Part 1: Planning and evaluating organic routes

Work backwards from a target, conserve the carbon skeleton and select conditions that produce the required functional group.

Begin with the last bond or group you need to make

A multistep synthesis is a sequence of justified transformations. First compare starting material and target: count carbons, mark the functional groups and identify any new C–C bonds. Work backwards by asking which familiar precursor could give the final group.

An ester suggests an alcohol plus a carboxylic acid or acyl derivative. A primary amine may come from haloalkane substitution, nitrile reduction or nitro-group reduction, depending on the skeleton. A one-carbon extension suggests cyanide chemistry. These are alternatives to evaluate, not interchangeable recipes.

Write each intermediate structure in full enough detail to show the reactive group. Only then add reagents, conditions and work-up. A correct list of reagents without a connected route may not demonstrate that the target can actually be made.

Use conditions to distinguish competing pathways

A haloalkane with aqueous hydroxide gives an alcohol by substitution; hot ethanolic hydroxide favours elimination to an alkene. The same named base in a different solvent can therefore lead to a different product.

A primary alcohol can be oxidised to an aldehyde using controlled oxidation with removal of the volatile aldehyde, or to a carboxylic acid using excess oxidant and reflux. A secondary alcohol gives a ketone. Do not write “dichromate” without identifying the acidified reagent and the conditions needed for the requested outcome.

Reflux heats a mixture while returning condensed vapour. Distillation separates a volatile component into another vessel. Neither term means simply “heat strongly”, and the two apparatus choices have different purposes in controlling a synthesis.

Worked route: bromoethane to methyl propanoate

The target ester contains a three-carbon acid-derived fragment and a one-carbon methoxy fragment. Starting bromoethane contains only two carbons, so direct oxidation after alcohol substitution would make the wrong acid skeleton.

Step 1: react bromoethane with CN⁻ in ethanol on heating to form propanenitrile, CH₃CH₂CN. Step 2: hydrolyse with dilute aqueous acid under reflux to propanoic acid, CH₃CH₂COOH. Step 3: react with methanol and an acid catalyst under suitable heating to form CH₃CH₂COOCH₃.

Track carbon origins: two come from bromoethane, one from cyanide and one from methanol. The final esterification is reversible, so an excess reactant or product removal can improve conversion. Isolation and purification still determine the final recovered yield.

Test every reagent against every group

For CH₃COCH₂COOH treated with NaBH₄ under the usual carbonyl conditions, reduce the ketone while retaining COOH. For a hydroxycarboxylic acid treated with NaOH, neutralise COOH; an ordinary alcohol OH is not converted into an alkoxide quantitatively by aqueous hydroxide in this model.

When a reagent can react at more than one site, show all appropriate changes or explain the supplied selectivity. Reaction order may matter: reducing NO₂ to NH₂ changes aromatic directing behaviour, and converting an acid to an acyl chloride creates a highly reactive group that should not be carried through arbitrary aqueous steps.

Evaluate a route using chemistry, step count, yield and the information supplied about safety, cost or waste. Fewer steps often help recovery, but one low-selectivity step can outweigh that advantage. Avoid claiming an “optimal” route without stating the criterion.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources