Aldehydes, Ketones, and Carboxylic Acids: Structure, Nomenclature, and Properties

Aldehydes, Ketones, and Carboxylic Acids

Introduction to Carbonyl Compounds

Organic compounds containing the carbonyl group (C=O) are fundamental in organic chemistry. The three main classes discussed here are aldehydes, ketones, and carboxylic acids. Each class is defined by the specific arrangement of atoms around the carbonyl group.

• Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom.

• Ketones: The carbonyl carbon is bonded to two other carbon atoms.

• Carboxylic acids: The carbonyl carbon is bonded to a hydroxyl group (-OH).

General Structures of Carbonyl Compounds

Function Group	Aldehydes	Ketones	Carboxylic Acids
General Structure	R-CHO	R-CO-R'	R-COOH

Nomenclature of Aldehydes

Aldehydes are named using the following IUPAC rules:

1. Identify the longest continuous chain of carbons with the acyl or carbonyl carbon as part of the chain.

2. Number the chain so that the carbonyl (acyl) carbon is always at position 1.

3. Locate and identify all substituents; indicate their positions by the appropriate numbers.

4. Name the compound by prefixing the substituents (in alphabetical order) and adding the suffix -al to the parent alkane name.

5. If the aldehyde group is attached to a ring, use the suffix carbaldehyde.

6. For common names (up to 5 carbons), use the parent chain name with the suffix -aldehyde (e.g., formaldehyde, acetaldehyde).

Example: 2-Bromo-3-methylpentanal

Common Names of Aldehydes

• Derived from Latin roots or the parent alkane name.

• Substituents are named as derivatives of the straight-chain parent compound.

• The carbon bonded to the carbonyl group is the α-carbon, the next is β, and so on.

IUPAC Name	Common Name	Formula
Methanal	Formaldehyde	HCHO
Ethanal	Acetaldehyde	CH3CHO
Propanal	Propionaldehyde	CH3CH2CHO
Butanal	Butyraldehyde	CH3CH2CH2CHO
Pentanal	Valeraldehyde	CH3CH2CH2CH2CHO

Nomenclature of Ketones

Ketones are named using the following IUPAC rules:

1. Identify the longest continuous chain of carbons with the carbonyl carbon as one of the carbons in the chain.

2. Number the chain so that the carbonyl carbon has the lowest possible number.

3. Identify and name substituents, listing them in alphabetical order and prefixing their carbon number.

4. Drop the 'e' from the parent alkane and add the suffix -one.

5. If more than one carbonyl group is present, use the appropriate Greek prefixes (di-, tri-, etc.).

Example: 3-Chloro-2-butanone

Common Names of Ketones

• The smallest ketone is propanone (acetone).

• For chains of 4 or more carbons, name the alkyl groups on either side of the carbonyl alphabetically, followed by 'ketone' (e.g., methyl ethyl ketone).

Common Name	Structure
Acetone	CH3COCH3
Methyl ethyl ketone	CH3COC2H5
Di-n-butyl ketone	C4H9COC4H9

Physical Properties of Aldehydes and Ketones

Boiling Points

• Aldehydes and ketones have higher boiling points than hydrocarbons and ethers of similar molecular weight due to dipole-dipole interactions.

• They have lower boiling points than alcohols because they cannot form intermolecular hydrogen bonds.

Example:

Compound	Boiling Point (°C)
Butane	-0.5
Propanal	49
1-Propanol	97

Water Solubility

• Short-chain aldehydes and ketones are soluble in water due to hydrogen bonding with water molecules.

• Solubility decreases as the hydrocarbon chain length increases.

Applications:

• Aldehydes and ketones are found in plants and animals.

• Many have distinctive odors and are used in perfumes and flavorings.

• Some are used in medicines.

Carboxylic Acids

Carboxylic acids contain the carboxyl group (-COOH). They are classified as aliphatic (alkyl group attached) or aromatic (aryl group attached).

• Carboxylic acids are very abundant in nature and were among the first organic substances isolated.

• They are formed by the hydrolysis of fats and oils.

• Carboxylic acids are weak acids.

Common Names for Carboxylic Acids

• Formic acid (from ants): HCOOH

• Acetic acid (from vinegar): CH3COOH

• Propionic acid: CH3CH2COOH

• Butyric acid: CH3CH2CH2COOH

• Caproic acid: C5H11COOH

IUPAC Nomenclature of Carboxylic Acids

1. Identify the longest carbon chain containing the carboxyl group.

2. Number the chain so that the carboxyl carbon is carbon 1.

3. Name substituents and indicate their positions.

4. Replace the 'e' of the parent alkane with 'oic acid'.

Important Carboxylic Acids

• Lactic acid (2-hydroxypropanoic acid): Found in sour milk.

• Oleic acid (9-octadecenoic acid): Found in butter; a monounsaturated fatty acid.

• Linoleic acid: A polyunsaturated fatty acid found in butter.

• Phthalic acids: Benzene derivatives with two carboxyl groups. Isomers include ortho- (phthalic), meta- (isophthalic), and para- (terephthalic) acids.

Summary Table: Key Functional Groups

Class	General Formula	Suffix	Example
Aldehyde	R-CHO	-al	Propanal
Ketone	R-CO-R'	-one	Butanone
Carboxylic Acid	R-COOH	-oic acid	Ethanoic acid

Key Equations and Concepts

• General formula for an aldehyde: $RCHO$

• General formula for a ketone: $RCOR'$

• General formula for a carboxylic acid: $RCOOH$

• Boiling point trend: $\text{Alcohol} > \text{Aldehyde/Ketone} > \text{Alkane}$ (for similar molecular weights)

Additional info: The study of these functional groups is essential for understanding organic reactivity, synthesis, and the properties of many biomolecules and industrial chemicals.