Infrared Spectroscopy (Cambridge (CIE) A Level Chemistry): Flashcards

A fingerprint region is the area of an IR spectrum below 1500 cm^-1. It contains many complex absorption peaks unique to each compound and is used to identify a specific molecule by comparison with reference spectra.

The C=O bond in a carboxyl group absorbs in the range 1670-1740 cm^-1. This appears as a strong, sharp peak. The carboxylic acid also shows a broad O-H peak at 2500-3000 cm^-1.

True.

Hydrogen bonding in alcohols causes the O-H bond to vibrate over a range of frequencies, producing a broad absorption peak in the region 3200-3600 cm^-1.

In IR spectroscopy, absorbed radiation is expressed as wavenumbers (units: cm^-1), which are the reciprocal of wavelength.

Both isomers have the same functional groups and show similar O-H and C-O absorptions above 1500 cm^-1. They are distinguished by comparing their fingerprint regions (below 1500 cm^-1) with reference spectra from a database.

True.

Ketone C=O groups absorb strongly and sharply in the range 1680-1750 cm^-1. A sharp peak around 1710 cm^-1 without a broad O-H absorption confirms a ketone rather than an alcohol.

A wavenumber is the number of wave cycles per centimetre, expressed in cm^-1. It is the reciprocal of wavelength and is used to describe the position of absorption peaks in an IR spectrum.

A C≡N nitrile bond absorbs in the range 2200-2250 cm^-1, while an N-H bond in an amine or amide absorbs in the range 3300-3500 cm^-1.

Some IR absorption bands overlap, making it impossible to assign all peaks to specific bonds with certainty. Mass spectrometry provides complementary information (molecular mass and fragmentation pattern) that helps confirm the identity of the compound.