Question

Discuss the differences observed in the IR and NMR spectra of (E)-cinnamic acid and 2,3-dibromo-3-phenylpropanoic acid that are consistent with addition of bromine to the double bond.

Answer 1

You can see the differences in the NMR peaks in the structures drawn above. The vinylc H atoms become shielded in the dibromo product. Hence the chemical shift decreases. You can see the values . All other NMR peaks are almost in a similar positions. So they are not shown , only differentiating peaks are discussed.

In the IR spectrum you will observe that there will be no streching frequency for C_sp3 - H bond for (E)- Cinnamic acid as all the carbon atoms are Sp2 . In the dibromo compound you will see a peak at ~ 2800-2950 cm-1 for that bond. All other peaks would appear in almost same position ,hence they are not worthy to do differentiation.