Question

7) What bands on the IR tell you if the product was formed? How would you know if there was any starting material contaminating the product?

Answer 1

7) Just like a human fingerprint, the pattern of absorbance peaks in the fingerprint region is unique to every molecule, meaning that the data from an unknown sample can be compared to the IR spectra of known standards in order to make a positive identification.

IR spectrum for 1-hexanol.

A very broad ‘mountain’ centered at about 3400 cm^-1. This signal is characteristic of the O-H stretching mode of alcohols, and is a dead giveaway for the presence of an alcohol group. The breadth of this signal is a consequence of hydrogen bonding between molecules.

Transmittance (%) 0-H 30 LL LLL 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm)

The characterization of contaminants is a common application for infrared spectroscopy. For this type of exercise it is necessary to know the importance of the contaminant and why it is believed to be a contaminant. Was it extracted from a liquid or solid matrix? if so, what type of matrix, and what was the composition of the matrix? Is it possible that the contaminant originated from the matrix itself, as a breakdown product? – if so, then there should be some chemical similarities or common heritage. Was it embedded within a matrix, such as a plastic/polymer film or a sheet of paper? – if so, is the contaminant itself potentially contaminated by the matrix? This is especially important for interpretation, because the matrix may have a very intense infrared spectrum of its own