Aldehydes, Ketones & Carboxylic Acids

  • Definition: A class of organic compounds containing a carbonyl group (C=O).
  • Aldehydes: Carbonyl group is bonded to at least one hydrogen atom.
  • Ketones: Carbonyl group is bonded to two carbon atoms.
  • Carboxylic Acids: Carbonyl group is bonded to a hydroxyl (-OH) group.
  • These compounds are important in organic synthesis and have numerous applications in industry.

Nomenclature of Aldehydes

  • Aldehydes are named by replacing the “-e” suffix of the corresponding parent alkane with “-al”.
  • The carbon chain is numbered starting from the carbonyl carbon atom.
  • If there are multiple functional groups, the aldehyde group is given the lowest number. Example:
  • Methanal (formaldehyde)
  • Ethanal (acetaldehyde)
  • Propanal (propionaldehyde)

Nomenclature of Ketones

  • Ketones are named by replacing the “-e” suffix of the corresponding parent alkane with “-one”.
  • The carbon chain is numbered starting from the end closest to the carbonyl group.
  • If there are multiple carbonyl groups, a prefix such as “di-” or “tri-” is used to indicate their positions. Example:
  • Propanone (acetone)
  • Butanone (methyl ethyl ketone)
  • Pentan-3-one (diethyl ketone)

Nomenclature of Carboxylic Acids

  • Carboxylic acids are named by replacing the “-e” suffix of the corresponding parent alkane with “-oic acid”.
  • The carbon chain is numbered starting from the carboxyl carbon, which is always carbon 1.
  • If there are multiple carboxyl groups, a prefix such as “di-” or “tri-” is used to indicate their positions. Example:
  • Formic acid (methanoic acid)
  • Acetic acid (ethanoic acid)
  • Benzoic acid

Physical Properties

  • Aldehydes and ketones have higher boiling points than hydrocarbons of similar molecular weight due to the presence of the polar carbonyl group.
  • Carboxylic acids have higher boiling points than aldehydes and ketones due to the presence of the additional –OH group, which can form hydrogen bonds.
  • Aldehydes and ketones are generally liquids at room temperature, while carboxylic acids are usually solids.

Chemical Reactions

  • Addition Reactions: Aldehydes and ketones can undergo addition reactions with nucleophiles such as Grignard reagents, hydrazines, and cyanohydrins.
  • Oxidation: Aldehydes can be oxidized to carboxylic acids using strong oxidizing agents such as potassium permanganate.
  • Reduction: Aldehydes and ketones can be reduced to alcohols using reducing agents like sodium borohydride or lithium aluminum hydride. Example:
  • Addition of Grignard reagent to formaldehyde produces primary alcohols.

Schiff’s Test

  • Schiff’s reagent is used to test for the presence of aldehydes.
  • Schiff’s reagent is a solution of fuchsin, which turns from colorless to pink/purple when it reacts with aldehydes.
  • The reaction involves the formation of a colored compound called a Schiff base. Example:
  • When Schiff’s reagent is added to an aldehyde like benzaldehyde, it turns pink/purple.

Tollens’ Test

  • Tollens’ reagent is used to test for the presence of aldehydes.
  • Tollens’ reagent is a solution of silver nitrate and ammonia.
  • Aldehydes are oxidized to carboxylic acids, while Tollens’ reagent is reduced to metallic silver, forming a silver mirror. Example:
  • When Tollens’ reagent is added to an aldehyde like glucose, a silver mirror is formed.

Acidity of Carboxylic Acids

  • Carboxylic acids are weak acids due to resonance stabilization of the conjugate base.
  • They can donate a proton to a base, forming a carboxylate anion.
  • The acidity of carboxylic acids increases with the presence of electron-withdrawing groups on the carbon chain. Example:
  • Acetic acid is a weak acid and can donate a proton to form the acetate ion.

Esterification Reaction

  • Carboxylic acids can undergo esterification reactions with alcohols, forming esters.
  • This reaction is acid-catalyzed and involves the condensation of the carboxylic acid and alcohol, followed by the elimination of water. Example:
  • Ethanoic acid reacts with ethanol to form ethyl ethanoate (a fruity-smelling ester).

Example- Addition of Grignard reagent

  • Grignard reagents (RMgX) can react with aldehydes and ketones to form alcohols.
  • The reaction involves the addition of the carbon chain from the Grignard reagent to the carbonyl group, forming an alcohol.
  • For example, the addition of phenylmagnesium bromide (C6H5MgBr) to formaldehyde (HCHO) produces primary alcohol (C6H5CH2OH). Equation: HCHO + C6H5MgBr –> C6H5CH2OH
  • This reaction is an important method for the synthesis of alcohols and is commonly used in organic chemistry.

Schiff’s Test

  • Schiff’s reagent is used to test for the presence of aldehydes.
  • It is a solution of fuchsin (a dye) in sulfuric acid.
  • When Schiff’s reagent is added to an aldehyde, it turns from colorless to pink/purple.
  • This color change is due to the formation of a colored compound called a Schiff base. Example:
  • When Schiff’s reagent is added to benzaldehyde, it turns pink/purple. Equation: C6H5CHO + Schiff’s reagent –> Pink/purple color
  • Schiff’s test can be used as a qualitative test for aldehydes in various organic compounds.

Tollens’ Test

  • Tollens’ reagent is used to test for the presence of aldehydes.
  • It is a solution of silver nitrate (AgNO3) in aqueous ammonia (NH3).
  • When Tollens’ reagent is added to an aldehyde, it forms a silver mirror.
  • This reaction involves the oxidation of aldehydes to carboxylic acids and the reduction of Tollens’ reagent to metallic silver. Example:
  • When Tollens’ reagent is added to glucose, a silver mirror is formed. Equation: C6H12O6 + Tollens’ reagent –> Silver mirror
  • Tollens’ test is used to distinguish aldehydes from ketones, as ketones do not react with Tollens’ reagent.

Acidity of Carboxylic Acids

  • Carboxylic acids are weak acids due to the resonance stabilization of the conjugate base.
  • They can donate a proton (H+) to a base, forming a carboxylate anion.
  • The acidity of carboxylic acids can be influenced by the presence of electron-withdrawing groups on the carbon chain.
  • Electron-withdrawing groups increase the acidity by destabilizing the conjugate base. Example:
  • Acetic acid (CH3COOH) is a weak acid that can donate a proton to form the acetate ion (CH3COO-). Equation: CH3COOH –> CH3COO- + H+
  • The acidity of carboxylic acids can be measured using pKa values, which represent the negative logarithm of the acid dissociation constant.

Esterification Reaction

  • Carboxylic acids can undergo esterification reactions with alcohols, forming esters.
  • This reaction is acid-catalyzed and involves the condensation of the carboxylic acid and alcohol, followed by the elimination of water.
  • The reaction occurs between the carboxyl group of the acid and the hydroxyl group of the alcohol. Example:
  • Ethanoic acid (CH3COOH) reacts with ethanol (CH3CH2OH) to form ethyl ethanoate (CH3COOCH2CH3). Equation: CH3COOH + CH3CH2OH –> CH3COOCH2CH3 + H2O
  • Esterification reactions are important in the synthesis of various organic compounds, including perfumes, flavors, and pharmaceuticals.
  1. Addition of Grignard Reagent:
  • Grignard reagents (RMgX) react with aldehydes and ketones.
  • The carbon chain from the Grignard reagent adds to the carbonyl group.
  • This reaction forms an alcohol. Example Equation:
  • Formaldehyde (HCHO) + Phenylmagnesium bromide (C6H5MgBr) → Phenylmethanol (C6H5CH2OH)
  1. Schiff’s Test:
  • Schiff’s reagent is used to detect the presence of aldehydes.
  • It contains fuchsin dye in sulfuric acid.
  • When Schiff’s reagent reacts with an aldehyde, a pink/purple color appears. Example Equation:
  • Benzaldehyde + Schiff’s reagent → Pink/purple color
  1. Tollens’ Test:
  • Tollens’ reagent is used to identify aldehydes.
  • It consists of silver nitrate (AgNO3) in aqueous ammonia (NH3).
  • When Tollens’ reagent reacts with an aldehyde, a silver mirror forms. Example Equation:
  • Glucose + Tollens’ reagent → Silver mirror
  1. Acidity of Carboxylic Acids:
  • Carboxylic acids are weak acids.
  • They can donate a proton (H+) to a base forming a carboxylate ion (Coo-).
  • Electron-withdrawing groups increase acidity by destabilizing the conjugate base. Example Equation:
  • Acetic acid (CH3COOH) → Acetate ion (CH3COO-) + H+
  1. Esterification Reaction:
  • Carboxylic acids undergo esterification with alcohols.
  • The reaction is acid-catalyzed.
  • It involves the condensation of carboxylic acid and alcohol, followed by water elimination. Example Equation:
  • Ethanoic acid (CH3COOH) + Ethanol (CH3CH2OH) → Ethyl ethanoate (CH3COOCH2CH3) + H2O
  1. Reduction of Aldehydes and Ketones:
  • Aldehydes and ketones can be reduced to alcohols.
  • Sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4) are commonly used reducing agents.
  • The carbonyl group is reduced to a hydroxyl group. Example Equation:
  • Propanal + Sodium borohydride → Propanol
  1. Oxidation of Aldehydes:
  • Aldehydes can be oxidized to carboxylic acids by strong oxidizing agents like potassium permanganate (KMnO4).
  • The carbonyl group is converted to a carboxyl group. Example Equation:
  • Ethanal + Potassium permanganate → Ethanoic acid
  1. Formation of Acetals:
  • Aldehydes and ketones can react with alcohols to form acetals.
  • Addition of alcohol to the carbonyl group followed by elimination of water forms acetal. Example Equation:
  • Pentanal + Methanol → 2,2-Dimethyl-1,3-dioxolane + Water
  1. Decarboxylation of Carboxylic Acids:
  • Carboxylic acids can undergo decarboxylation.
  • Carbon dioxide is lost, leading to the formation of a hydrocarbon. Example Equation:
  • Benzoic acid → Benzene + Carbon dioxide
  1. Hydrolysis of Esters:
  • Esters can be hydrolyzed to form carboxylic acids and alcohols.
  • This reaction is catalyzed by either acid or base. Example Equation:
  • Ethyl ethanoate + Water (in acidic conditions) → Ethanoic acid + Ethanol