9701 · 22.1
Infrared spectroscopy flashcards
Revision flashcards for Cambridge 9701 Infrared spectroscopy (syllabus 22.1). Flip, recall, then mark a real past-paper question.
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What is the primary use of infrared (IR) spectroscopy in organic chemistry?
To identify the functional groups present in a molecule.
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What happens to a covalent bond when it absorbs infrared radiation?
It vibrates with increased amplitude. This can be a stretching or a bending motion.
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What is 'wavenumber' and what are its units in IR spectroscopy?
Wavenumber is the reciprocal of wavelength ($1/\lambda$). It is directly proportional to frequency and energy. The unit is reciprocal centimetres (cm⁻¹).
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On an IR spectrum, what does a downward 'peak' (a trough) signify?
It signifies that radiation of a specific wavenumber has been absorbed by the sample. A low percentage transmittance means high absorbance.
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What is the 'fingerprint region' of an IR spectrum?
The complex region of the spectrum below 1500 cm⁻¹. It is unique to a particular molecule and is used for identification by comparison with known spectra.
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What is the characteristic absorption for a carbonyl group (C=O)?
A strong, sharp absorption peak in the range of 1680–1750 cm⁻¹.
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What is the characteristic absorption for the hydroxyl group (O-H) in an alcohol?
A broad absorption peak in the range of 3200–3600 cm⁻¹.
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Why is the O-H absorption peak in an IR spectrum typically broad?
Due to hydrogen bonding between molecules. This creates a variety of bond environments, causing absorption over a wide range of frequencies.
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How does the O-H absorption in a carboxylic acid differ from that in an alcohol?
The O-H peak in a carboxylic acid is very broad, appearing in the range 2500–3300 cm⁻¹. It is much wider than the alcohol O-H peak and often overlaps with C-H absorptions.
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An IR spectrum has no significant absorption above 3000 cm⁻¹ and no strong peak around 1700 cm⁻¹. What can you deduce?
The molecule likely does not contain O-H (alcohol/acid) or N-H groups, nor does it contain a C=O (carbonyl) group. It is likely an alkane, haloalkane, or ether.
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What two absorptions would you look for to identify an aldehyde?
1. A strong, sharp C=O peak at ~1720-1740 cm⁻¹. 2. Two C-H stretches of the aldehyde group at ~2720-2820 cm⁻¹ and ~2820-2920 cm⁻¹.
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Why is the x-axis of an IR spectrum plotted with wavenumber decreasing from left to right?
This is a historical convention. It places the higher energy, higher frequency absorptions (like O-H and C-H stretches) on the left.