Aldehydes, Ketones & Carboxylic Acids

  • Structure and polarity of carbonyl group

Aldehydes

  • Contain a carbonyl group (-C=O) bonded to one alkyl or aryl group and one hydrogen atom
  • General formula: RCHO
  • Examples:
    • Formaldehyde (HCHO)
    • Acetaldehyde (CH3CHO)
    • Benzaldehyde (C6H5CHO)

Ketones

  • Contain a carbonyl group (-C=O) bonded to two alkyl or aryl groups
  • General formula: RCOR'
  • Examples:
    • Acetone (CH3COCH3)
    • Propanone (CH3COCH2CH3)
    • Butanone (CH3COCH2CH2CH3)

Carboxylic Acids

  • Contain a carbonyl group (-C=O) and a hydroxyl group (-OH) bonded to the same carbon atom
  • General formula: RCOOH
  • Examples:
    • Formic acid (HCOOH)
    • Acetic acid (CH3COOH)
    • Benzoic acid (C6H5COOH)

Polarity of Carbonyl Group

  • Oxygen atom is more electronegative than carbon atom
  • Carbonyl group has a polar covalent bond
  • Oxygen partially withdraws electron density from carbon, resulting in a slight positive charge on carbon and a slight negative charge on oxygen
  • Makes carbonyl compounds reactive and capable of forming hydrogen bonds

Structure of Carbonyl Group

  • Consists of a carbon atom that is double bonded to an oxygen atom
  • Both carbon and oxygen have sp2 hybridization
  • The bond angle around the carbonyl carbon is approximately 120 degrees
  • The carbon-oxygen bond length is shorter than a typical carbon-carbon single bond

Examples of Aldehydes

  • Formaldehyde (HCHO) is used as a disinfectant and preservative
  • Acetaldehyde (CH3CHO) is a starting material in the production of acetic acid
  • Benzaldehyde (C6H5CHO) is used in the synthesis of various aromatic compounds

Examples of Ketones

  • Acetone (CH3COCH3) is commonly used as a solvent and nail polish remover
  • Propanone (CH3COCH2CH3) is used in the production of plastics and pharmaceuticals
  • Butanone (CH3COCH2CH2CH3) is used as a solvent in many industrial applications

Examples of Carboxylic Acids

  • Formic acid (HCOOH) is found in the venom of certain ants and can be used as a reducing agent
  • Acetic acid (CH3COOH) is commonly known as vinegar and is used in food preservation and as a cleaning agent
  • Benzoic acid (C6H5COOH) is used as a food preservative and in the manufacture of dyes and perfumes

Summary

  • Aldehydes have a carbonyl group bonded to one alkyl or aryl group and one hydrogen atom
  • Ketones have a carbonyl group bonded to two alkyl or aryl groups
  • Carboxylic acids have a carbonyl group and a hydroxyl group bonded to the same carbon atom
  • The carbonyl group in these compounds is polar and makes them reactive

Chemical Reactions of Aldehydes

  • Aldehydes undergo various chemical reactions due to the presence of a reactive carbonyl group.
  • Oxidation: Aldehydes can be oxidized to carboxylic acids using oxidizing agents such as potassium dichromate (K2Cr2O7) and acidified potassium permanganate (KMnO4).
  • Reduction: Aldehydes can be reduced to primary alcohols using reducing agents such as lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).
  • Nucleophilic Addition: Aldehydes can undergo nucleophilic addition reactions with nucleophiles such as water, alcohols, and primary amines to form hemiacetals, acetals, and imines, respectively.
  • Reaction with Grignard Reagents: Aldehydes react with Grignard reagents (RMgX) to form secondary alcohols after hydrolysis.
  • Aldol Condensation: Aldehydes can undergo aldol condensation reactions to form β-hydroxy aldehydes or β-hydroxy ketones.

Chemical Reactions of Ketones

  • Ketones also exhibit various chemical reactions due to the presence of a carbonyl group.
  • Oxidation: Ketones are resistant to oxidation and do not undergo oxidation reactions under normal conditions.
  • Reduction: Ketones can be reduced to secondary alcohols using reducing agents such as lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).
  • Nucleophilic Addition: Ketones can undergo nucleophilic addition reactions with nucleophiles such as water, alcohols, and primary amines to form hemiketals, ketals, and imines, respectively.
  • Reaction with Grignard Reagents: Ketones react with Grignard reagents (RMgX) to form tertiary alcohols after hydrolysis.
  • Crossed Aldol Condensation: Ketones can undergo crossed aldol condensation reactions with aldehydes to form a mixture of α,β-unsaturated carbonyl compounds.

Chemical Reactions of Carboxylic Acids

  • Carboxylic acids are highly reactive due to the presence of both carbonyl and hydroxyl groups.
  • Acid-Base Reactions: Carboxylic acids can donate a proton to a base to form a carboxylate ion and water.
  • Esterification: Carboxylic acids can react with alcohols in the presence of an acid catalyst to form esters.
  • Reduction: Carboxylic acids can be reduced to primary alcohols by using strong reducing agents such as lithium aluminum hydride (LiAlH4).
  • Decarboxylation: Carboxylic acids undergo decarboxylation under certain conditions to form carbon dioxide and an alkane or an alkene.
  • Reaction with Metal Hydroxides: Carboxylic acids react with metal hydroxides to form carboxylate salts and water.

Importance of Aldehydes in Daily Life

  • Formaldehyde is used in the production of resins, plastics, and textiles.
  • Acetaldehyde is used as a flavoring and fragrance agent in foods and beverages.
  • Benzaldehyde is used in the synthesis of dyes, perfumes, and pharmaceuticals.
  • Vanillin, a popular flavoring agent in foods, is an aldehyde compound.

Importance of Ketones in Daily Life

  • Acetone is commonly used as a solvent and nail polish remover.
  • Methyl ethyl ketone (MEK) is used in the production of paints, adhesives, and coatings.
  • Acetophenone is used as a flavoring agent in foods and beverages.
  • Cholesterol, a vital component of cell membranes, is a ketone compound.

Importance of Carboxylic Acids in Daily Life

  • Acetic acid is used as a preservative in pickles, and as a solvent in the production of vinegar.
  • Citric acid is widely used as a flavoring agent in foods and beverages.
  • Ascorbic acid (vitamin C) is an essential nutrient involved in various metabolic processes.
  • Salicylic acid is used in the production of aspirin and other pharmaceuticals.

Common Reagents for Aldehydes, Ketones & Carboxylic Acids

  • Tollens’ Reagent: Used to test for the presence of aldehydes, forms a silver mirror when reduced.
  • Fehling’s Solution: Used to test for aldehydes, forms a red precipitate (Cu2O) when reduced.
  • Brady’s Reagent: Used to test for ketones, forms a yellow precipitate when reduced.
  • Lucas Test: Used to distinguish between primary, secondary, and tertiary alcohols based on the rate of reaction with hydrochloric acid.
  • Benedicts’ Reagent: Used to test for reducing sugars, turns from blue to brick red in the presence of sugars.

Spectroscopic Methods for Aldehydes, Ketones & Carboxylic Acids

  • Infrared (IR) Spectroscopy: Used to identify functional groups present in a compound, such as the carbonyl group in aldehydes, ketones, and carboxylic acids.
  • Nuclear Magnetic Resonance (NMR) Spectroscopy: Provides information about the carbon and hydrogen atoms in a compound, helping to determine the structure and connectivity of aldehydes, ketones, and carboxylic acids.
  • Mass Spectrometry (MS): Used to determine the molecular weight and molecular formula of a compound, aiding in the identification of aldehydes, ketones, and carboxylic acids.

Industrial Applications of Aldehydes, Ketones & Carboxylic Acids

  • Formaldehyde is used in the production of adhesives, plastics, and textiles.
  • Acetone is widely used as a solvent in industries and laboratories.
  • Citric acid is employed as an acidulant and flavoring agent in the food and beverage industry.
  • Acetic acid is used in the production of vinyl acetate, which is further used for making adhesives and paints.

Environmental Impacts of Aldehydes, Ketones & Carboxylic Acids

  • Formaldehyde is a volatile organic compound (VOC) and a known carcinogen, contributing to indoor air pollution.
  • Acetone is considered a volatile organic compound (VOC) and can contribute to air pollution.
  • Some carboxylic acids, such as formic acid and acetic acid, are naturally occurring in the environment and serve as significant contributors to the acidity of rainwater.

Chemical Properties of Aldehydes

  • Aldehydes undergo oxidation reactions to form carboxylic acids.
  • They can react with nucleophiles in nucleophilic addition reactions.
  • Aldehydes can undergo condensation reactions to form larger molecules.
  • They can undergo reduction reactions to form primary alcohols.
  • Aldehydes can react with Grignard reagents to form secondary alcohols.

Examples of Aldehyde Reactions

  • Oxidation: Ethanal (CH3CHO) can be oxidized to ethanoic acid (CH3COOH).
  • Nucleophilic Addition: Formaldehyde (HCHO) reacts with water to form methylene glycol.
  • Condensation: Acetaldehyde (CH3CHO) undergoes aldol condensation to form crotonaldehyde.
  • Reduction: Benzaldehyde (C6H5CHO) can be reduced to benzyl alcohol (C6H5CH2OH).
  • Grignard Reaction: Formaldehyde reacts with phenylmagnesium bromide (C6H5MgBr) to form benzyl alcohol.

Chemical Properties of Ketones

  • Ketones do not undergo oxidation reactions under normal conditions.
  • They can react with nucleophiles in nucleophilic addition reactions.
  • Ketones can undergo reduction reactions to form secondary alcohols.
  • They can undergo condensation reactions to form larger molecules.
  • Ketones can react with Grignard reagents to form tertiary alcohols.

Examples of Ketone Reactions

  • Reductive Amination: Acetone (CH3COCH3) can undergo reductive amination to form methylamine (CH3NH2).
  • Nucleophilic Addition: Propanone (CH3COCH2CH3) reacts with hydrazine (N2H4) to form a hydrazone compound.
  • Condensation: Acetone undergoes aldol condensation to form mesityl oxide.
  • Reduction: Propanone can be reduced to isopropanol (CH3CH(OH)CH3) using sodium borohydride (NaBH4).
  • Grignard Reaction: Acetone reacts with phenylmagnesium bromide (C6H5MgBr) to form tertiary alcohol.

Chemical Properties of Carboxylic Acids

  • Carboxylic acids can donate a proton in acid-base reactions.
  • They can undergo esterification reactions to form esters.
  • Carboxylic acids can be reduced to primary alcohols.
  • They undergo decarboxylation under certain conditions.
  • They react with metal hydroxides to form carboxylate salts.

Examples of Carboxylic Acid Reactions

  • Acid-Base Reaction: Ethanoic acid (CH3COOH) can react with sodium hydroxide (NaOH) to form sodium acetate (CH3COONa) and water.
  • Esterification: Ethanoic acid reacts with ethanol (C2H5OH) in the presence of a strong acid catalyst to form ethyl acetate (CH3COOC2H5) and water.
  • Reduction: Propanoic acid (CH3CH2COOH) can be reduced to propanol (CH3CH2CH2OH).
  • Decarboxylation: Ethanoic acid can undergo decarboxylation to form methane (CH4) and carbon dioxide (CO2).
  • Reaction with Metal Hydroxides: Acetic acid reacts with sodium hydroxide (NaOH) to form sodium acetate (CH3COONa) and water.

Differences Between Aldehydes and Ketones

  • Aldehydes have a hydrogen atom bonded to the carbonyl carbon, while ketones have two alkyl or aryl groups bonded to the carbonyl carbon.
  • Aldehydes are more reactive than ketones due to the presence of a more electron-withdrawing hydrogen atom.
  • Aldehydes have a lower boiling point than ketones of similar molecular weight due to the ability of aldehydes to form intermolecular hydrogen bonds.
  • Aldehydes are easily oxidized to carboxylic acids, while ketones are resistant to oxidation.

Differences Between Aldehydes and Carboxylic Acids

  • Aldehydes have one carbon atom bonded to a carbonyl group, whereas carboxylic acids have a carbonyl group bonded to a hydroxyl group on the same carbon atom.
  • Aldehydes are less acidic than carboxylic acids since they do not have a hydroxyl group that can easily donate a proton.
  • Aldehydes can be oxidized to form carboxylic acids, while carboxylic acids do not undergo this oxidation reaction.
  • Aldehydes have a sweet or fruity smell, while carboxylic acids have a strong, pungent odor.

Differences Between Ketones and Carboxylic Acids

  • Ketones have two alkyl or aryl groups bonded to the carbonyl carbon, whereas carboxylic acids have a carbonyl group and a hydroxyl group bonded to the same carbon atom.
  • Ketones do not have acidic properties and do not readily donate protons, unlike carboxylic acids.
  • Ketones are resistant to oxidation, while carboxylic acids can be easily oxidized to produce carbon dioxide and water.
  • Ketones have a pleasant smell, whereas carboxylic acids have a pungent, vinegar-like odor.

Summary

  • Aldehydes and ketones both contain a carbonyl group, but aldehydes have a hydrogen atom bonded to the carbonyl carbon, while ketones have two alkyl or aryl groups.
  • Carboxylic acids have a carbonyl group and a hydroxyl group bonded to the same carbon atom.
  • Aldehydes, ketones, and carboxylic acids undergo various chemical reactions due to the presence of the carbonyl group.
  • Aldehydes are easily oxidized to carboxylic acids, while ketones and carboxylic acids are resistant to oxidation.
  • Nucleophilic addition, reduction, condensation, and reaction with Grignard reagents are common reactions of aldehydes, ketones, and carboxylic acids.
  • Understanding the differences between these functional groups helps to distinguish their properties and reactivity.