---
title: "Aldehydes and Ketones: Nucleophilic Addition and the Tests That Separate Them"
description: "The carbonyl carbon carries a partial positive charge, so nucleophiles add across the double bond. Aldehydes oxidise further and ketones do not, which is exactly what the bench tests exploit."
canonical: https://lightmysky.com/learn/science/aldehydes-and-ketones-nucleophilic-addition-and-the-tests-that-separate-them-mt_bVcJ0UaXk6
source: https://lightmysky.com/learn/science/aldehydes-and-ketones-nucleophilic-addition-and-the-tests-that-separate-them-mt_bVcJ0UaXk6.md
retrieved: 2026-09-12
---

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# Aldehydes and Ketones: Nucleophilic Addition and the Tests That Separate Them

The carbonyl carbon carries a partial positive charge, so nucleophiles add across the double bond. Aldehydes oxidise further and ketones do not, which is exactly what the bench tests exploit.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/aldehydes-and-ketones-nucleophilic-addition-and-the-tests-that-separate-them-mt_bVcJ0UaXk6

## Ready when they can

- Draws the addition mechanism for cyanide, showing the carbonyl double bond breaking
- Explains why the product of cyanide addition exists as two mirror-image forms
- Predicts the result of Tollens' reagent and of Fehling's solution for an aldehyde and for a ketone
- Names the reduction product of each and the reagent that gets there

## Lesson: Aldehydes, ketones and their tests

Oxygen drags the shared electrons toward itself, leaving the carbonyl carbon short of charge and open to attack. When cyanide adds, the double bond breaks and the CN lands on carbon while oxygen ends up as OH: a hydroxynitrile.

The carbonyl is flat, so cyanide strikes as easily from above as from below. With propanal the new carbon then holds four different groups, so the two attacks give mirror forms in equal amounts: a racemic mixture. Propanone breaks the pattern: its new carbon holds two identical methyl groups, so only one product forms and there is no mirror pair.

**Example.** Hold propanal next to propanone. Tollens reagent gives propanal a silver mirror and leaves propanone unchanged, while Fehling solution turns from blue to brick red with propanal and stays blue with propanone.

Reduction draws the line again: aldehydes give primary alcohols and ketones give secondary alcohols, using a reducing agent such as sodium tetrahydridoborate.

**Recap.** A charged carbon invites attack, a flat carbonyl can give mirror pairs, oxidation tests split the families, and reduction names the alcohol.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Alcohols: Classification, Oxidation and Dehydration](https://lightmysky.com/learn/science/alcohols-classification-oxidation-and-dehydration-mt_FleYaf40y3)
- [Nucleophilic Substitution in Haloalkanes](https://lightmysky.com/learn/science/nucleophilic-substitution-in-haloalkanes-mt_QG-nKlnVvW)
- [Complex Ions, Ligands and Their Shapes](https://lightmysky.com/learn/science/complex-ions-ligands-and-their-shapes-mt_xxWYYqgZp_)

## Opens up

- [Nucleophilic Addition and the Tetrahedral Intermediate](https://lightmysky.com/learn/science/nucleophilic-addition-and-the-tetrahedral-intermediate-mt_N9Eydek3s7)
- [Carboxylic Acids, Esters and Hydrolysis](https://lightmysky.com/learn/science/carboxylic-acids-esters-and-hydrolysis-mt_QLxUs5cOH2)