Question 1

Why is the O-H absorption peak in an IR spectrum so broad?

Accepted Answer

The broadness of the O-H peak is caused by hydrogen bonding. In a bulk sample of an alcohol or carboxylic acid, molecules are hydrogen-bonded to varying degrees. This creates a wide range of O-H bond environments, each with a slightly different vibrational frequency, smearing the absorption over a broad range of wavenumbers.

Question 2

Can IR spectroscopy distinguish between isomers?

Accepted Answer

Yes, it is excellent for distinguishing between functional group isomers (e.g., propanal and propanone vs. prop-2-en-1-ol) because they have different functional groups that absorb at different frequencies. However, it is generally not used to distinguish between positional isomers (e.g., propan-1-ol and propan-2-ol) or stereoisomers, as their key functional groups are the same and their spectra are often very similar in the functional group region.

Question 3

What is the difference between the C=O peak in an aldehyde and a ketone?

Accepted Answer

The C=O absorption for both aldehydes and ketones falls in a similar range (~1700-1740 cm⁻¹) and they cannot be reliably distinguished by this peak alone. However, an aldehyde also has characteristic C-H stretches for the aldehyde proton at ~2720 cm⁻¹ and ~2820 cm⁻¹. The presence of these weak but distinct peaks, in addition to the C=O peak, confirms an aldehyde.

Question 4

Do I need to memorise all the wavenumber values?

Accepted Answer

No. You will be provided with a Data Booklet in the exam that contains a table of characteristic infrared absorption values for common bonds. Your task is to learn how to use that data to interpret a given spectrum, not to memorise the numbers themselves. You should, however, be familiar with the general regions and shapes (e.g., 'broad O-H', 'strong, sharp C=O').

Infrared spectroscopy

What you need to know

The Molecular Vibe Check