Controlling Oxidation and Identifying Carbonyls Revision Notes | A Level AQA Chemistry

1. The Oxidation Problem: Why Method Matters

You already know that primary alcohols can be oxidised — but here is the catch: the same starting material can give you two completely different products depending on how you carry out the reaction. Get the method wrong in an exam question and you will name the wrong product, lose the marks, and wonder where it went.

The oxidising agent used throughout this topic is acidified potassium dichromate(VI), written as $K_2Cr_2O_7$ in acidic solution (typically dilute $H_2SO_4$). When it oxidises an organic compound, the dichromate(VI) ion ($Cr_2O_7^{2-}$, orange) is reduced to chromium(III) ($Cr^{3+}$, green) — a colour change you can observe directly. The key variable is not the reagent itself, but the physical setup of the reaction.

2. Primary Alcohol to Aldehyde: Distil as You Go

A primary alcohol has the $-OH$ group on a carbon bonded to only one other carbon group. Ethanol ($CH_3CH_2OH$) is the classic example. When you oxidise a primary alcohol, the first product is an aldehyde — but aldehydes are themselves susceptible to further oxidation to carboxylic acids.

To stop at the aldehyde stage, you need to remove it from the reaction mixture before it can be oxidised further. The technique is distillation during the reaction: the oxidising agent is added to the alcohol in a flask fitted for distillation, and the aldehyde — which has a lower boiling point than the alcohol or carboxylic acid — is distilled off as it forms. Because the aldehyde leaves the reaction vessel immediately, it never accumulates long enough to be oxidised to the acid.

For ethanol, the product collected is ethanal ($CH_3CHO$). The overall transformation is: primary alcohol $\rightarrow$ aldehyde, using a limited supply of oxidising agent and continuous removal of the product.

3. Primary Alcohol to Carboxylic Acid: Reflux

If you want the carboxylic acid instead, you use the opposite approach: keep the aldehyde in contact with excess oxidising agent for as long as possible. The technique is reflux — the reaction mixture is heated under a condenser so that volatile components evaporate, condense, and fall back into the flask. Nothing escapes; the aldehyde is continuously re-exposed to the oxidising agent until it is fully oxidised to the carboxylic acid.

For ethanol, reflux with excess acidified potassium dichromate(VI) gives ethanoic acid ($CH_3COOH$). The aldehyde is an intermediate that you never isolate — it is oxidised onward before it can accumulate.

Notice that the colour change is the same in both cases — the difference lies entirely in the experimental setup, not in the appearance of the dichromate. This is a point examiners test directly: you must state the technique, not just the reagent.

4. What About Secondary Alcohols and Ketones?

Secondary alcohols — where the $-OH$ group is on a carbon bonded to two other carbon groups — are oxidised to ketones by acidified potassium dichromate(VI). Propan-2-ol gives propanone, for example. Crucially, ketones cannot be oxidised further under these conditions because there is no hydrogen on the carbonyl carbon to remove. The reaction stops at the ketone regardless of whether you use distillation or reflux.

Tertiary alcohols, where the carbon bearing $-OH$ has no hydrogen attached, are resistant to oxidation by acidified dichromate(VI) altogether — the solution remains orange. This distinction between primary, secondary, and tertiary alcohols underpins the chemical tests you will use in the next section.

5. Identifying Carbonyls: Tollens' Reagent and Fehling's Solution

Both aldehydes and ketones contain a carbonyl group ($C=O$), so they share some chemical properties. The challenge is distinguishing between them. Two reagents exploit the fact that aldehydes are reducing agents — they can donate electrons to an oxidising agent — whereas ketones cannot be oxidised under mild conditions.

When an aldehyde is warmed gently with Tollens' reagent in a clean test tube, the aldehyde reduces the silver(I) complex to silver metal, which deposits as a silver mirror on the inside of the glass. The aldehyde is itself oxidised to a carboxylate ion. Ketones do not reduce Tollens' reagent, so no silver mirror forms — the solution remains colourless.

Worked example: Tollens' Reagent Test: What Happens at Each Stage

Describe and explain the result when ethanal is warmed with Tollens' reagent, and contrast this with the result for propanone.

1. $[Ag(NH_3)_2]^+$ (the diamminesilver(I) complex ion) — Silver(I) ions are the oxidising agent. The ammonia keeps them in solution at alkaline pH, preventing silver oxide precipitation.
2. $Ag^+(aq) + e^- \rightarrow Ag(s)$ — Each silver ion gains one electron and is reduced to silver metal. This is the species that forms the mirror.
3. $RCHO \rightarrow RCOO^-$ (aldehyde oxidised to carboxylate ion) — The aldehyde is the reducing agent — it supplies the electrons that reduce Ag+ to Ag. The carbonyl carbon is oxidised from oxidation state +1 to +3.
4. Silver mirror forms on the inside of the test tube; solution may become colourless. — This is a positive result. The silver mirror is the key observation to state in an exam answer.
5. No silver mirror forms; solution remains colourless. — Propanone is a ketone and cannot be oxidised under these mild conditions. No electrons are donated to Ag+, so no silver deposits.

Answer: Ethanal gives a silver mirror (positive result); propanone gives no change (negative result). The difference arises because aldehydes are oxidised by Tollens' reagent whereas ketones are not.

Fehling's solution works on the same principle — it uses a different mild oxidising agent to distinguish aldehydes from ketones. Fehling's solution is an alkaline solution containing copper(II) ions complexed with tartrate ions (to keep them in solution at high pH). When warmed with an aldehyde, the copper(II) ions ($Cu^{2+}$, blue) are reduced to copper(I) oxide ($Cu_2O$), which precipitates as a brick-red solid. Ketones give no reaction — the solution remains blue.

Both tests rely on the same underlying chemistry: the aldehyde acts as a reducing agent, donating electrons to the metal ion in the reagent. Ketones have no hydrogen atom on the carbonyl carbon, so they cannot be oxidised to a carboxylic acid under these mild conditions.

7. Exam Technique and Common Mistakes

The most common mark-losing errors on this topic fall into two categories: imprecise observations and missing the link between experimental technique and product identity. The guidance below targets both.

Across this lesson, the central thread has been control: controlling the oxidation method to select your product, and using controlled chemical tests to identify what you have made. The technique — distillation versus reflux — is as important as the reagent itself, and the colour changes in Tollens' and Fehling's tests are precise observations that must be stated accurately to score marks.