Slide 1: Aldehydes, Ketones & Carboxylic Acids - Acidity of alpha-Hydrogen

  • The alpha-hydrogen in aldehydes, ketones, and carboxylic acids is weakly acidic.
  • The acidity is due to the presence of a highly electronegative oxygen atom in the alpha position.
  • The acidity is enhanced by the electron-withdrawing groups attached to the carbonyl group.
  • The pKa values of alpha-hydrogens in these compounds range from approximately 15-20.
  1. Aldehydes, Ketones & Carboxylic Acids - Keto-Enol Tautomerism
  • Keto-enol tautomerism is a reversible reaction between the keto form (carbonyl) and the enol form (enol-OH) of aldehydes, ketones, and certain carboxylic acids.
  • The equilibrium between the two forms is governed by the stability of the enol form.
  • Enols are less stable than the corresponding keto forms due to the presence of the sp^2 hybridized carbon in the enol form.
  • The keto-enol tautomerism is catalyzed by acids or bases, and can also be induced by heat or light.
  1. Aldehydes, Ketones & Carboxylic Acids - Keto-Enol Tautomerism (Example)
  • An example of keto-enol tautomerism is the equilibrium between acetylacetone (keto form) and its enol form.
  • The keto form of acetylacetone is more stable than the enol form due to the resonance stabilization of the carbonyl group.
  • However, in the presence of acid or base, the enol form can be formed through protonation or deprotonation, respectively.
  1. Aldehydes, Ketones & Carboxylic Acids - Nucleophilic Addition Reactions
  • Aldehydes and ketones undergo nucleophilic addition reactions due to the electrophilic nature of the carbonyl carbon.
  • Nucleophiles attack the carbonyl carbon, forming a tetrahedral intermediate.
  • The carbonyl oxygen is protonated, followed by elimination of a leaving group, resulting in the formation of a new bond.
  1. Aldehydes, Ketones & Carboxylic Acids - Nucleophilic Addition Reactions (Example)
  • An example of a nucleophilic addition reaction is the reaction between an aldehyde or ketone and a Grignard reagent.
  • The nucleophilic carbon of the Grignard reagent attacks the carbonyl carbon, forming a tetrahedral intermediate.
  • The addition of an acid or water protonates the oxygen, yielding an alcohol as the final product.
  1. Aldehydes, Ketones & Carboxylic Acids - Oxidation Reactions
  • Aldehydes can be oxidized to carboxylic acids by strong oxidizing agents such as potassium permanganate (KMnO4) or chromic acid (H2CrO4).
  • Ketones, however, are resistant to oxidation due to the absence of an alpha-hydrogen.
  • The oxidation of aldehydes involves the breaking of the carbon-oxygen double bond and formation of a carbon-oxygen single bond.
  1. Aldehydes, Ketones & Carboxylic Acids - Oxidation Reactions (Example)
  • An example of an oxidation reaction is the conversion of propanal to propanoic acid using potassium permanganate.
  • In the presence of KMnO4, the aldehyde group is oxidized to a carboxylic acid group, resulting in the formation of propanoic acid.
  1. Aldehydes, Ketones & Carboxylic Acids - Reduction Reactions
  • Aldehydes and ketones can be reduced to alcohols by various reducing agents such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4).
  • Reduction involves the addition of electrons to the carbonyl carbon, resulting in the formation of a new bond.
  • The carbonyl oxygen is protonated, followed by elimination of a leaving group, leading to the formation of an alcohol.
  1. Aldehydes, Ketones & Carboxylic Acids - Reduction Reactions (Example)
  • An example of a reduction reaction is the conversion of propanone (acetone) to propan-2-ol using sodium borohydride.
  • NaBH4 reduces the carbonyl group of propanone to an alcohol group, resulting in the formation of propan-2-ol.
  1. Aldehydes, Ketones & Carboxylic Acids - Nomenclature
  • Aldehydes are named by replacing the -e ending of the corresponding parent alkane with -al.
  • Ketones are named by replacing the -e ending of the corresponding parent alkane with -one.
  • Carboxylic acids are named by replacing the -e ending of the corresponding parent alkane with -oic acid.
  1. Aldehydes, Ketones & Carboxylic Acids - Nomenclature (Examples)
  • Formaldehyde (CH2O) is an aldehyde named systematically as methanal.
  • Acetone (CH3C(O)CH3) is a ketone named systematically as propan-2-one.
  • Ethanoic acid (CH3COOH) is a carboxylic acid named systematically as ethanoic acid.
  1. Aldehydes, Ketones & Carboxylic Acids - Acidity of alpha-Hydrogen
  • The alpha-hydrogens in aldehydes, ketones, and carboxylic acids are weakly acidic.
  • The acidity is due to the presence of a highly electronegative oxygen atom in the alpha position.
  • The presence of electron-withdrawing groups attached to the carbonyl group enhances the acidity.
  • The pKa values of alpha-hydrogens in these compounds range from approximately 15-20.
  • The acidity of alpha-hydrogens can be measured using a pKa scale.
  1. Aldehydes, Ketones & Carboxylic Acids - Acidity of alpha-Hydrogen (Example)
  • An example of acidity of alpha-hydrogen is acetic acid.
  • The alpha-hydrogen in acetic acid is weakly acidic with a pKa value of approximately 19.
  • The acidity of the alpha-hydrogen can be attributed to the electronegativity of the oxygen atom and the resonance stabilization of the resulting carbanion.
  1. Aldehydes, Ketones & Carboxylic Acids - Keto-Enol Tautomerism
  • Keto-enol tautomerism is a reversible reaction between the keto form (carbonyl) and the enol form (enol-OH) of aldehydes, ketones, and certain carboxylic acids.
  • The equilibrium between the two forms is governed by the stability of the enol form.
  • Enols are less stable than the corresponding keto forms due to the presence of the sp^2 hybridized carbon in the enol form.
  • The keto-enol tautomerism can be catalyzed by acids or bases and can also be induced by heat or light.
  • Tautomers are constitutional isomers that can interconvert rapidly, usually through a reversible intramolecular reaction.
  1. Aldehydes, Ketones & Carboxylic Acids - Keto-Enol Tautomerism (Example)
  • An example of keto-enol tautomerism is the equilibrium between acetylacetone (keto form) and its enol form.
  • The keto form of acetylacetone is more stable than the enol form due to the resonance stabilization of the carbonyl group.
  • However, in the presence of acid or base, the enol form can be formed through protonation or deprotonation, respectively.
  • Keto-enol tautomerism is important in reactions like condensation, isomerization, and formation of cyclic compounds.
  1. Aldehydes, Ketones & Carboxylic Acids - Nucleophilic Addition Reactions
  • Aldehydes and ketones undergo nucleophilic addition reactions due to the electrophilic nature of the carbonyl carbon.
  • Nucleophiles attack the carbonyl carbon, forming a tetrahedral intermediate.
  • The carbonyl oxygen is protonated, followed by the elimination of a leaving group, resulting in the formation of a new bond.
  • Nucleophilic addition reactions are commonly observed in reactions like Grignard reactions, nucleophilic addition of water, and nucleophilic addition of alcohol.
  1. Aldehydes, Ketones & Carboxylic Acids - Nucleophilic Addition Reactions (Example)
  • An example of a nucleophilic addition reaction is the reaction between an aldehyde or ketone and a Grignard reagent.
  • The nucleophilic carbon of the Grignard reagent attacks the carbonyl carbon, forming a tetrahedral intermediate.
  • The addition of an acid or water protonates the oxygen, yielding an alcohol as the final product.
  • Nucleophilic addition reactions can also occur with other nucleophiles like ammonia or primary amines.
  1. Aldehydes, Ketones & Carboxylic Acids - Oxidation Reactions
  • Aldehydes can be oxidized to carboxylic acids by strong oxidizing agents such as potassium permanganate (KMnO4) or chromic acid (H2CrO4).
  • Ketones, however, are resistant to oxidation due to the absence of an alpha-hydrogen.
  • The oxidation of aldehydes involves the breaking of the carbon-oxygen double bond and formation of a carbon-oxygen single bond.
  • Oxidation reactions are important in the synthesis of carboxylic acids from aldehydes and ketones.
  1. Aldehydes, Ketones & Carboxylic Acids - Oxidation Reactions (Example)
  • An example of an oxidation reaction is the conversion of propanal to propanoic acid using potassium permanganate.
  • In the presence of KMnO4, the aldehyde group is oxidized to a carboxylic acid group, resulting in the formation of propanoic acid.
  • Oxidation reactions can also be used to distinguish between aldehydes and ketones in a chemical reaction.
  1. Aldehydes, Ketones & Carboxylic Acids - Reduction Reactions
  • Aldehydes and ketones can be reduced to alcohols by various reducing agents such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4).
  • Reduction involves the addition of electrons to the carbonyl carbon, resulting in the formation of a new bond.
  • The carbonyl oxygen is protonated, followed by elimination of a leaving group, leading to the formation of an alcohol.
  • Reduction reactions are commonly used in the synthesis of alcohols from aldehydes and ketones.
  1. Aldehydes, Ketones & Carboxylic Acids - Reduction Reactions (Example)
  • An example of a reduction reaction is the conversion of propanone (acetone) to propan-2-ol using sodium borohydride.
  • NaBH4 reduces the carbonyl group of propanone to an alcohol group, resulting in the formation of propan-2-ol.
  • Reduction reactions can also be used to convert carboxylic acids to primary alcohols by converting the carboxylic acid to an aldehyde intermediate.