Mass spectra and infrared spectroscopyEdexcel International A Level Chemistry: Revision notes

In mass spectrometry a molecule loses an electron to form the molecular ion, M⁺. Its m/z value (mass-to-charge ratio, with charge +1) is the relative molecular mass, and it is the peak with the highest m/z (ignoring tiny isotope peaks).

The molecular ion is unstable and breaks into smaller pieces. Only the positively charged fragments are detected, so a spectrum shows a fragmentation pattern. The base peak is the tallest peak (100% abundance). Fragments are identified from the difference between m/z values: M − 15 is loss of CH₃, M − 17 loss of OH, M − 29 loss of C₂H₅ or CHO.

Useful fragments:

• 15: CH₃⁺ · 29: C₂H₅⁺ (or CHO⁺) · 31: CH₂OH⁺ · 43: CH₃CO⁺ or C₃H₇⁺ · 45: COOH⁺ or CH₃CHOH⁺ · 57: C₄H₉⁺ or C₂H₅CO⁺

To deduce a structure: (1) read Mr from M⁺, (2) match it to a molecular formula, (3) assign fragments by subtracting from M, (4) check the proposed structure would break to give those peaks. For example, a strong peak at 43 with M⁺ = 58 and a C=O in the IR spectrum suggests propanone, CH₃COCH₃.

Bonds absorb infrared radiation at frequencies that make them vibrate (stretch). Absorptions are quoted as wavenumbers in cm⁻¹, and the data booklet gives ranges:

• O–H in alcohols: about 3200–3550, broad
• O–H in carboxylic acids: about 2500–3300, very broad
• N–H in amines: about 3300–3500
• C–H: alkanes and alkenes about 2850–3100; aldehydes about 2700–2900
• C=O: about 1680–1750, strong and sharp
• C=C: about 1620–1680
• C–X (halogenoalkanes): below about 1000, in the fingerprint region (for example C–Cl about 600–800 cm⁻¹)

A spectrum shows which bonds, and so which functional groups, are present.

Use both techniques together:

1. The molecular ion gives Mr and a molecular formula.
2. IR shows the functional groups (O–H, C=O, C=C, N–H).
3. Fragments show how the carbon skeleton is arranged, to choose between isomers.

Example: Mr 60, broad 3200–3550 cm⁻¹, no C=O and a base peak at 45 suggest propan-2-ol; propan-1-ol would give a peak at 31 (CH₂OH⁺).

Identify inorganic and organic unknowns by systematic tests and record observations.

Inorganic: flame tests for cations; carbonate (acid gives CO₂); sulfate (barium chloride); halide (silver nitrate, then dilute and concentrated ammonia); ammonium (warm with sodium hydroxide; ammonia turns damp red litmus blue).

Organic: alkene (bromine water decolourised); alcohol (PCl₅ steamy fumes); aldehyde (Fehling's or Benedict's brick-red precipitate; dichromate orange to green); carboxylic acid (effervescence with carbonate); halogenoalkane (hydrolysis then silver nitrate in ethanol).

Record each observation and the conclusion it supports.