Q.55. - Organic Chemistry

Question

Question: Propose a structure consistent with each set of data.

a. ${\mathit{C}}_{\mathbf{10}}{\mathit{H}}_{\mathbf{14}}$ : IR absorptions at $\mathbf{3150}\mathbf{-}\mathbf{2850}$, $\mathbf{1600}$ , and $\mathbf{1500}\mathbf{ }\mathit{c}{\mathit{m}}^{\mathbf{-}\mathbf{1}}$ 

b. ${\mathit{C}}_{\mathbf{9}}{\mathit{H}}_{\mathbf{12}}$: ${}^{\mathbf{13}}\mathit{C}$NMR signals at 21, 127, and 138 ppm

c. ${\mathit{C}}_{\mathbf{8}}{\mathit{H}}_{\mathbf{10}}$ : IR absorptions at $\mathbf{3108}\mathbf{-}\mathbf{2875}\mathbf{ }\mathit{c}{\mathit{m}}^{\mathbf{-}\mathbf{1}}$ , 1606, and  $\mathbf{1496}\mathbf{ }\mathit{c}{\mathit{m}}^{\mathbf{-}\mathbf{1}}$

Step-by-Step Solution

Answer

Answer

Step 1: NMR Spectroscopy

The physical as well as chemical properties of a particular molecule can be understood with the help of NMR spectroscopy. There are several kinds of NMR spectroscopy, and the important ones include   ${}^{\mathbf{1}}\mathit{H}$NMR and ${}^{\mathbf{13}}\mathit{C}$ NMR.

Step 2: Predicting the structure from NMR spectra

Several ways can be employed to evaluate the structures from the spectral data. The count of hydrogen atoms and carbon atoms can be acquired from  ${}^{\mathbf{1}}\mathit{H}$NMR and ${}^{\mathbf{13}}\mathit{C}$ NMR, respectively.

 

The functional groups can be predicted from the IR spectra.

Step 3: Proposing the structure from the given data

a. $C_{10}{H}_{14}$ :The IR absorption at $3150-2850 c{m}^{-1}$ is because of the $s{p}^2$ and $s{p}^3$ hybridized C-H. The absorption at $1600$ and $1500 c{m}^{-1}$ is due to the benzene ring.

 

The ${}^{1}H$  NMR data can be given as:

This data indicates the presence of  ${\left(C{H}_3\right)}_{2}CH$ group. 

The location of the two groups on the benzene ring cannot be understood from this data. They are not situated para as the para arrangement gives two sets of distinct peaks resembling two doublets. Hence, the two possible structures can be given as:

Possible structures of spectral data in question a

 

b. $C_9{H}_{12}$: ${}^{13}C$NMR signals at 21, 127, and 138 ppm indicate three different carbon types are present in this compound. 

The ${}^{1}H$  NMR indicates the two types of hydrogens; nine hydrogens indicate three $C{H}_3$ groups,; and the other three hydrogens are very deshielded, so they are bonded to a benzene ring. The possible structure of the compound can be given as:

Possible structure of spectral data in question b

c. $C_8{H}_{10}$: The IR absorption at $3108-2875 c{m}^{-1}$ indicates the presence of $s{p}^2$ and $s{p}^3$  hybridized C-H. The absorption at $1606 and 1496 c{m}^{-1}$ is because of the benzene ring. The ${}^{1}H$ NMR data can be given as:

The structure of the compound can be given as:

Possible structure of spectral data in question c