Question

You will be given two IR’s.  One for 4-hydroxybenzaldehyde and one for 4-hydroxybenzoic acid.  Explain the differences by labeling the distinguishing peak(s).

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Answer 1

The main difference we can identify only with two functional groups aldehyde and carboxylic acid, remaining all are same.

4-hydroxybenzaldehyde:

C=O stretch at 1669 cm-1 (due to conjugation C=O stretch lowered)

medium, two peaks at 2831 & 2760 cm-1 due to aldehyde =C-H stretch

P-HYDROXYBENZALDEHYDE C7H502 LOD TRANSHETTANCET 2 medium, two peaks at 2831 & 2760 cm-1 aldehyde =C-H stretch CEO stretch at 1669 cm-1 DT 4000 2000 150g 1000 500 HAVENUHBERI- 3199 41 3171 37 3044 70 2960 77 2980 77 2831 79 278981 2750 81 2622 86 1908 85 1669 7 1649 64 1604 4 161966 1466 28 1220 26 1413 74 | 1164 15 1389 70 1115 77 1316 44 869 70 12918 835 36 1265 74 823 70 1241 62 78974 712 70 644 74 606 42 607 62 HO -C- O= H O

4-hydroxybenzoic acid:

strong, very broad in the region 2500-3300 cm-1 (3393 cm-1 due to para-OH electron donating group), we know hydrogen bonding lowers the wavenumber of absorption and at the same time electron donating groups raises wavenumber of absorption.

C=O stretch at 1677 cm-1 (due to conjugation C=O stretch lowered)

P-HYDROXYBENZOIC ACID C7H503 LOD TRANSHETTANCET 2 C=O stretch at 1677 cm- strong, very broad signal 3393 cm-1 due to O-H stretch D + 4000 Boop 2000 IS DO 1000 500 HAVENUHBERI-1 | 3393 33 2979 57 2969 57 2950 67 2919 55 2849 53 2670 68 2664 60 1449 38 1694 14 | 1426 29 1677 B 1386 70 1693 741366 70 1609 12 1320 13 1597 10 1297 25 1610 62 1292 26 1248 4 1171 26 112972 1102 60 931 57 856 18 896 74 770 34 693 70 641 68 621 26 591 77 550 506 O= C-OH O

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