Question

How do you read infrared spectrometry graphs and determine the types of bonds that are shown at each peak? Also, how would you read a graph and determine which compound is being shown?

Answer 1

IR spectroscopy is used to determine the structure of the compounds by determining the varioud types of bonds, functional groups present with the help of various peaks.

However some of the peaks are broad whereas some of them are pointed or sharpened peaks as they determines the polarity of he following groups present in the compound. The more polar a molecule is, the stronger the peak is. Repetition sometimes also determines the strength, as many of the same functional group leads to a larger peak.

First of all look out the common absorption table to determine the ranges of the peaks that falls in a particular region. As first check out the longest peak with the maximum range like for eg. Alcohol it falls in the range of 3580-3650cm^-1 which is usaually very broad.

Terminal alkyne which falls in the range 3340 ? 3250 cm?1 are generally with the sharper amnd stronger peak.

Similary look it for other groups also by noticing in the graph carefully.

for eg. if we look out for ethanol first check out the peaks carefully and mark for the functional groups if present like O-H

1. The O-H(stretch) bond in an alcohol absorbs at a higher wavenumber somewhere between 3230 - 3550 cm^-1.

2. The absorption due to the C-H (strech)bonds falls under 3000 cm^-1, and also the troughs between 1000 and 1100 cm^-1 as one of which is due to the C-O (bend) bond.

3. Also O-H(bend ) falls in the range of around 1450-1380cm^-1