Aldehydes, Ketones & Carboxylic Acids - General Preparation of Aldehydes and Ketones

• Aldehydes and ketones are organic compounds that contain a carbonyl group (C=O).
• The carbonyl group in aldehydes is located at the end of the carbon chain, while in ketones it is located in the middle.

Preparation of Aldehydes

1. Oxidation of Primary Alcohols:
   • Primary alcohols can be oxidized to form aldehydes using mild oxidizing agents such as pyridinium chlorochromate (PCC) or pyridinium dichromate (PDC).
   • Example: Ethanol can be oxidized to produce acetaldehyde.

2. Oxidation of Primary Aldehydes:
   • Primary aldehydes can be further oxidized to form carboxylic acids using stronger oxidizing agents like potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7).
   • Example: Acetaldehyde can be oxidized to produce acetic acid.

3. Oxidation of Alkyl Benzene:
   • Alkyl benzene compounds can be oxidized using powerful oxidizing agents like chromic acid mixture to form benzoic acid.
   • Example: Toluene can be oxidized to produce benzoic acid.

4. By Ozonolysis of Alkenes:
   • Alkenes can undergo ozonolysis, where ozone (O3) reacts with the double bond to produce aldehydes.
   • Example: Propene can be ozonolyzed to produce propanal.

5. From Cyanohydrins:
   • Cyanohydrins, which are compounds containing both a cyano group (C≡N) and a hydroxyl group (OH), can be hydrolyzed to form aldehydes.
   • Example: Benzoin cyanohydrin can be hydrolyzed to produce benzaldehyde.

Preparation of Ketones

1. Oxidation of Secondary Alcohols:
   • Secondary alcohols can be oxidized using oxidizing agents like potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) to form ketones.
   • Example: Isopropanol can be oxidized to produce acetone.

2. Oxidation of Alkyl Amines:
   • Alkyl amines can be oxidized using strong oxidizing agents like potassium permanganate (KMnO4) to form ketones.
   • Example: Methylethylamine can be oxidized to produce methyl ethyl ketone (MEK).

3. Friedel-Crafts Acylation:
   • Aromatic compounds can be reacted with acyl halides (such as acetyl chloride or benzoyl chloride) in the presence of a Lewis acid catalyst (such as aluminum chloride) to produce ketones.
   • Example: Benzene can be reacted with acetyl chloride to produce acetophenone.

4. Preparation from Carboxylic Acids:
   • Carboxylic acids can be heated with calcium salts to undergo decarboxylation and form ketones.
   • Example: Ethanoic acid can be heated with calcium acetate to produce acetone.

5. From Nitriles:
   • Nitriles can be hydrolyzed using acidic or basic conditions to form ketones.
   • Example: Propionitrile can be hydrolyzed to produce acetone.

Preparation of Aldehydes and Ketones from Alcohols

1. Dehydration of Alcohols:
   • Alcohols can be dehydrated in the presence of acid catalysts to form aldehydes and ketones.
   • Example: Ethanol can be dehydrated to produce ethanal.

2. Clemmensen Reduction:
   • Ketones can be reduced to form aldehydes using zinc amalgam and hydrochloric acid.
   • Example: Propanone can be reduced to produce propanal.

3. Wolff-Kishner Reduction:
   • Aldehydes and ketones can be reduced using hydrazine followed by heating with base to form alkanes.
   • Example: Propanal can be reduced to produce propane.

4. Cannizzaro Reaction:
   • Aldehydes which do not have an alpha hydrogen atom can undergo self-disproportionation in the presence of a strong base to form an alcohol and a carboxylic acid.
   • Example: Benzaldehyde can undergo Cannizzaro reaction to form benzyl alcohol and benzoic acid.

5. Reduction of Nitriles:
   • Nitriles can be reduced using lithium aluminum hydride (LiAlH4) or sodium borohydride (NaBH4) to form aldehydes and ketones.
   • Example: Propionitrile can be reduced to produce propanal.

Preparation of Aldehydes and Ketones from Carboxylic Acids

1. Decarboxylation:
   • Carboxylic acids can undergo decarboxylation when heated to form aldehydes.
   • Example: Oxalic acid can be heated to produce formaldehyde.

2. Oxidative Cleavage of α-Hydroxy Acids:
   • α-Hydroxy acids can be oxidatively cleaved using an oxidizing agent like nitric acid (HNO3) to form aldehydes.
   • Example: Glycolic acid can be oxidized to produce glyoxylic acid.

3. Perkin Reaction:
   • Carboxylic acids can be condensed with anhydrides in the presence of an acid catalyst to form α,β-unsaturated carboxylic acids, which can be further reduced to form aldehydes or ketones.
   • Example: Benzoic acid can be condensed with acetic anhydride to produce cinnamic acid, which can be reduced to produce benzaldehyde.

4. Ozonolysis of Alkynes:
   • Alkynes can undergo ozonolysis, where ozone (O3) reacts with the triple bond to produce carboxylic acids.
   • Example: Ethyne can be ozonolyzed to produce ethanedioic acid.

5. Vilsmeier-Haack Reaction:
   • Aromatic compounds can be reacted with a mixture of formamide (HCONH2) and a strong acid (such as phosphorus oxychloride or phosphoryl chloride) to form aldehydes or ketones.
   • Example: Benzene can be reacted with formamide to produce benzaldehyde.

Mechanism of Oxidation Reactions

1. Oxidation of Primary Alcohols to Aldehydes:
   • In the presence of an oxidizing agent, primary alcohols undergo oxidation to form aldehydes.
   • The mechanism involves the formation of a reactive intermediate known as a Swern complex, followed by rearrangement to produce the aldehyde.
   • Example: Oxidation of ethanol to acetaldehyde.

2. Oxidation of Primary Aldehydes to Carboxylic Acids:
   • Primary aldehydes can be further oxidized to form carboxylic acids.
   • The mechanism involves the formation of a geminal diol intermediate, which further reacts with the oxidizing agent to produce the carboxylic acid.
   • Example: Oxidation of acetaldehyde to acetic acid.

3. Oxidation of Secondary Alcohols to Ketones:
   • Secondary alcohols undergo oxidation to form ketones by directly removing the hydrogen atoms from the alcohol molecule.
   • Example: Oxidation of isopropanol to acetone.

4. Oxidation of Alkyl Benzene to Carboxylic Acids:
   • Alkyl benzene compounds can be oxidized to form carboxylic acids by breaking the aromatic ring and adding an oxygen atom.
   • Example: Oxidation of toluene to benzoic acid.

5. Oxidation of Aldehydes to Carboxylic Acids:
   • Aldehydes can be oxidized to form carboxylic acids by the addition of oxygen atoms.
   • The mechanism involves the formation of an intermediate known as a hemiacetal, which further reacts to form the carboxylic acid.
   • Example: Oxidation of acetaldehyde to acetic acid.

Summary and Key Points

• Aldehydes and ketones can be prepared through various methods, including oxidation of alcohols, amines, and alkyl benzene compounds.
• Aldehydes are often synthesized through the oxidation of primary alcohols or the reduction of oxides.
• Ketones can be synthesized through the oxidation of secondary alcohols or the reduction of carbonyl compounds.
• Other methods such as Friedel-Crafts acylation, hydrolysis of nitriles, and reduction reactions are also used for the preparation of aldehydes and ketones.
• The mechanism of oxidation reactions involves the formation of reactive intermediates, such as Swern complexes and geminal diols.
• Understanding the synthesis of aldehydes and ketones is essential for understanding their properties and reactions in organic chemistry.

Reactions of Aldehydes

• Aldehydes can undergo a variety of reactions due to the presence of the carbonyl group.
• Some important reactions of aldehydes include:

1. Nucleophilic Addition Reactions:
   • Aldehydes can react with nucleophiles such as water, alcohols, and primary amines to form hydrates, hemiacetals, and imines, respectively.
   • Example: Formaldehyde can react with water to form methanediol.

2. Reduction to Primary Alcohols:
   • Aldehydes can be reduced by reducing agents such as hydrides to form primary alcohols.
   • Example: Formaldehyde can be reduced to produce methanol.

3. Reaction with Grignard Reagents:
   • Aldehydes can react with Grignard reagents, which are organomagnesium compounds, to form secondary alcohols.
   • Example: Formaldehyde can react with ethylmagnesium bromide to produce 2-propanol.

4. Oxidation to Carboxylic Acids:
   • Aldehydes can be further oxidized to form carboxylic acids using strong oxidizing agents like potassium permanganate or acidified potassium dichromate.
   • Example: Formaldehyde can be oxidized to produce formic acid.

5. Reaction with Ammonia:
   • Aldehydes can react with ammonia to form a class of compounds called aldimines or imines.
   • Example: Benzaldehyde can react with ammonia to produce benzylamine.