Question

Please identify which of the seven unknowns is represented in the data provided and why

Lab 8: Instrumental Analysis You are given seven unknowns, all of which are white or brown powders. Use the provided IR and 1H NMR spectra to assign Unknowns 1-7 to the given compounds. Label all spectra and briefly explain how you made your determinations. NH ОН OR foncommon - 0 o-vanillin Benzoic acid p-toluidine 4-tert-butylphenol OH Y OH H2NY COH salicylic acid 9-fluorenol 4-aminobenzoic acid

Answer 1

Let's see NMR spectra carefully, here two aromatic proton splitted as doublet, clearly indicates spectra belong to para-substituted compound. So let's see chemical shift and splitting pattern in detail-

1) 5.5 ppm (2H , sharp singlet )

This peak belong to -NH₂ proton, that is why peak deshielded up to 5.5 ppm.

2)6.8 ppm (2H ,dd)

This peak belong to aromatic proton , it split as doublet of doublet due to presence of para-substituted compound.

3)7.9 ppm (2H ,dd)

This peak belong to aromatic proton, it split as doublet of doublet due to presence of chemical equivalent proton at its vicinity.

4)12.4 ppm (1H ,singlet)

This peak belong to -COOH group proton, it gets deshielded due to vicinity of electronegative oxygen atom.

So based on number of proton , splitting pattern and it's chemical shift suggest that, compound would be

4-Aminobenzoic acid.

Let's see label proton-

# H NMK peak assignment -04 12.4 ppm p.gppm

Prominent IR peak that support the structure of 4-Aminobenzoic acid-

1)3400-3600 cm^-1 = support the presence of -COOH and -NH₂ group.

2)2900-3000 cm^-1 shows stretching between sp³ hybridised carbon and hydrogen atom.

3) 1640 cm^-1 is the C=O stretching in carboxylic acid .

4)1200-1400 cm^-1 is the C=C ' stretching in aromatic compound.

Based on above finding, both NMR and IR spectra belong to 4-Aminobenzoic acid.