Chapter 12: Aldehydes and Ketones – Chemistry 2nd Year Notes

This chapter covers Aldehydes and Ketones from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Both contain the carbonyl group (C=O). In an aldehyde the carbonyl carbon is at the end of the chain and carries a hydrogen (R-CHO), while in a ketone it is in the middle, joined to two carbon groups (R-CO-R). The carbonyl group is polar, which makes these compounds react readily with nucleophiles.

A central theme of this chapter is how to tell aldehydes and ketones apart using tests such as Tollens, Fehling, Benedict and the iodoform test.

Learning Objectives

• Identify the carbonyl group and distinguish aldehydes from ketones.
• Describe methods of preparing aldehydes and ketones.
• Explain nucleophilic addition reactions of the carbonyl group.
• Describe the oxidation and reduction of aldehydes and ketones.
• Use Tollens, Fehling, Benedict and the iodoform tests to distinguish them.
• State the important uses of methanal and acetone.

Key Concepts

The Carbonyl Group

The carbonyl group, C=O, is the functional group of both aldehydes and ketones. Because oxygen is more electronegative than carbon, the group is polar: the carbon carries a partial positive charge and the oxygen a partial negative charge. The positive carbon is open to attack by nucleophiles, so nucleophilic addition is the characteristic reaction.

Preparation and Nucleophilic Addition

Aldehydes and ketones are simply prepared by the controlled oxidation of alcohols: a primary alcohol gives an aldehyde and a secondary alcohol gives a ketone. Their typical reaction is nucleophilic addition, in which a nucleophile adds to the positive carbon and the pi bond opens; examples include addition of hydrogen cyanide to give cyanohydrins, addition of sodium hydrogen sulphite, and addition of Grignard reagents which give alcohols. Aldehydes are generally more reactive than ketones because ketones have two bulky groups that hinder the nucleophile.

Oxidation, Reduction and Tests

Aldehydes are easily oxidised to carboxylic acids because the carbonyl carbon carries a hydrogen, whereas ketones are not oxidised under mild conditions. On reduction aldehydes give primary alcohols and ketones give secondary alcohols. The distinguishing tests rely on the easy oxidation of aldehydes: Tollens reagent gives a silver mirror with aldehydes; Fehling and Benedict solutions give a red precipitate with aldehydes; and the iodoform test gives a yellow precipitate with compounds containing the methyl ketone group and with ethanol.

Uses

Methanal (formaldehyde) is used as a preservative (formalin) and to make Bakelite and other plastics; ethanal is used to make ethanoic acid; and acetone (propanone) is a very common solvent, for example in nail-polish remover.

Important Definitions

Formulas & Rules

Item	Fact
Aldehyde / Ketone	R-CHO (end) / R-CO-R (middle); both contain C=O
Reactivity	aldehydes greater than ketones in nucleophilic addition
Oxidation	aldehyde to acid (easy); ketone not easily
Reduction	aldehyde to primary alcohol; ketone to secondary alcohol
Tests	Tollens silver mirror and Fehling red ppt for aldehydes; iodoform for methyl ketones

Diagrams & Illustrations

The carbonyl group: a diagram showing the polar C=O bond with a positive carbon and negative oxygen, and the difference between aldehydes (carbonyl at the chain end) and ketones (carbonyl in the middle).

Distinguishing tests: a chart showing Tollens (silver mirror), Fehling and Benedict (red precipitate) positive for aldehydes and negative for ketones, plus the iodoform test for methyl ketones.

Solved Examples & Numericals

Example 1: Distinguishing test

How would you distinguish ethanal from propanone? Use Tollens reagent; ethanal (an aldehyde) gives a silver mirror while propanone (a ketone) does not. Fehling solution can be used the same way.

Example 2: Reduction

What is formed when propanone is reduced? Propanone (a ketone) is reduced to propan-2-ol, a secondary alcohol.

Example 3: Iodoform test

Which of ethanal and benzaldehyde gives a positive iodoform test? Ethanal, because it contains the methyl ketone type group; benzaldehyde does not.

Short Questions & Answers

Long Questions & Answers

Q1: Explain the nucleophilic addition reactions of aldehydes and ketones.

Aldehydes and ketones react mainly by nucleophilic addition because their carbonyl group is polar, with a positive carbon and a negative oxygen. A nucleophile attacks the electron-poor carbon, the pi bond breaks and the electrons move onto the oxygen to give an alkoxide, which then picks up a proton. In this way hydrogen cyanide adds to give a cyanohydrin with one more carbon; sodium hydrogen sulphite adds to give a crystalline addition compound useful for purification; and a Grignard reagent adds and, after hydrolysis, gives an alcohol. Aldehydes generally undergo these additions more readily than ketones, because the two carbon groups of a ketone hinder the approaching nucleophile.

Q2: Describe the tests used to distinguish aldehydes from ketones.

The key difference is that aldehydes are easily oxidised while ketones are not, and the tests are based on this. In the Tollens test the compound is warmed with an ammoniacal silver nitrate solution; an aldehyde reduces the silver ions to metallic silver, which deposits as a bright silver mirror, whereas a ketone gives no mirror. In the Fehling test (and the similar Benedict test) the compound is warmed with a blue copper solution; an aldehyde reduces the copper to a brick-red precipitate of copper(I) oxide, whereas a ketone gives none. A further test is the iodoform test: a compound containing the methyl ketone group, or ethanol, warmed with iodine and sodium hydroxide gives a yellow precipitate of iodoform. Together these tests identify aldehydes, methyl ketones and other ketones.

Q3: Compare the reactivity of aldehydes and ketones and explain the difference.

Both react through their carbonyl group, but aldehydes are generally more reactive, both in nucleophilic addition and in oxidation. There are two reasons. First, a ketone has two alkyl or aryl groups on the carbonyl carbon, and these bulky groups get in the way of an approaching nucleophile, whereas an aldehyde has only one such group and a small hydrogen atom. Second, the alkyl groups of a ketone push electron density toward the carbonyl carbon, reducing its positive charge and making it less attractive to nucleophiles. In oxidation the difference is clearer: the aldehyde carbonyl carbon carries a hydrogen that can be removed, so aldehydes are easily oxidised to carboxylic acids, whereas ketones have no such hydrogen and resist mild oxidation. This is why aldehydes give positive Tollens and Fehling tests while ketones do not.

MCQs with Answers

Quick Revision Summary

• Carbonyl C=O: aldehyde R-CHO (end), ketone R-CO-R (middle); polar (positive carbon, negative oxygen).
• Characteristic reaction is nucleophilic addition (HCN, sodium hydrogen sulphite, Grignard).
• Aldehydes more reactive than ketones (steric plus electronic).
• Oxidation: aldehyde to acid (easy); ketone resists.
• Reduction: aldehyde to primary alcohol; ketone to secondary alcohol.
• Tests: Tollens silver mirror and Fehling red ppt for aldehydes; iodoform for methyl ketones.

Exam Tips

• Learn the three distinguishing tests and their exact positive results.
• State clearly why aldehydes are more reactive and easily oxidised.
• Know which compounds give the iodoform test (methyl ketone group and ethanol).
• Reduction products: aldehyde to primary alcohol, ketone to secondary alcohol.
• Remember uses: formalin and Bakelite (methanal), solvent (acetone).