Study the NMR spectrum of 1-bromobutane (C4H9Br) (Figure 20).
1H-NMR (300 MHz): δ 0.92 (3H, t, J = 6 Hz, –Me), 1.45 (2H, h, J = 6Hz, CH2), 1.85 (2H, q, J = 6Hz, CH2), 3.41 (2H, t, J = 6Hz, CH2Br) ppm.
13C-NMR (75.43 MHz): δ 13, 22, 31, 53 ppm.
The Correct Answer and Explanation is:
The NMR spectra provided—both ¹H-NMR and ¹³C-NMR—are consistent with the structure of 1-bromobutane (CH₃CH₂CH₂CH₂Br). Let’s analyze each in detail to support the structure.
¹H-NMR (300 MHz)
Chemical shifts (δ in ppm) and splitting patterns:
- 0.92 ppm (3H, triplet, J = 6 Hz)
- This is a methyl group (–CH₃) attached to a CH₂ group.
- The triplet arises due to coupling with the two neighboring protons on the adjacent –CH₂–.
- This corresponds to the terminal methyl group in the chain.
- 1.45 ppm (2H, multiplet – appears as a hextet, J = 6 Hz)
- A –CH₂– group next to both a CH₂ and a CH₃.
- The multiplicity suggests coupling with five equivalent protons (two on one side, three on the other).
- This is the penultimate –CH₂– next to the terminal methyl.
- 1.85 ppm (2H, quartet, J = 6 Hz)
- A –CH₂– group adjacent to the CH₂Br group.
- Coupling likely comes from the neighboring CH₂ (middle of chain).
- Shifted downfield slightly due to the proximity of the electronegative Br (via the next carbon).
- 3.41 ppm (2H, triplet, J = 6 Hz)
- This is the –CH₂Br group.
- The deshielded shift (downfield) is due to the electronegative bromine.
- Triplet due to coupling with the adjacent CH₂.
¹³C-NMR (75.43 MHz)
Chemical shifts:
- 13 ppm – methyl carbon (–CH₃)
- 22 ppm – CH₂ next to CH₃
- 31 ppm – middle CH₂
- 53 ppm – CH₂ bonded to Br (most deshielded)
Conclusion (Correct Answer):
The ¹H-NMR and ¹³C-NMR spectra fully support the structure of 1-bromobutane. Each hydrogen and carbon environment is distinct and corresponds with expected chemical shifts and splitting due to neighboring protons. The NMR data confirms the linear structure of 1-bromobutane with a terminal bromine on a four-carbon chain.
