Aldehydes, Ketones, and Carboxylic Acids

Definitions, Structures, Properties, Preparation, Nomenclature, and Uses

Aldehydes, ketones, and carboxylic acids are among the most important classes of organic compounds in chemistry. These compounds contain the carbonyl group (C=O) and play a crucial role in biological systems, industrial manufacturing, pharmaceuticals, food processing, and laboratory chemistry.

Because they share the carbonyl functional group, aldehydes and ketones exhibit similar chemical behavior. However, aldehydes are generally more reactive than ketones due to the presence of a hydrogen atom attached directly to the carbonyl carbon.

What Are Carbonyl Compounds?

Carbonyl compounds are organic molecules that contain a carbon–oxygen double bond (C=O).
This group is central to many biochemical processes and industrial reactions.

Major carbonyl compounds include:

• Aldehydes

• Ketones

• Carboxylic acids

Aldehydes

Definition of Aldehydes

Aldehydes are organic compounds that contain the aldehyde functional group (–CHO).

General Formula

R–CHO, where:

• R = alkyl or aryl group

• –CHO = carbonyl carbon bonded to hydrogen

Preparation of Aldehydes

Rosenmund Reduction

Aldehydes are prepared by the controlled reduction of acid chlorides using hydrogen in the presence of palladium catalyst poisoned with barium sulfate.

Reaction:

$$\text{RCOCl} + \text{H}_2 \xrightarrow[\text{BaSO}_4]{\text{Pd}} \text{RCHO} + \text{HCl}$$

• Used mainly for aromatic aldehydes

• Not suitable for ketones or formaldehyde

Properties of Aldehydes

• Carbonyl carbon is sp² hybridized

• Small aldehydes are soluble in water

• Polar due to electron-withdrawing oxygen

• Tend to undergo polymerization

• More reactive than ketones

Nomenclature of Aldehydes (IUPAC)

1. Aliphatic aldehydes: suffix –al

   • CH₃CH₂CH₂CHO → Butanal

2. Cyclic aldehydes: suffix –carbaldehyde

   • C₆H₁₁CHO → Cyclohexanecarbaldehyde

3. As a substituent: prefix oxo-

   • HOOC–CH₂–CHO → 3-Oxopropanoic acid

Common Aldehydes

Formula	Common Name	IUPAC Name
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
(CH₃)₂CHCHO	Isobutyraldehyde	2-Methylpropanal
CH₃CH=CHCHO	Crotonaldehyde	But-2-enal

Uses of Aldehydes

• Formaldehyde as disinfectant and preservative

• Manufacturing plastics and resins

• Silvering of mirrors

• Benzaldehyde in perfumes and dyes

Ketones

Definition of Ketones

Ketones are organic compounds containing the carbonyl group where the carbonyl carbon is bonded to two carbon atoms.

General Formula

R–CO–R′

Structure and Nature

• Carbonyl carbon is sp² hybridized

• Trigonal planar structure (≈120°)

• Polar due to C=O bond

• Less reactive than aldehydes

Preparation of Ketones

From Acid Chlorides using Dialkyl Cadmium

Step 1: Formation of Dialkyl Cadmium

$$2\text{R–MgX} + \text{CdCl}_2 → \text{R}_2\text{Cd} + 2\text{MgXCl}$$

Step 2: Ketone Formation

$$2\text{RCOCl} + \text{R}_2\text{Cd} → 2\text{R–CO–R} + \text{CdCl}_2$$

• Useful for preparing mixed ketones

Properties of Ketones

• High boiling points

• Cannot form intermolecular hydrogen bonds

• Large dipole moments

• Form cyanohydrins with HCN

• Form bisulfite addition compounds

Nomenclature of Ketones

• Suffix: –anone

• Position of C=O indicated by number

Formula	Common Name	IUPAC Name
CH₃COCH₃	Acetone	Propanone
CH₃COCH₂CH₃	Methyl ethyl ketone	Butanone
CH₃CH₂COCH₂CH₃	Diethyl ketone	Pentan-3-one
(CH₃)₂CHCOCH₃	Isopropyl methyl ketone	3-Methylbutan-2-one

Uses of Ketones

• Solvents (acetone, butanone)

• Pharmaceutical synthesis

• Polymer production (Perspex)

Carboxylic Acids

Definition of Carboxylic Acids

Carboxylic acids are organic compounds containing the carboxyl group (–COOH).

General Formula

R–COOH

Preparation of Carboxylic Acids

Oxidation of Primary Alcohols

$$\text{RCH}_2\text{OH} + [O] → \text{RCOOH}$$

Example:

$$\text{CH}_3(\text{CH}_2)_8\text{CH}_2\text{OH} → \text{CH}_3(\text{CH}_2)_8\text{COOH}$$

Oxidizing agents:

• KMnO₄

• K₂Cr₂O₇

• CrO₃

Properties of Carboxylic Acids

• Strong intermolecular hydrogen bonding

• Form stable dimers

• Higher boiling points than alcohols

• More acidic than phenols, less than mineral acids

• Soluble in organic solvents

Nomenclature of Carboxylic Acids

• Suffix: –oic acid

• –COOH always at position 1

Formula	Common Name	IUPAC Name
HCOOH	Formic acid	Methanoic acid
CH₃COOH	Acetic acid	Ethanoic acid
CH₃CH₂COOH	Propionic acid	Propanoic acid
C₆H₅COOH	Benzoic acid	Benzenecarboxylic acid

Frequently Asked Questions (FAQs)

1. Do aldehydes react to form carboxylic acids?

Yes. Aldehydes are readily oxidized to carboxylic acids.

2. Are aldehydes and ketones carboxylic acid derivatives?

No. Carboxylic acid derivatives contain a heteroatom directly attached to the carbonyl carbon.

3. Which is more acidic: aldehydes or ketones?

Aldehydes are more acidic due to lower electron-donating effects.

4. What is Schiff’s test used for?

Schiff’s test is used to identify aldehydes.

5. How are aldehydes purified?

By steam distillation, sublimation, or crystallization.

Final Conclusion

Aldehydes, ketones, and carboxylic acids are indispensable carbonyl compounds with wide-ranging applications in chemistry, biology, medicine, and industry. Mastery of their structures, reactions, nomenclature, and uses is essential for success in organic chemistry.

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