← Properties of selected organic compounds

Distinguishing several compounds with limited reagents

A harder version of the pairwise distinguishing test: given several compounds and only ONE or TWO reagents, devise a sequence that identifies every one of them.

The method for distinguishing MORE than two compounds

  • Apply the FIRST reagent to all compounds at once, and sort them into groups by their observation (e.g. those that decolourise bromine water vs those that do not).
  • Within each group that still contains more than one compound, apply the SECOND reagent to separate them further.
  • Continue until every compound has a UNIQUE combination of observations that identifies it — if two compounds still share identical observations after all reagents, the method has not fully distinguished them.
  • State the structural formula of any product formed at each positive reaction, not just the observation.
Worked example Distinguishing four compounds with two reagents Devise a method to distinguish a haloalkane, a carboxylic acid, an amine, and an alcohol using only red litmus paper and acidified potassium dichromate. REAGENT 1 (litmus paper): the carboxylic acid turns BLUE litmus RED (acidic); the amine turns RED litmus BLUE (basic); the haloalkane and alcohol cause no colour change (neutral) — this separates the four into three groups. REAGENT 2 (dichromate) applied to the remaining neutral group: the alcohol turns the dichromate from orange to green (oxidised); the haloalkane produces no colour change (haloalkanes are not oxidised by dichromate). RESULT: every one of the four compounds now has a unique combination of observations — acid (red litmus), amine (blue litmus), alcohol (orange→green with dichromate), haloalkane (no change with either reagent). ✓ The two reagents together produce four distinct observation combinations for four compounds — the method is only complete once every compound is uniquely identified, not just partially sorted. ✕

Stopping once compounds are partially sorted

Applying the first reagent and noting that it separates the four compounds into three groups is not yet a complete answer if one group still contains two compounds with identical observations. The method must continue with a further reagent or test until every single compound has its own unique, distinguishing result.

Sketching the decision tree before writing the answer

  • Draw a quick branching diagram: each reagent is a branch point, and each observation is a branch — this makes it visually obvious whether every compound reaches a unique end point.
  • If two compounds end up on the same final branch, a further test is needed before the answer is complete.

Practise just this