The IR, NMR, and mass spectra are provided for an organic compound.

1. Consider each spectrum individually, and tell what characteristics of the molecule are apparent from that spectrum.
2. Propose a structure for the compound, and show how your structure fits the spectral data.
3. Explain why an important signal is missing from the proton NMR spectrum.

The technique used to understand the organic molecules and their characteristics is IR, mass, and NMR spectra.

The spectral details are usually given in the graphical form in which IR has a wavelength, NMR has a chemical shift, and the mass spectra has the m/z values.

(a) The characteristics of the spectrum are listed below.

IR:

• 2400 - 2400cm^-1 corresponds to the broad O_H stretch of the carboxylic acid (COOH).
• 1700 cm^-1 corresponds to the carbonyl carbon.
• 1240 cm^-1 corresponds to the C-O bond.
• 1600 cm^-1corresponds to the aromatic C=C

The m/z value 152 corresponds to the molecular ion peak. The molecular weight is 152. The value 107 corresponds to the loss of the COOH group. The m/Z value corresponding to 77 is the mono-substituted benzene ring.

There are two signals in the range of 6.8-7.3 ppm in the ratio of 2H to 3H. This value is for the mono-substituted benzene ring. The chemical shift value, 4.6 ppm corresponds to the group which is deshielded. There is a 2H singlet.

The carbon NMR spectrum shows that 170 ppm has a small peak of the carbonyl group. The chemical shift value 115-157 ppm has four peaks which are of the monosubstituted benzene ring in which the deshielded peak indicates the oxygen substituted on the ring.

(b) The spectral details show that there is benzene ring, COOH group and group. The m/z peak in the mass spectra shows that the structure is phenoxy acetic acid.

The group is downfield because it is between the two electron-withdrawing groups that is O and COOH.

The structure of phenoxyacetic acid is given below.

Phenoxy acetic acid

(c) The proton present in the carboxylic acid group (COOH) is missing from the proton NMR spectra. This group is attached to the group through an oxygen atom.

The signal is missing because it may be beyond 10 or the scanning of the NMR is not clear. Another possibility is that the peak gets broadened beyond the detection due to the hydrogen bonding with DMSO.