Chapter 20: Q.13-56 (page 1038)

Show how you would distinguish among the following three compounds
Using infrared spectroscopy and no other information.
Using proton NMR spectroscopy and no other information.
Using, including DEPT, and no other information.

Short Answer

Expert verified

Based on stretching frequency in IR spectrum, 1 has aldehyde stretching frequency at 2710 and 2810 . 2 has stretching frequency at 3100-3500 . 3 has stretching frequency around 2500-3000 .

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.

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

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 stretching frequency at 2710 and 2810 . 2 has stretching frequency at 3100-3500 . 3 has stretching frequency around 2500-3000 with a shoulder around 2700 .

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 at6.8 while compound 3 do not show peak at this chemical shift. Compound 1 shows sharp singlet at9-10 due to aldehydic hydrogen while compound 3 shows broad singlet at10-14 due to presence of carboxylic group.

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