Chapter 14: Q45P (page 527)

Label the signals due to \({{\bf{H}}_{\bf{a}}}\), \({{\bf{H}}_{\bf{b}}}\), and \({{\bf{H}}_{\bf{c}}}\) in the 1 H NMR spectrum of acrylonitrile \(\left( {{\bf{C}}{{\bf{H}}_{\bf{2}}}{\bf{ = CHCN}}} \right)\). Draw a splitting diagram for the absorption due to the \({{\bf{H}}_{\bf{c}}}\) proton.

Short Answer

Expert verified

\({{\rm{H}}_{\rm{a}}}\): doublet of doublets at 5.7 ppm.

\({{\rm{H}}_{\rm{b}}}\): doublet of doublets at ~6.2 ppm.

\({{\rm{H}}_{\rm{c}}}\): doublet of doublets at ~6.6 ppm.

Step by step solution

1H NMR spectroscopy

The proton NMR spectroscopy gives all the types of proton’s signals. The geminal protons couple and give doublet according to their chemical environment.

The vicinal protons also couple to give doublet according to their chemical environment.

Splitting pattern

The splitting pattern of a proton adjacent to two different protons gives doublet of doublet according to their coupling constant value.

The splitting pattern of a proton adjacent to the methylene group and a proton gives triplet of doublet.

Explanation

\({{\rm{H}}_{\rm{a}}}\): doublet of doublets at 5.7 ppm. Two large J values are seen for the H’s cis\(\left( {{{\rm{J}}_{{\rm{ab}}}} = 11.8\,{\rm{Hz}}} \right)\) and trans\(\left( {{{\rm{J}}_{{\rm{ac}}}} = 18\,{\rm{Hz}}} \right)\)to\({{\rm{H}}_{\rm{a}}}\).

\({{\rm{H}}_{\rm{b}}}\): doublet of doublets at ~6.2 ppm. One large J value is seen for the cis H\(\left( {{{\rm{J}}_{{\rm{ab}}}} = 11.8\,{\rm{Hz}}} \right)\). The geminal coupling\(\left( {{{\rm{J}}_{{\rm{bc}}}} = 0.9\,{\rm{Hz}}} \right)\)is hard to see.

\({{\rm{H}}_{\rm{c}}}\): doublet of doublets at ~6.6 ppm. One large J value is seen for the trans H\(\left( {{{\rm{J}}_{{\rm{ac}}}} = 18\,{\rm{Hz}}} \right)\). The geminal coupling\(\left( {{{\rm{J}}_{{\rm{bc}}}} = 0.9\,{\rm{Hz}}} \right)\)is hard to see.

The splitting pattern of the proton is doublet of doublet.