CIE AS/A Level Chemistry 22.1 Infrared spectroscopy Study Notes- 2025-2027 Syllabus

Detailed Analysis of Infrared (IR) Spectra

Infrared spectroscopy is used to identify functional groups present in an organic molecule. It works by measuring the absorption of infrared radiation, which causes bonds to vibrate.

Each type of bond absorbs IR radiation at a characteristic wavenumber, measured in \( \mathrm{cm^{-1}} \).

Key Principles

• Only bond vibrations are detected, not full molecular structures
• Different functional groups absorb at distinct regions of the spectrum
• Peak position, shape, and intensity are all important

Important Regions of an IR Spectrum

• 4000–2500 cm\(^{-1}\) → O–H and N–H stretching
• 2500–2000 cm\(^{-1}\) → triple bonds (C≡C, C≡N)
• 2000–1500 cm\(^{-1}\) → C=O and C=C stretching
• 1500–500 cm\(^{-1}\) → fingerprint region (not analysed at AS)

Functional Groups from the Data Section

• Alcohol O–H: broad peak at 3200–3600 cm\(^{-1}\)
• Carboxylic acid O–H: very broad peak at 2500–3300 cm\(^{-1}\)
• C=O: strong sharp peak at ~1700 cm\(^{-1}\)
• C=C: medium peak at ~1600 cm\(^{-1}\)
• C–H: peaks at 2850–3000 cm\(^{-1}\)

Broad peaks indicate hydrogen bonding, while sharp peaks usually indicate double bonds.

Using Absence of Peaks

The absence of an expected absorption is often as important as its presence.

• No broad O–H peak → not an alcohol or carboxylic acid
• No C=O peak → not an aldehyde, ketone, ester or carboxylic acid
• No C=C peak → molecule is saturated

Exam Technique for IR Questions

• Quote approximate wavenumber values
• Describe peak shape (broad or sharp)
• Link each peak directly to a specific bond
• Use Data Booklet wording
• Avoid guessing the whole structure