13.7.5 Aldehydes and Ketones

This topic explains aldehydes and ketones — two classes of organic compounds that both contain the carbonyl group (C=O).

Definition

Aldehydes

An aldehyde has at least one hydrogen atom attached to the carbonyl carbon atom.

Simple explanation: In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen.

Ketones

A ketone has two hydrocarbon groups (alkyl) bonded to the carbonyl carbon atom.

Simple explanation: In a ketone, the carbonyl carbon is bonded to two carbon groups.

Main Idea

Both aldehydes and ketones contain the carbonyl group (C=O).

• Aldehyde: R–CHO (at least one H attached to C=O)
• Ketone: R–CO–R' (two alkyl groups attached to C=O)

Simple explanation: The difference is what is attached to the C=O group — H (aldehyde) or two carbons (ketone).

Core Concept

The Carbonyl Group

The carbonyl group is:

C=O

• A carbon atom double-bonded to an oxygen atom.
• Present in both aldehydes and ketones.
• The functional group that gives these compounds their characteristic properties.

Simple explanation: The C=O group is the "heart" of aldehydes and ketones.

Key Difference

Detailed Explanation

Aldehydes

Definition: An aldehyde has at least one hydrogen atom attached to the carbonyl carbon atom.

Functional group: –CHO

General formula: R–CHO (where R = H or alkyl group)

Structure:

R–C(=O)–H

Examples from the book:

Simple explanation: Aldehydes always have an H attached to the C=O group.

Ketones

Definition: A ketone has two hydrocarbon groups (alkyl) bonded to the carbonyl carbon atom.

Functional group: –CO–

General formula: R–CO–R' (where R and R' are alkyl groups)

Structure:

R–C(=O)–R'

Example from the book:

Simple explanation: Ketones have two carbon groups attached to the C=O.

The Book's Question

The book asks:

> "Which of the above compound is an aldehyde? Which is a ketone?"

• **H–C(=O)–H** (formaldehyde)
• **CH₃–C(=O)–CH₃** (acetone)

• **H–C(=O)–H** is an **aldehyde** (H attached to C=O).
• CH₃–C(=O)–CH₃ is a ketone (two CH₃ groups attached to C=O).

Simple explanation: If C=O has an H, it's an aldehyde; if it has two carbons, it's a ketone.

Naming Aldehydes and Ketones

• Named by replacing the -e of the alkane with -al.
• Examples: methanal, ethanal, propanal.

• Named by replacing the -e of the alkane with -one.
• Examples: propanone, butanone.

Simple explanation: Aldehydes end in -al; ketones end in -one.

Physical Properties

Simple explanation: Both have lower boiling points than alcohols because they cannot form hydrogen bonds with each other.

Chemical Properties

Both aldehydes and ketones undergo addition reactions and oxidation reactions.

• Aldehydes are easily oxidized to carboxylic acids.
• Ketones are resistant to oxidation.

• Tollens' test — aldehydes give a silver mirror; ketones do not.
• Fehling's test — aldehydes give a red precipitate; ketones do not.

Simple explanation: Aldehydes are easily oxidized; ketones are not. This is how we tell them apart.

Uses of Aldehydes and Ketones

Simple explanation: Aldehydes and ketones are used as solvents, preservatives, and in making plastics.

Intermediate Level

Structure of Aldehydes and Ketones

Aldehyde:

R–C(=O)–H

- Carbonyl carbon bonded to R (alkyl or H) and H.

Ketone:

R–C(=O)–R'

- Carbonyl carbon bonded to two alkyl groups.

Simple explanation: The difference is what's attached to the carbonyl carbon.

Naming in Detail

Simple explanation: Aldehydes end in -al; ketones end in -one.

Why Aldehydes Are More Reactive Than Ketones

• Aldehydes have less steric hindrance (only one alkyl group) → easier for reagents to attack the carbonyl carbon.
• Aldehydes have a partial positive charge on the carbonyl carbon that is more accessible.
• So aldehydes undergo oxidation easily, while ketones are resistant.

Simple explanation: Aldehydes are less crowded, so they react more easily.

Tests to Distinguish Aldehydes from Ketones

Simple explanation: Aldehydes react with Tollens' and Fehling's reagents; ketones do not.

Uses in Detail

Important Points

• - Aldehydes and ketones both contain the carbonyl group (C=O).
• - Aldehyde: at least one H atom attached to the carbonyl carbon; functional group –CHO.
• - Ketone: two hydrocarbon groups (alkyl) bonded to the carbonyl carbon; functional group –CO–.
• - Examples:
• - Methanal (formaldehyde): HCHO — aldehyde
• - Ethanal (acetaldehyde): CH₃CHO — aldehyde
• - Propanone (acetone): CH₃COCH₃ — ketone
• - Naming: aldehydes end in -al; ketones end in -one.
• - Physical properties: lower boiling points than alcohols; soluble in water (small).
• - Chemical properties: aldehydes are easily oxidized; ketones are resistant.
• - Tests: Tollens' test (silver mirror) and Fehling's test (red precipitate) distinguish aldehydes from ketones.
• - Uses: preservatives, disinfectants, solvents, production of plastics.

Remember

• - **Aldehyde** = **H** on C=O.
• - **Ketone** = **two C's** on C=O.
• - Both have C=O (carbonyl group).
• - Functional groups: –CHO (aldehyde); –CO– (ketone).
• - Examples:
• - Formaldehyde: HCHO
• - Acetaldehyde: CH₃CHO
• - Acetone: CH₃COCH₃
• - Naming: -al (aldehyde); -one (ketone).
• - Aldehydes are easily oxidized; ketones are resistant.

Common Mistakes

• 1. Confusing aldehydes with ketones. Aldehydes have H on C=O; ketones have two C's on C=O.
• 2. Confusing aldehydes with alcohols. Alcohols have –OH; aldehydes have –CHO.
• 3. Confusing ketones with ethers. Ketones have –CO–; ethers have –O–.
• 4. Forgetting the carbonyl group. Both aldehydes and ketones contain C=O.
• 5. Thinking aldehydes and ketones are the same. They have different structures and different reactivity.
• 6. Forgetting the tests. Aldehydes react with Tollens' and Fehling's reagents; ketones do not.

Quick Revision

• Aldehydes and ketones both contain the carbonyl group (C=O).
• Aldehyde: an aldehyde has at least one hydrogen atom attached to the carbonyl carbon atom; functional group –CHO.
• Ketone: a ketone has two hydrocarbon groups (alkyl) bonded to the carbonyl carbon atom; functional group –CO–.
• Examples:
• Methanal (formaldehyde): HCHO — aldehyde
• Ethanal (acetaldehyde): CH₃CHO — aldehyde
• Propanone (acetone): CH₃COCH₃ — ketone
• Naming: aldehydes end in -al; ketones end in -one.
• Physical properties: lower boiling points than alcohols; soluble in water (small).
• Chemical properties: aldehydes are easily oxidized to carboxylic acids; ketones are resistant to oxidation.
• Tests: Tollens' test (silver mirror) and Fehling's test (red precipitate) distinguish aldehydes from ketones.
• Uses: preservatives, disinfectants, solvents, production of plastics.

60-Second Revision

• **Aldehyde** = **H** on C=O.
• **Ketone** = **two C's** on C=O.
• Both have C=O (carbonyl group).
• Examples: HCHO (aldehyde), CH₃COCH₃ (ketone).
• Warning: aldehydes are easily oxidized; ketones are not — this is how we tell them apart.