Aldehydes, Ketones & Carboxylic Acids - Structure and Preparation Mechanisms

  • Aldehydes, ketones, and carboxylic acids are important classes of organic compounds.
  • They contain a carbonyl group (-C=O) as their functional group.
  • The carbonyl carbon in aldehydes and ketones is sp2 hybridized, while in carboxylic acids, it is sp3 hybridized.
  • The presence of the carbonyl group gives these compounds unique chemical properties.
  • In this lecture, we will discuss the structures and preparation mechanisms of aldehydes, ketones, and carboxylic acids.

Aldehydes

  • An aldehyde has a carbonyl group (-C=O) with a hydrogen atom attached to the carbonyl carbon.
  • Aldehydes are named by replacing the -e ending of the parent alkane with -al.
  • Example: Methane → Methanal (Formaldehyde)

Preparation of Aldehydes

  1. Oxidation of primary alcohols: Primary alcohols can be oxidized to form aldehydes using an oxidizing agent such as pyridinium chlorochromate (PCC) or potassium dichromate (K2Cr2O7) in the presence of an acid.
  1. Oxidation of alkyl benzenes: Alkyl benzenes can be oxidized with a strong oxidizing agent, such as potassium permanganate (KMnO4), to form aldehydes.

Ketones

  • A ketone has a carbonyl group (-C=O) with two alkyl or aryl groups attached to the carbonyl carbon.
  • Ketones are named by replacing the -e ending of the parent alkane with -one.
  • Example: Propane → Propanone (Acetone)

Preparation of Ketones

  1. Oxidation of secondary alcohols: Secondary alcohols can be oxidized to form ketones using an oxidizing agent, such as chromic acid (H2CrO4).
  1. Friedel-Crafts acylation: Aromatic compounds can be reacted with acyl chlorides in the presence of a Lewis acid catalyst, such as aluminum chloride (AlCl3), to form ketones.

Carboxylic Acids

  • A carboxylic acid has a carbonyl group (-C=O) and a hydroxyl group (-OH) attached to the same carbon atom, known as the carboxyl group (-COOH).
  • Carboxylic acids are named by replacing the -e ending of the parent alkane with -oic acid.
  • Example: Ethane → Ethanoic acid (Acetic acid)

Preparation of Carboxylic Acids

  1. Oxidation of primary alcohols: Primary alcohols can be further oxidized to form carboxylic acids using a strong oxidizing agent like potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4).
  1. Hydrolysis of nitriles: Nitriles can be hydrolyzed using acid or base to form carboxylic acids.

References

  • Organic Chemistry, 7th Edition by Paula Yurkanis Bruice
  • Organic Chemistry I For Dummies, 2nd Edition by Arthur Winter ``markdown

Reaction of Aldehydes and Ketones with Grignard Reagents

  • Aldehydes and ketones can react with Grignard reagents to form alcohols.
  • The reaction proceeds via the nucleophilic addition of the carbon chain of the Grignard reagent to the carbonyl carbon of the aldehyde or ketone.
  • Example: Ketone + Grignard Reagent → Alcohol Acetone + Ethylmagnesium bromide → 2-Butanol

Reduction of Aldehydes and Ketones

  • Aldehydes and ketones can be reduced to alcohols using reducing agents such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4).
  • The reducing agent donates a hydride ion (H-) to the carbonyl carbon, resulting in the formation of an alcohol.
  • Example: Aldehyde + Sodium borohydride → Alcohol Formaldehyde + Sodium borohydride → Methanol ``

``markdown

Oxidation of Aldehydes and Ketones

  • Aldehydes can be further oxidized to carboxylic acids using strong oxidizing agents, such as potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4).
  • Ketones, on the other hand, are resistant to oxidation since they do not have a hydrogen atom attached to the carbonyl carbon.
  • Example: Aldehyde + Oxidizing Agent → Carboxylic Acid Ethanal + Potassium dichromate → Ethanoic acid

Aldol Condensation

  • Aldehydes and ketones can undergo aldol condensation to form β-hydroxyaldehydes or β-hydroxyketones.
  • The reaction involves the nucleophilic addition of an enolate ion, generated from the carbonyl compound, to the carbonyl carbon of another molecule of the same carbonyl compound.
  • Example: Aldehyde or Ketone + Base → Aldol Acetaldehyde + Sodium hydroxide → Acetaldehyde aldol ``

``markdown

Reactions of Carboxylic Acids

  • Carboxylic acids can undergo various reactions due to the presence of both a carbonyl group and a hydroxyl group.
  • Some important reactions of carboxylic acids include:
    1. Esterification: Reaction with alcohols in the presence of an acid as a catalyst to form esters.
    2. Decarboxylation: Thermal or catalytic decarboxylation of carboxylic acids to form carbon dioxide and an alkane.
    3. Substitution: Reaction with a nucleophile to replace the hydroxyl group with another functional group.
    4. Acid-Base Reactions: Reacting with bases to form carboxylate salts. ``

``markdown

Hydrolysis of Esters

  • Esters can be hydrolyzed to form carboxylic acids and alcohols in the presence of an acid or a base.
  • Acidic hydrolysis: Esters are treated with a dilute acid, such as sulfuric acid (H2SO4), to produce a carboxylic acid and an alcohol.
  • Basic hydrolysis: Esters are treated with a strong base, such as sodium hydroxide (NaOH), to produce a carboxylate salt and an alcohol.
  • Example: Ethyl acetate + Hydrochloric acid → Acetic acid + Ethanol ``

``markdown

Reaction of Carboxylic Acids with Alcohols

  • Carboxylic acids can react with alcohols in the presence of an acid catalyst to form esters.
  • This reaction is known as esterification.
  • The hydroxyl group of the carboxylic acid reacts with the hydroxyl group of the alcohol, resulting in the formation of an ester and water.
  • Example: Carboxylic Acid + Alcohol → Ester Ethanoic acid + Methanol → Methyl ethanoate ``

``markdown

Decarboxylation

  • Decarboxylation is the process of removing a carboxyl group (-COOH) from a carboxylic acid to form carbon dioxide (CO2) and an alkane.
  • This reaction usually requires high temperatures or catalysts.
  • Example: Carboxylic Acid → CO2 + Alkane Ethanoic acid → CO2 + Methane ``

``markdown

Reaction of Carboxylic Acids with Amines

  • Carboxylic acids can react with amines to form amides.
  • The reaction involves the replacement of the hydroxyl group of the carboxylic acid with an amino group from the amine.
  • Example: Carboxylic Acid + Amine → Amide Acetic acid + Methylamine → N-Methylacetamide ``

``markdown

Reaction of Carboxylic Acids with Metals

  • Carboxylic acids react with reactive metals, such as sodium (Na), to form carboxylate salts and hydrogen gas (H2).
  • The carboxylate salts have the general formula RCOO-Na+, where R represents the alkyl or aryl group.
  • Example: Carboxylic Acid + Metal → Carboxylate Salt + Hydrogen Gas Ethanoic acid + Sodium → Sodium ethanoate + Hydrogen gas ``

``markdown

Acid-Base Reactions of Carboxylic Acids

  • Carboxylic acids react with bases to form carboxylate salts.
  • The carboxylate salts have the general formula RCOO-, where R represents the alkyl or aryl group.
  • Example: Carboxylic Acid + Base → Carboxylate Salt + Water Acetic acid + Sodium hydroxide → Sodium acetate + Water ``

``markdown

Summary

  • Aldehydes, ketones, and carboxylic acids are important classes of organic compounds.
  • Aldehydes have a carbonyl group (-C=O) with a hydrogen atom attached, while ketones have two alkyl or aryl groups attached to the carbonyl carbon.
  • Carboxylic acids have a carbonyl group and a hydroxyl group attached to the same carbon atom.
  • These compounds can be prepared through various synthetic methods and undergo numerous reactions.
  • Understanding their structures and preparation mechanisms is crucial for understanding the behavior and reactivity of these organic compounds. ``

``markdown

Key Concepts

  • Aldehydes, ketones, and carboxylic acids are organic compounds containing a carbonyl group (-C=O) as their functional group.
  • Aldehydes have a hydrogen atom attached to the carbonyl carbon, while ketones have two alkyl or aryl groups.
  • Carboxylic acids have a hydroxyl group attached to the carbonyl carbon.
  • The carbonyl group gives these compounds unique chemical properties.

General Structure

  • The general structure of an aldehyde can be represented as RCHO, where R represents an alkyl or aryl group.
  • The general structure of a ketone can be represented as RCOR’, where R and R’ represent alkyl or aryl groups.
  • The general structure of a carboxylic acid can be represented as RCOOH, where R represents an alkyl or aryl group.

Naming Aldehydes

  • Aldehydes are named by replacing the -e ending of the parent alkane with -al.
  • Example: Methane → Methanal (Formaldehyde)

Naming Ketones

  • Ketones are named by replacing the -e ending of the parent alkane with -one.
  • Example: Propane → Propanone (Acetone)

Naming Carboxylic Acids

  • Carboxylic acids are named by replacing the -e ending of the parent alkane with -oic acid.
  • Example: Ethane → Ethanoic acid (Acetic acid)

Examples

  • Butane → Butanoic acid (Butyric acid)
  • Hexane → Hexanoic acid (Caproic acid)

Oxidation of Primary Alcohols to Aldehydes

  • Primary alcohols can be oxidized to aldehydes using an oxidizing agent such as pyridinium chlorochromate (PCC) or potassium dichromate (K2Cr2O7).
  • Example: Ethanol → Ethanal

Oxidation of Aldehydes to Carboxylic Acids

  • Aldehydes can be further oxidized to carboxylic acids using strong oxidizing agents such as potassium dichromate (K2Cr2O7).
  • Example: Ethanal → Ethanoic acid

Oxidation of Secondary Alcohols to Ketones

  • Secondary alcohols can be oxidized to ketones using an oxidizing agent such as chromic acid (H2CrO4).
  • Example: 2-Propanol → Propanone

Reduction of Aldehydes and Ketones to Alcohols

  • Aldehydes and ketones can be reduced to alcohols using reducing agents such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4).
  • Example: Ethanal → Ethanol

Grignard Reaction with Aldehydes and Ketones

  • Aldehydes and ketones can react with Grignard reagents to form alcohols.
  • The reaction proceeds via the nucleophilic addition of the carbon chain of the Grignard reagent to the carbonyl carbon of the aldehyde or ketone.
  • Example: Acetone + Ethylmagnesium bromide → 2-Butanol

Aldol Condensation

  • Aldehydes and ketones can undergo aldol condensation to form β-hydroxyaldehydes or β-hydroxyketones.
  • The reaction involves the nucleophilic addition of an enolate ion, generated from the carbonyl compound, to the carbonyl carbon of another molecule of the same carbonyl compound.
  • Example: Acetaldehyde + Sodium hydroxide → Acetaldehyde aldol

Hydrolysis of Nitriles to Carboxylic Acids

  • Nitriles can be hydrolyzed using acid or base to form carboxylic acids.
  • Acid hydrolysis: Nitrile + Acid → Carboxylic acid
  • Base hydrolysis: Nitrile + Base → Carboxylate salt

Reaction of Carboxylic Acids with Alcohols

  • Carboxylic acids can react with alcohols in the presence of an acid catalyst to form esters.
  • Example: Ethanoic acid + Methanol → Methyl ethanoate

Esterification of Carboxylic Acids

  • Esterification is the reaction between a carboxylic acid and an alcohol to form an ester and water.
  • The reaction is catalyzed by an acid, such as sulfuric acid (H2SO4) or hydrochloric acid (HCl).
  • Example: Ethanoic acid + Methanol → Methyl ethanoate + Water

Decarboxylation

  • Decarboxylation is the process of removing a carboxyl group (-COOH) from a carboxylic acid to form carbon dioxide (CO2) and an alkane.
  • This reaction usually requires high temperatures or catalysts.
  • Example: Ethanoic acid → CO2 + Methane

Summary

  • Aldehydes, ketones, and carboxylic acids are important classes of organic compounds.
  • They have distinct structures and naming conventions.
  • Aldehydes and ketones can be prepared through various oxidation and reduction reactions.
  • Carboxylic acids can be obtained through oxidation of primary alcohols or hydrolysis of nitriles.
  • Understanding the reactions and properties of these compounds is essential for their application in various fields. ``