Chapter 13: Q56P (page 707)

Show how you would distinguish among the following threecompounds
(a)Using infrared spectroscopy and no other information.
(b)Using proton NMR spectroscopy and no other information.
(c)Using ¹³C-NMR, including DEPT, and no other information.

Short Answer

Expert verified

(a)Based on stretching frequency in IR spectrum, 1 has aldehyde C-H stretchingfrequency at 2710 and 2810 cm^-1 . 2 has OH stretching frequency at 3100-3500 cm^-1. 3 has COOH stretching frequency around 2500-3000 cm^-1.

(b)On the basis of proton NMR, 1, 2 and 3 all have benzene para pattern, but 1 and 2 have peaks at chemical shift value 6.8 , whereas 3 does not. 1 has sharp singlet from chemical shift value 9-10 whereas 2 does not.

(c)On the basis of carbon NMR, the methyl peaks in 1 and 2 are from chemical shift value 50-60 whereas 3 has methyl signal around chemical shift value 20.

Step by step solution

Step-1. Explanation of part (a):

The absorption bands in IR spectra have different intensity and frequencies. Based on stretching frequency in IR spectrum, 1 has aldehyde C-H stretching frequency at 2710 and 2810 cm^-1. 2 has OH stretching frequency at 3100-3500 cm^-1. . 3 has COOH stretching frequency around 2500-3000 cm^-1.with a shoulder around 2700 cm^-1.

Step-2. Explanation of part (b) and (c):

Based on proton NMR spectrum, all three have benzene para pattern showing two doublets but compounds 1 and 2 have peaks at δ 6.8 while compound 3 do not show peak at this chemical shift. Compound 1 shows sharp singlet at δ 9-10 due to aldehydic hydrogen while compound 3 shows broad singlet at δ 10-14 due to presence of carboxylic group.

On the basis of ¹³C-NMR, compound 1 shows C=O around δ 190 while compounds 2 and 3 have C=O around δ 170. Compound 2 shows benzene peak at δ 160 and all of the benzene signals in compound 3 are from δ 120-140. The methyl peaks in compounds 1 and 2 are from δ 50-60 whereas compound 3 has around δ 20.