Question

The ${}^{\mathbf{1}}\mathbf{H}$NMR spectra for two esters with molecular formula ${\mathbf{C}}_8{\mathbf{H}}_8{\mathbf{O}}_2$ are shown next. Which of the esters is hydrolyzed more rapidly in an aqueous solution with a pH of 10?

Short answer

Following ester will hydrolyze more rapidly:

Step-by-step solution

Step-by-step-solutionStep 1: H1NMR spectroscopy

The chemical shift in ${}^{\mathbf{1}}\mathbf{H}$NMR spectroscopy denoted as role="math" localid="1652430408976" $\mathbf{\delta }$tells about the environment around the atomic nuclei. The $\mathbf{\delta }$scale comprises a value in the range of 0-10 ppm. A high chemical shift value indicates the proton is deshielded and a low chemical shift value shows the proton is shielded.

Identifying the desired ester

Each of the NMR spectra possesses signals between 7 and 8 ppm (integrating to five hydrogens).This indicates that the compound has a benzene ring with one substituent.

There is an additional signal that is a singlet. The singlet in each spectrum is due to the methyl group. The methyl group is at a higher frequency on the second spectrum. This is the spectrum of the compound on the right, as the methyl group is near to an electron-withdrawing oxygen. The compound on the left is hydrolyzed more rapidly as its OR group is a weaker base. ( The ${\mathrm{pK}}_{\mathrm{a}}$of phenol is approximately equal to 10 and that of methanol is approximately equal to 16).

Different esters