Aldehydes, ketones, and carboxylic acids 

1.0What are Aldehydes, Ketones, and Carboxylic Acids?

Aldehydes, ketones, and carboxylic acids are three important classes of organic compounds that contain a carbonyl group (>C=O). The difference between them lies in the atoms attached to this carbonyl carbon.

• In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom.
• In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups.
• In carboxylic acids, the carbonyl carbon is bonded to a hydroxyl group (-OH) and an alkyl or aryl group.

2.0What are Aldehydes?

Aldehydes are organic compounds with the general formula R-CHO. The functional group is the formyl group, which consists of a carbonyl group with one side attached to a hydrogen atom and the other to an alkyl (R) or aryl group.

Preparation of Aldehydes

1. Oxidation of Primary Alcohols: Controlled oxidation of primary alcohols using mild oxidizing agents like PCC (Pyridinium chlorochromate) or anhydrous $Cr{O}_3$ in a non-aqueous solvent. 

• $R-C{H}_{2}OH\stackrel{PCC}{\to }R-CHO$

2. Rosenmund's Reduction: Hydrogenation of an acyl chloride over a palladium catalyst supported on barium sulfate .  This is a selective reduction. 

• $R-COCl+H_2\stackrel{Pd/BaS{O}_4}{\to }R-CHO+HCl$

3. Stephen Reaction: Reduction of nitriles with and HCl followed by hydrolysis. 

• $R-C\equiv N\stackrel{SnC{l}_2/HCl}{\to }R-CH=NH\stackrel{H_{2}O}{\to }R-CHO+N{H}_3$

Properties of Aldehydes

Physical Properties:

• Lower aldehydes are soluble in water due to hydrogen bonding; solubility decreases with increasing molecular weight.
• They have higher boiling points than hydrocarbons of similar molecular weight due to dipole-dipole interactions.

Chemical Properties:

• Nucleophilic Addition Reactions: Aldehydes undergo addition reactions with nucleophiles like HCN, resulting in cyanohydrins.
• Oxidation: Aldehydes are readily oxidized to carboxylic acids using oxidizing agents like KMnO₄.
• Reduction: Reduction of aldehydes with reducing agents like LiAlH₄ yields primary alcohols.

Nomenclature of Aldehydes

• IUPAC Naming: Aldehydes are named by replacing the '-e' of the parent alkane with '-al'. For example, CH₃CHO is named ethanal.
• Common Names: Often derived from the corresponding carboxylic acids; for instance, HCHO is formaldehyde.

Uses of Aldehydes

• Formaldehyde (HCHO) is used in making Bakelite, a plastic, and as a disinfectant.
• Acetaldehyde $(C{H}_{3}CHO)$ is used in the synthesis of acetic acid and various other chemicals.

3.0What are Ketones?

Ketones are organic compounds with the general formula R-CO-R', where R and R' are alkyl or aryl groups. The carbonyl carbon is bonded to two carbon atoms.

Preparation of Ketones

1. Oxidation of Secondary Alcohols: Secondary alcohols are oxidised using strong oxidizing agents like $K_{2}C{r}_2{O}_{7}orKMn{O}_4$

• $R-CH(OH)-R^{\prime }\stackrel{Oxidation}{\to }R-Co-R^{\prime }$

2. Friedel-Crafts Acylation: Reaction of an acyl chloride or acid anhydride with an aromatic ring in the presence of a Lewis acid, like $AlC{l}_3$

• $C_6{H}_6+C{H}_{3}COCl\stackrel{AlC{l}_3}{\to }{C}_6{H}_{5}COC{H}_3(\text{Acetophenone})$

3. From Acyl Chlorides and Organocadmium Compounds: Reaction of acyl chlorides with dialkylcadmium, which is a less reactive organometallic reagent. 

• $2R^{\prime }COCl+(R{)}_{2}Cd\to 2R^{\prime }COR+CdC{l}_2$

Properties of Ketones

• Physical Properties: Ketones are generally liquids at room temperature. They have higher boiling points than aldehydes of similar mass due to a stronger dipole-dipole interaction. They are not as reactive as aldehydes.
• Chemical Properties: They undergo nucleophilic addition reactions. Ketones are resistant to mild oxidation and can be oxidized only by strong oxidizing agents. They are reduced to secondary alcohols.

Nomenclature of Ketones

• IUPAC Naming: Ketones are named by replacing the '-e' of the parent alkane with '-one'. For example, CH₃COCH₃ is named propanone.
• Common Names: Derived from the names of the alkyl groups attached to the carbonyl group; for instance, CH₃COCH₃ is acetone.

Uses of Ketones

• Formaldehyde: Making bakelite, urea-formaldehyde resin, disinfectant, and preservative for biological specimens.
• Benzaldehyde: Perfumes, dye industry, flavouring agent.
• Acetaldehyde: Production of acetic acid, drugs, and perfumes.

4.0Carboxylic Acids

What are Carboxylic Acids?

Carboxylic acids are organic compounds containing the carboxyl functional group, -COOH. The general formula is R-COOH, where R is an alkyl, alkenyl, aryl, or other group.

Preparation of Carboxylic Acids

1. Oxidation of Primary Alcohols and Aldehydes: Using strong oxidizing agents like 

• $KMn{O}_4/H^{+}or{K}_{2}C{r}_2{O}_7/H^{+}$
• $R-C{H}_{2}OH\stackrel{KMn{O}_4}{\to }R-COOH$
• $R-CHO\stackrel{KMn{O}_4}{\to }R-COOH$

2. From Nitriles and Amides: Hydrolysis of nitriles with an acid or base catalyst, followed by acidification.

• $R-C\equiv N\stackrel{H_3{O}^{+}orO{H}^{-}}{\to }R-COOH$

3. From Grignard Reagents: Reaction of a Grignard reagent with carbon dioxide, followed by hydrolysis.

• $R-MgX+C{O}_2\to COO-MgX\stackrel{H_{2}O/H^{+}}{\to }R-COOH$

Properties of Carboxylic Acids

• Physical Properties: Lower carboxylic acids are colorless liquids with sharp, sour smells. Due to the presence of the -OH group, they can form strong intermolecular hydrogen bonds, often existing as stable dimers. This gives them significantly higher boiling points than aldehydes, ketones, and even alcohols of similar molar mass.
• Chemical Properties: They are acidic in nature and can react with bases, metals, and carbonates to form salts. They undergo nucleophilic substitution reactions to form derivatives like esters, amides, and acid chlorides.

Nomenclature of Carboxylic Acids

• The IUPAC name is derived by replacing the suffix '-e' of the parent alkane with '-oic acid'. The carboxyl carbon is always C-1. Example: $C{H}_{3}COOH$ is named Ethanoi

Uses of carboxylic Acid

• Acetic acid is used in vinegar and as a solvent.
• Carboxylic acids are precursors in the synthesis of polymers, pharmaceuticals, and perfumes.