Q41E - Organic Chemistry (Mcmurry)

Question

Propose structures for compounds that fit the following descriptions:

 (a) ${\mathbf{C}}_{10}{\mathbf{H}}_{14}$  ${}^1\mathbf{H}$NMR: $\mathbf{7}\mathbf{.}\mathbf{18}\mathbf{\delta }$ (4 H, broad singlet); $\mathbf{2}\mathbf{.}\mathbf{70}\mathbf{\delta }$(4 H, quartet, J = 7 Hz);  $\mathbf{1}\mathbf{.}\mathbf{20}\mathbf{\delta }$(6 H, triplet, J =7 Hz) IR:$\mathbf{745}{\mathbf{cm}}^{-1}$ .

(b)${\mathbf{C}}_{10}{\mathbf{H}}_{14}$ ${}^1\mathbf{H}$NMR:  $\mathbf{7}\mathbf{.}\mathbf{0}\mathbf{\delta }$ (4 H, broad singlet); $\mathbf{2}\mathbf{.}\mathbf{85}\mathbf{\delta }$ (1 H, septet, J = 8 Hz); $\mathbf{2}\mathbf{.}\mathbf{28}\mathbf{\delta }$(3 H, singlet);  $\mathbf{1}\mathbf{.}\mathbf{20}\mathbf{\delta }$(6 H, doublet, J = 8 Hz) IR:$\mathbf{825}{\mathbf{cm}}^{-1}$ .

Step-by-Step Solution

Answer

a)

Given possible structure for a

b)

     

Step 1: NMR spectroscopy

NMR spectroscopy deals with the chemical shift of protons. By knowing the chemical shift of protons or protons attached to the chemical element we can propose the structure of the compound.

Step 2: a) Based on NMR data

The chemical shift value and the presence of 4H, quartet, and 6H, triplet indicate the presence of two $-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ groups in a similar chemical environment. Four proton broad singlets in the aromatic region indicate that all the aromatic protons are chemically equivalent, from this date the possible structure is given as follows:

Step 3: b) Based on the NMR data

The two NMR signals doublet and septet with J=8 Hz indicate the presence of an isopropyl group that is attached to a carbon that doesn’t possess any hydrogen atoms. Four hydrogen broad singlet in the aromatic region indicates the presence of the para di-substituted aromatic ring. The singlet at $\mathrm{\delta }=2.28$, 3 H indicates the presence of methyl substituent on the aromatic ring.

So, the structure of the molecule base on the chemical shift values and the splitting pattern is as follows: