Planning multi-step synthesis routes between functional groups using practical techniques (reflux, distillation, purification), and the analytical techniques of infrared spectroscopy (functional-group absorptions) and mass spectrometry (molecular ion and fragmentation).

What this topic is asking

OCR specification points 4.2.3 and 4.2.4 want you to plan multi-step synthesis routes between the functional groups of Module 4 using the correct reagents, conditions and practical techniques, and to identify functional groups and structures from infrared spectra and mass spectra. This is where the separate reactions become a connected reaction map and where instruments confirm what you have made.

Planning a synthesis

To plan a route, identify the functional groups in the starting material and target, then choose a chain of conversions connecting them, stating the reagent, conditions and mechanism type at each step. Shorter routes with higher atom economy are preferred.

Infrared spectroscopy

Key absorptions to recognise:

• $\text{O-H}$ in an alcohol: broad, about $3200$ to $3550\ \text{cm}^{-1}$.
• $\text{O-H}$ in a carboxylic acid: very broad, about $2500$ to $3300\ \text{cm}^{-1}$.
• $\text{C=O}$ in aldehydes, ketones, acids and esters: sharp, about $1680$ to $1750\ \text{cm}^{-1}$.
• $\text{C-H}$ in alkanes: about $2850$ to $2960\ \text{cm}^{-1}$.

Infrared absorption by atmospheric $\text{CO}_2$, $\text{H}_2\text{O}$ and $\text{CH}_4$ is also the molecular basis of the greenhouse effect.

Mass spectrometry

Examples in context

Example 1. Quality control in a pharmaceutical plant. Infrared and mass spectra are run on every batch of a drug intermediate to confirm the functional groups and relative molecular mass match the target, exactly the structure-determination skill tested here.

Example 2. Forensic identification. A mass spectrum of an unknown sample is matched against a database of fragmentation patterns to identify substances, relying on the same molecular-ion and fragment reasoning.

Try this

Q1. State what the molecular ion peak in a mass spectrum tells you. [1 mark]

• Cue. Its $m/z$ value equals the relative molecular mass of the compound.

Q2. A compound absorbs strongly at $1715\ \text{cm}^{-1}$ but shows no broad band above $3000\ \text{cm}^{-1}$. State the functional group present. [1 mark]

• Cue. A carbonyl group $\text{C=O}$ (an aldehyde or ketone, since there is no $\text{O-H}$).