Tests for alkenes, alcohols, aldehydes and carboxylic acids. Use of bromine water, acidified potassium dichromate(VI), Fehling's and Tollens' reagents, and sodium carbonate. Determination of empirical and molecular formulae from combustion or composition data. The principle of mass spectrometry and infrared spectroscopy for structure determination.

What this dot point is asking

AQA wants you to identify functional groups using simple chemical tests, write the observations, and use mass spectrometry and infrared (IR) spectroscopy to deduce or confirm structures.

Tests for functional groups

When several tests are used together, the order matters: start with the test that gives the most decisive result. Sodium carbonate is a good first test because only the carboxylic acid fizzes, immediately separating acids from the rest. Tollens' or Fehling's then separates aldehydes from ketones, and acidified dichromate confirms an oxidisable alcohol. A clear record of reagent, conditions (warm, reflux) and the exact observation is what scores marks, since a vague "it changed colour" is not credited.

Mass spectrometry

The molecular ion peak (M) at the highest m/z gives the relative molecular mass. The M+1 peak (from carbon-13) and the pattern of fragment peaks help deduce structure: e.g. a peak at m/z 15 indicates a $CH_3^+$ fragment, m/z 29 a $CHO^+$ or $C_2H_5^+$, m/z 43 a $CH_3CO^+$ (acylium) fragment. The difference between the molecular ion and a fragment peak gives the mass of the neutral group lost, so a loss of 15 points to a methyl group, a loss of 29 to an ethyl or aldehyde group, and a loss of 45 to a carboxyl group. Combined with the molecular mass, these losses build up the structure.

Determining empirical and molecular formulae

Combustion or composition data is converted into an empirical formula by finding the moles of each element (mass or percentage divided by $A_r$) and reducing to the simplest whole-number ratio. The molecular formula is then found by comparing the empirical formula mass with the relative molecular mass from the mass spectrum and multiplying up. This links organic analysis back to the mole calculations of the physical chemistry unit.

Infrared spectroscopy

Bonds absorb IR radiation at characteristic wavenumbers because they vibrate.

• $O-H$ (carboxylic acid): broad, $2500-3300 \text{ cm}^{-1}$
• $O-H$ (alcohol): broad, $3230-3550 \text{ cm}^{-1}$
• $C=O$: strong, $1680-1750 \text{ cm}^{-1}$
• $C-H$: around $2850-3100 \text{ cm}^{-1}$

The fingerprint region (below $1500 \text{ cm}^{-1}$) is unique to a molecule and is matched against a database.

Try this

Q1. Which reagent distinguishes an aldehyde from a ketone, and what is seen? [2 marks]

• Cue. Tollens' reagent; aldehyde gives a silver mirror, ketone gives no change.

Q2. What does a strong absorption at $1715 \text{ cm}^{-1}$ in an IR spectrum suggest? [1 mark]

• Cue. A $C=O$ (carbonyl) bond.