Question

The following NMR spectrum was obtained for a compound with the chemical formula C3H7Br. Propose a reasonable structure for the compound. Identify all of the resonances in the 1H NMR spectrum and explain your reasoning.

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Answer 1

The structure of the compound is CH3-CH2-CH2-Br

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1. The first peak at around 1.0 ppm is a triplet, it indicates that neighboring carbons have 2 protons (N+1 splitting). The peak position at 1.0 ppm indicates that it is attached only to C atom and not to any electron withdrawing functional group (If that is the case it will get de-shielded and peak will be at higher ppm).

2. The second peak around 1.9 ppm is a sextet, it indicates that neighboring carbons have 5 protons. Which is only possible if the C is attached to two Carbon atoms on either side (maximum H on a C-atom is 3). Which is also confirmed by slightly more de-shielding than the first peak (which was only attached to single C atom). Hence the functional group will be CH3-[CH2]-CH2-

3. The third peak around 3.4 ppm is a triplet, it indicates that neighboring carbons have 2 protons. The peak position at 3.4 ppm indicates that it is attached to an electron withdrawing functional group, hence high de-shielding compared to other C-atom. From formula -Br is present in the compound, hence it can be confirmed the presence of Br on the C atom.

From the above observation the structure, consists can be confirmed to be CH3 - CH2 - CH2-Br