Quick questions on Infrared spectroscopy of organic compounds explained: HSC Chemistry Module 8

A covalent bond behaves like a tiny spring connecting two atoms. It has a natural vibration frequency given (to first approximation) by:

A vibration only absorbs IR if it changes the molecular dipole moment. Symmetrical stretches of nonpolar bonds (like the symmetric stretch of $CO_2$, or the stretch of $H_2$) do not appear. Polar bonds (O-H, N-H, C=O) give the strongest absorptions.

The single most useful peak is the C=O stretch at 1700 to 1750 cm$^{-1}$, because it is intense, narrow, and only present when there is a carbonyl.

1. Is there a strong C=O around 1700 to 1750? If yes, a carbonyl is present (aldehyde, ketone, acid, ester, amide). 2. Is there a broad O-H/N-H above 3000? Broad and centred on 3300 (alcohol O-H), very broad from 2500 (acid O-H), or sharper around 3400 (amine N-H).

Below 1500 cm$^{-1}$, the spectrum is dominated by C-C, C-O and skeletal bending vibrations. The pattern is too complex to assign peak by peak, but it is unique to each compound. To confirm an identification, compare the fingerprint region to a reference spectrum: a match across both functional-group region and fingerprint region is conclusive.

Strengths. Fast (seconds per spectrum on a modern FTIR), non-destructive (the sample can be recovered), identifies functional groups directly, and works on solids, liquids and gases.

Fast (seconds per spectrum on a modern FTIR), non-destructive (the sample can be recovered), identifies functional groups directly, and works on solids, liquids and gases.

Does not give exact carbon counts (use mass spectrometry). Cannot distinguish enantiomers (use chiral chromatography or polarimetry). Heavily overlapping peaks in the fingerprint region need a reference library to resolve.

Wavenumber decreases to the right. The O-H stretch (high wavenumber) is on the left of the spectrum.

Alcohol O-H is broad but centred around 3300; acid O-H is very broad, dragging down to 2500 cm$^{-1}$, often obscuring the C-H stretches.

Two small peaks at 2720 and 2820 cm$^{-1}$ are diagnostic of an aldehyde C-H.

C-H stretches are always strong but rarely diagnostic. Look at the diagnostic regions (carbonyl, O-H, N-H) first.

IR sees bonds, not chirality. Enantiomers give identical IR spectra.