72 - Organic Chemistry

Question

Three isomeric compounds, A, B, and C, all have molecular formula $C_8{H}_{11}N$ . The  ${}^{1}H$NMR and IR spectral data of A, B, and C are given below. What are their structures?

Step-by-Step Solution

Answer

Compound A:

Compound B:

Compound C:

H 1 NMR spectrum

The hydrogen atoms present in a compound show the characteristic absorption peaks in a  ${}^{1}H$ NMR spectrum.

The multiplicity of the peaks is determined using the n+1 rule, where n represents the number of neighboring/adjacent protons.

Identifying compound A

The molecular formula of compound A is $C_8{H}_{11}N$  .

The IR absorption at $3400 c{m}^{-1}$  is characteristic of a secondary amine.

NMR data:

1.3 ppm (triplet, 3H): the $C{H}_3$  group adjacent to a $C{H}_2$   group.

3.1 ppm (quartet, 2H): the  $C{H}_2$ group adjacent to a  $C{H}_3$ group.

3.6 ppm (singlet, 1H): the amine hydrogen.

6.8 - 7.2 ppm (multiplet, 5H): the aromatic hydrogens.

Therefore, the given compound is an aromatic amide.

The predicted structure of the compound is

                                        Compound A

Identifying compound B

The molecular formula of compound B is  $C_8{H}_{11}N$.

The IR absorption at  $3310 c{m}^{-1}$ is characteristic of a secondary amine.

NMR data:

1.4 ppm (singlet, 1H): the amine hydrogen. 

The $C{H}_3$  group is adjacent to a $C{H}_2$  group.

2.4 ppm (singlet, 3H): the $C{H}_3$   group connected to the amine group.

3.8 ppm (singlet, 2H): the $C{H}_2$  group. 

7.2 ppm (multiplet, 5H): the aromatic hydrogens.

The predicted structure of the compound is

                               Compound B

Identifying compound C

The molecular formula of compound C is $C_8{H}_{11}N$  .

The IR absorption at  $3430 c{m}^{-1}$ and  $3350 c{m}^{-1}$  is characteristic of a primary amine.

NMR data:

1.3 ppm (triplet, 3H): the  $C{H}_3$ group adjacent to a $C{H}_2$  group.

2.5 ppm (quartet, 2H): the  $C{H}_2$ group adjacent to a  $C{H}_3$ group.

3.6 ppm (singlet, 2H): the amine hydrogens.

6.7 ppm (doublet, 2H): the aromatic hydrogens.

7.0 ppm (doublet, 2H): the aromatic hydrogens.

As two signals correspond to aromatic hydrogens, the compound is para-disubstituted.

The predicted structure of the compound is

                               Compound C