Question

Propose structures for the two compounds whose¹H NMR spectra are

shown.

(a) C₄H₉Br

(b) C₄H₈CI₂

Short answer

Proton nuclear magnetic resonance (₁H NMR) is an application of NMR spectroscopy and uses hydrogen nuclei to determine the structure of molecules.Every compound has different hydrogens exhibit different values of chemical shift.

Step-by-step solution

1H NMR spectra

Proton nuclear magnetic resonance (₁H NMR) is an application of NMR spectroscopy and uses hydrogen nuclei to determine the structure of molecules.Every compound has different hydrogens exhibit different values of chemical shift.

Step 2: Structure of the compound that fits the hydrogen NMR data

(a) The degree of unsaturation of the compound C₄H₉Br can be given as:

The compound has zero degrees of unsaturation.

The NMR shows three signals:

• 2 H , doublet at 3.31 ppm
• 1H, multiplet at 1.97 ppm.
• 6H, doublet at 1.05 ppm.

The signal at 1.05 ppm corresponds to 6 protons and is split into a doublet. The signal for the six equivalent protons at 1.05 ppm is split into a doublet by the proton C-H unit. The splitting pattern of this signal indicates 2 methyl groups connected to a carbon bearing only one hydrogen atom. Therefore the group is isopropyl group.

The signal at 1.97 ppm corresponds to one proton and is split into a multiplet. This is typical of a C-H proton connected to multiple groups.

The signal at 3.31 ppm corresponds to two protons and is split into a doublet. This is typical of a group connected to a C-H unit.

The structure of the compound can be given as:

Structure of compound (a)

(b) has zero degrees of unsaturation. The NMR shows four signals

• 1 H, multiplet at 4.25 ppm.
• 2 H, signal at 3.72 ppm
• 2H, quartet at 2.13 ppm.
• 3 H, doublet at 1.56 ppm.

The signal at 4.25 ppm corresponds to one proton and is split into a multiplet. This is typical of a C-H proton connected to a halogen group like chlorine and also connected to multiple groups. The proton signal at 4.25 ppm shows that there is diastereotopicity due to the chiral center.

There is diastereotopicity in the compound due to the chiral center. Because of this, -CH₂proton signal at 3.72 ppm are diastereotopic in nature and create different signals. The -CH₂ group is connected to the halogen group and another -CH₂unit. The signal at 2.13 ppm corresponds to 2 protons and is split into a quartet.

This is typical of a -CH₂ group connected to three neighboring protons. The signal at 1.56 ppm corresponds to three protons and is split into a doublet. This is typical of a -CH₃group connected to a C-H unit.

The structure of the compound can be given as:

Structure of compound (b)