Question

List three changes you would expect in the IR spectrum of 2,3-dibromocinnamic acid as compared to the IR spectrum of trans-cinnamic acid. Make sure to reference the approximate wavenumber of any relevant peaks and whether they correspond to a stretch or a bend.

Answer 1

Functional group Frequency values in cm^-1    Reason/ explanation

trans cinnammic acid

OH 2500-3400 cm^-1   broad peak of acid OH

C=O 1682-1715 cm^-1 it is alpha-beta unsaturated acid and so value decreases than normal acid carbonyl frequency

C=C 1630-1635 cm-1   alkene streching frequency which is next to carbonyl   

C=C 1480 cm^-1(bending), charachteristic of aromatic alkene streching and bending

157 cm^-1(streching)

C-O-C 1285 cm^-1   c-o-c of acid

mono substituted ring 683 cm^-1(bending) , C-H bending of monosubstituted ring

710 cm^-1(streching)   

trans 980-960 cm^-1    C-H bending of 1,2 di substituted trans alkene

2,3-dibromocinnammic acid

changes that will occur in 2,3-dibromocinnammic acid are,

C=O 1680-1685 cm^-1

C-Br (streching) 560-600 cm^-1

^{trans C-H bending will have changes in value and aromatic alkene streching also.}