Infrared Spectroscopy

· Infrared (IR) spectroscopy is used to identify functional groups in simple molecules.
· Molecules absorb infrared radiation when bonds vibrate at characteristic frequencies.
· Absorptions are shown as troughs/peaks on an IR spectrum.
· The x-axis is wavenumber / cm⁻¹.
· In exams, match the wavenumber of key absorptions to the Data section to identify functional groups.
· Focus on strong, clear absorptions rather than every small peak.
· IR usually shows which functional groups are present, not the full structure by itself.

Key Exam Absorptions to Recognise

· O–H in alcohols: broad absorption around 3200–3600 cm⁻¹.
· O–H in carboxylic acids: very broad absorption around 2500–3300 cm⁻¹.
· N–H in amines/amides: absorption around 3300–3500 cm⁻¹.
· C–H in alkanes: absorption around 2850–3000 cm⁻¹.
· C=C in alkenes: absorption around 1620–1680 cm⁻¹.
· C=O in carbonyl compounds: strong absorption around 1640–1750 cm⁻¹.
· C≡N in nitriles: sharp absorption around 2200–2250 cm⁻¹.
· C–O in alcohols, carboxylic acids and esters: absorption around 1000–1300 cm⁻¹.
· Strong C=O peak is one of the most important exam clues.
· Broad O–H + strong C=O strongly suggests a carboxylic acid.

These example spectra show how different functional groups create recognisable absorption patterns. They help students practise identifying groups from the presence or absence of key peaks. Source

How to Analyse an IR Spectrum

· First check for a broad O–H absorption:
· 3200–3600 cm⁻¹ = likely alcohol.
· 2500–3300 cm⁻¹ = likely carboxylic acid.
· Then check for a strong C=O absorption around 1640–1750 cm⁻¹.
· If C=O is present, consider aldehyde, ketone, carboxylic acid or ester.
· If C=O is absent, rule out most carbonyl compounds.
· Check for C≡N around 2200–2250 cm⁻¹ to identify a nitrile.
· Use multiple clues together, not one peak alone.
· Ignore the fingerprint region below about 1500 cm⁻¹ unless the question gives specific guidance.
· State conclusions clearly: “The absorption at ___ cm⁻¹ shows the presence of ___.”

Distinguishing Common Functional Groups

· Alcohol: broad O–H peak, but usually no strong C=O peak.
· Carboxylic acid: very broad O–H peak plus strong C=O peak.
· Aldehyde/ketone: strong C=O peak but no broad O–H peak.
· Ester: strong C=O peak plus C–O absorption in the 1000–1300 cm⁻¹ region.
· Nitrile: sharp C≡N peak around 2200–2250 cm⁻¹.
· Alkene: C=C absorption around 1620–1680 cm⁻¹ and possible =C–H just above 3000 cm⁻¹.
· Alkane: mainly C–H absorptions around 2850–3000 cm⁻¹; lacks distinctive O–H, C=O or C≡N peaks.

This spectrum shows the two key carboxylic acid features: a very broad O–H absorption and a strong C=O absorption. It is ideal for learning how to distinguish carboxylic acids from alcohols and ketones. Source

Common Exam Phrases

· “Absorption at ___ cm⁻¹ indicates…”
· “The broad absorption suggests an O–H bond.”
· “The strong absorption around 1700 cm⁻¹ suggests a C=O bond.”
· “The absence of a broad O–H absorption means the compound is unlikely to be an alcohol or carboxylic acid.”
· “The IR spectrum supports the presence of the ___ functional group.”
· “Use the Data section to match absorptions to functional groups.”