Q49E - Organic Chemistry (Mcmurry)

Question

The proton NMR spectrum for a compound with formula ${\mathbf{C}}_{10}{\mathbf{H}}_{12}{\mathbf{O}}_2$  is shown below. The infrared spectrum has a strong band at $\mathbf{1711}{\mathbf{cm}}^{-1}$ . The normal carbon-13 NMR spectral results are tabulated along with the DEPT-135 and DEPT-90 information. Draw the structure of this compound. 

Step-by-Step Solution

Answer

${ }^{13}C {}^1\text{H}$NMR- Spectrum    

                                                ${}^{13}\text{C}$ NMR Spectrum

Step-by-Step Solution Step 1: Determination of the structure

By observing the ${}^1\mathbf{H}$NMR and ${}^{13}\mathbf{C}$NMR spectrum the structure of the compound is drawn. Based on the chemical shift value the hydrogens are attached and possible structures are considered  

Step 2: Proton NMR features

The assignment of  ${}^1\mathrm{H}$NMR spectrum of the compound with molecular formula ${\mathrm{C}}_{10}{\mathrm{H}}_{12}{\mathrm{O}}_2$  is as follows:

The key features of this ${}^1\mathrm{H}$NMR spectrum are:

1. The methyl protons at $\delta 3.80$ as a 3H, singlet as the protons are attached to electronegative oxygen atom and have no adjacent protons.
2. The benzylic proton gives a singlet at $\delta 2.15$.
3. The methylene protons give a peek around $\delta 3.65$ as these protons are flanked by a benzene ring and a carbonyl carbon.
4.  The aromatic protons are marked and their value depends on the electronic environment.

The IR peak at $1711c{m}^{-1}$ corresponds to the carbonyl stretching frequency

For ${}^{13}C$ NMR, $C{H}_2$ protons give the negative signal at DEPT-135 whereas ${\mathrm{CH}}_3$ protons give the positive signal at DEPT-135. The assignment of ${}^{13}C$ peaks is as shown:

                                                         ${}^{13}\text{C}$NMR -Spectra