Distinguishing Aldehydes and Ketones Revision Notes | A Level AQA Chemistry

1. The Problem: Aldehydes and Ketones Look Almost Identical

Both aldehydes and ketones contain a carbonyl group, $C=O$. Their IR spectra both show a strong C=O absorption in the region 1680–1750 cm$^{-1}$, making them indistinguishable by IR alone. Their physical properties overlap. So how do you tell them apart in the lab?

The answer lies in one crucial structural difference: the aldehyde has a hydrogen atom directly bonded to the carbonyl carbon, and that hydrogen makes all the difference chemically. Ketones lack this feature entirely.

Because of that C–H bond on the carbonyl carbon, aldehydes can be oxidised to carboxylic acids by mild oxidising agents. Ketones cannot be oxidised under the same conditions — they have no hydrogen atom bonded to the carbonyl carbon, meaning the carbonyl carbon cannot be further oxidised to a carboxylate without breaking C–C bonds. This difference in reactivity is the foundation of both tests you need to know for AQA.

2. The Oxidation Principle Behind Both Tests

Think of it this way: the oxidising agent in each test is a metal ion in a high oxidation state. When the aldehyde reduces it, the metal ion drops to a lower oxidation state, and that change produces a dramatic colour change you can observe directly. Ketones cannot do this — they have no available hydrogen on the carbonyl carbon to facilitate oxidation under mild conditions.

This is why both Tollens' reagent and Fehling's solution are described as mild oxidising agents. They are selective — strong enough to oxidise aldehydes, but not strong enough to oxidise ketones. That selectivity is what makes them useful as distinguishing tests.

3. Tollens' Reagent — The Silver Mirror Test

Tollens' reagent is prepared by dissolving silver nitrate in aqueous ammonia to form the diamminesilver(I) complex ion, $[Ag(NH_3)_2]^+$. This colourless solution is the oxidising agent. The silver is in the +1 oxidation state, and when an aldehyde reduces it, the silver ions are reduced to silver metal.

The silver metal deposits as a thin, reflective layer on the inside of the test tube — the classic silver mirror. This is the positive result for an aldehyde. If a ketone is tested instead, the solution remains colourless and no mirror forms.

In practice, the test tube is warmed gently in a water bath at around 60 °C. This provides enough activation energy for the reaction to proceed at a visible rate without decomposing the reagent. The silver mirror is unmistakable — it is one of the most visually striking results in A-level organic chemistry.

4. Fehling's Solution — The Brick-Red Precipitate Test

Fehling's solution is made by mixing two separate solutions: Fehling's A (aqueous copper(II) sulphate) and Fehling's B (sodium hydroxide and sodium potassium tartrate). The tartrate ions act as ligands, keeping the $Cu^{2+}$ ions in solution as a deep blue complex. This blue colour is the starting point of the test.

When an aldehyde is added and the mixture is heated, the aldehyde reduces the $Cu^{2+}$ ions (copper in the +2 oxidation state) to $Cu^+$ ions (copper in the +1 oxidation state). The $Cu^+$ ions precipitate out of solution as copper(I) oxide, $Cu_2O$, which is a brick-red solid.

Heating is essential for Fehling's solution — the reaction does not proceed at room temperature. The colour change from deep blue to brick-red is the positive result. You may also see the solution pass through a green intermediate as the blue and red colours mix during the reaction, but the final positive result is the brick-red precipitate.

5. Comparing the Two Tests Side by Side

Both tests exploit the same chemical principle — the ability of aldehydes to act as reducing agents — but they use different oxidising agents and produce different observable results. Knowing both tests, and being able to compare them precisely, is a common AQA exam demand.

One practical note: Tollens' reagent must be freshly prepared and used promptly. On standing, it can decompose to form explosive silver nitride (sometimes referred to as silver azide in more precise chemical contexts), which is a real laboratory safety consideration, not a straightforward detail.

6. Worked Exam-Style Walkthrough

AQA questions on this topic often present an unknown compound and ask you to plan or interpret a series of tests. Here is a typical scenario worked through step by step, modelling the kind of reasoning that earns full marks.

Worked example: Identifying an Unknown Carbonyl Compound

A student has two unlabelled bottles, X and Y. Both contain colourless liquids with a carbonyl group confirmed by IR spectroscopy. Bottle X gives a silver mirror with Tollens' reagent. Bottle Y gives no change with Tollens' reagent and remains blue when heated with Fehling's solution. Identify X and Y, and explain your reasoning. [4 marks]

1. Bottle X produces a silver mirror with Tollens' reagent. — A silver mirror means $Ag^+$ ions have been reduced to $Ag(s)$. Only an aldehyde can reduce Tollens' reagent under these conditions. [B1]
2. Bottle X is an aldehyde. — The aldehyde has been oxidised to a carboxylate ion while reducing the silver complex. [B1]
3. Bottle Y gives no silver mirror with Tollens' reagent and no brick-red precipitate with Fehling's solution. — No reaction with either mild oxidising agent means Y cannot reduce $Ag^+$ or $Cu^{2+}$. This is consistent with a ketone, which cannot be oxidised under mild conditions. [B1]
4. Bottle Y is a ketone. — Ketones have no hydrogen bonded to the carbonyl carbon, so they cannot be oxidised by Tollens' reagent or Fehling's solution. Both tests give negative results. [B1]

Answer: X is an aldehyde (positive Tollens' test: silver mirror formed). Y is a ketone (negative results with both Tollens' reagent and Fehling's solution, consistent with inability to be oxidised by mild oxidising agents).

7. Exam Technique and Common Mistakes

Questions on this topic are often worth 3–6 marks and combine 'describe' and 'explain' command words. Describing the observation alone will not earn the explanation marks — you must link the colour change to the underlying redox chemistry.

The logic is comparison-based: aldehydes are oxidised by these mild oxidising reagents, but ketones are not, so a negative result in both tests is strong evidence for a ketone.