine and the lack of evidence for a vinylic C-H bond definitively rule out 1-pentene.
- Compound A (Butylamine): This molecule has a primary amine group (-NH₂) and a butyl group (a saturated alkyl chain).
Conclusion:
The presence of the two N-H stretching peaks near 3300-3400 cm⁻¹ is conclusive evidence for a primary amine. The C-H stretching peaks confirm an alkyl structure. Therefore, the spectrum corresponds to Compound A, butylamine.
