Aldehydes, Ketones & Carboxylic Acids

Special Preparation of Aldehydes

  • Aldehydes can be prepared by the oxidation of primary alcohols.
  • The oxidation can be carried out using a variety of oxidizing agents such as potassium dichromate (K2Cr2O7), chromic acid (H2CrO4), or potassium permanganate (KMnO4).
  • The reaction is typically carried out under acidic conditions.
  • For example, the oxidation of ethanol yields acetaldehyde as the product. Equation:
    CH3CH2OH + [O] → CH3CHO + H2O
  • Another method of aldehyde synthesis is the ozonolysis of alkenes followed by reduction of the ozonide formed.
  • This method is especially useful for the preparation of aldehydes with a terminal carbon.
  • For example, the ozonolysis of ethene yields formaldehyde. Equation:
    CH2=CH2 + O3 → CH2O + O2
  • Aldehydes can also be prepared by the oxidation of primary amines.
  • The amine is oxidized to an imine intermediate, which is then hydrolyzed to yield the aldehyde.
  • This method is known as the formylation reaction.
  • For example, the oxidation of methylamine yields formaldehyde. Equation:
    CH3NH2 + [O] → HCHO + H2O
  • Aldehydes can also be prepared by the hydrolysis of geminal dihalides in the presence of a strong base.
  • The dihalide undergoes elimination to form a carbonyl compound.
  • This method is known as the haloform reaction.
  • For example, the hydrolysis of chloroform yields chloral, which is an important aldehyde. Equation:
    CHCl3 + OH- → Cl3CCHO + Cl- + H2O
  1. Common Methods of Ketone Synthesis
  • Ketones can be prepared by the oxidation of secondary alcohols.
  • Similar to the oxidation of primary alcohols, various oxidizing agents can be used such as potassium dichromate, chromic acid, or potassium permanganate.
  • The reaction is typically carried out under acidic conditions.
  • For example, the oxidation of propan-2-ol yields propanone (acetone) as the product. Equation:
    (CH3)2CHOH + [O] → (CH3)2CO + H2O
  • Ketones can also be prepared by the dehydration of secondary alcohols.
  • The alcohol undergoes elimination to form a carbonyl compound.
  • This reaction is typically carried out using a strong acid catalyst, such as sulfuric acid or phosphoric acid.
  • For example, the dehydration of 2-propanol yields propanone. Equation:
    (CH3)2CHOH → (CH3)2CO + H2O
  • Ketones can be prepared by the oxidation of alkylbenzenes.
  • Alkylbenzenes are aromatic compounds that contain an alkyl group attached to a benzene ring.
  • The oxidation reaction is typically carried out using powerful oxidizing agents such as potassium permanganate or chromic acid.
  • For example, the oxidation of ethylbenzene yields acetophenone. Equation:
    C6H5CH2CH3 + [O] → C6H5COCH3 + H2O
  • Ketones can also be prepared by the reaction of certain Grignard reagents with carbonyl compounds.
  • Grignard reagents are organometallic compounds that contain a carbon-metal bond.
  • The reaction between a Grignard reagent and a carbonyl compound is known as a Grignard reaction.
  • For example, the reaction between phenylmagnesium bromide and formaldehyde yields benzyl alcohol. Equation:
    C6H5MgBr + HCHO → C6H5CH2OH
  • Ketones can be prepared by the oxidation of alkyl halides.
  • The alkyl halide is oxidized to a carbonyl compound using a strong oxidizing agent such as potassium permanganate or chromic acid.
  • The reaction is typically carried out under basic conditions.
  • For example, the oxidation of 2-chloropropane yields propanone. Equation:
    CH3CHClCH3 + [O] → CH3COCH3 + HCl
  1. Oxidation of Alcohols to Carboxylic Acids
  • Primary alcohols can be oxidized to carboxylic acids using a variety of oxidizing agents such as potassium permanganate or chromic acid.
  • The reaction is typically carried out under acidic conditions.
  • For example, the oxidation of ethanol yields ethanoic acid. Equation:
    CH3CH2OH + [O] → CH3COOH + H2O
  • Secondary alcohols can also be oxidized to ketones using the same oxidizing agents.
  • However, the reaction conditions may vary, and the reaction is typically carried out under milder conditions compared to the oxidation of primary alcohols.
  • For example, the oxidation of propan-2-ol yields propanone. Equation:
    (CH3)2CHOH + [O] → (CH3)2CO + H2O
  • Tertiary alcohols cannot be oxidized to carboxylic acids using conventional oxidizing agents.
  • This is because tertiary alcohols do not have a hydrogen atom attached to the carbon bearing the hydroxyl group.
  • Instead, tertiary alcohols can be oxidized to give ketones.
  • For example, the oxidation of 2-methyl-2-propanol yields 2-methyl-2-propanone. Equation:
    (CH3)3COH + [O] → (CH3)3CO + H2O
  • The oxidation of alcohols to carboxylic acids can also proceed via a two-step process.
  • In the first step, the alcohol is oxidized to an aldehyde using a mild oxidizing agent such as pyridinium chlorochromate (PCC).
  • In the second step, the aldehyde is further oxidized to a carboxylic acid using a stronger oxidizing agent.
  • This method is useful when selective oxidation is desired, and the risk of over-oxidation to the carboxylic acid is minimized.
  • For example, the oxidation of ethanol using PCC yields acetaldehyde, and further oxidation with potassium dichromate yields ethanoic acid. Equation:
    CH3CH2OH + PCC → CH3CHO + H2O
    CH3CHO + [O] → CH3COOH
  • Another method for the oxidation of alcohols to carboxylic acids is the use of Tollens’ reagent or Fehling’s solution.
  • These reagents allow for oxidation without the need for an acidic or basic environment.
  • Tollens’ reagent contains silver ions, which are reduced by the alcohol to give a silver mirror.
  • Fehling’s solution contains copper ions, which are reduced by the alcohol to give a red precipitate.
  • For example, the oxidation of ethanol using Fehling’s solution yields ethanoic acid. Equation:

2Cu2+ + 2OH- + CH3CH2OH → 2Cu+ + 2H2O + CH3COOH

  1. Reduction of Aldehydes and Ketones
  • Aldehydes and ketones can be reduced to alcohols using a variety of reducing agents.
  • One commonly used reducing agent is sodium borohydride (NaBH4).
  • In the presence of water or an alcohol solvent, NaBH4 reacts with the carbonyl compound to yield the corresponding alcohol.
  • For example, the reduction of propanal (an aldehyde) yields propan-1-ol. Equation:
    CH3CH2CHO + NaBH4 → CH3CH2CH2OH + NaB(OH)4
  • Another commonly used reducing agent is lithium aluminum hydride (LiAlH4).
  • LiAlH4 is a more powerful reducing agent compared to NaBH4 and can reduce both aldehydes and ketones to alcohols.
  • The reaction conditions for the reduction with LiAlH4 typically involve a non-aqueous solvent such as diethyl ether or tetrahydrofuran (THF).
  • For example, the reduction of propanone (a ketone) yields propan-2-ol. Equation:
    (CH3)2CO + LiAlH4 → (CH3)2CHOH + LiAlO2
  • Catalytic hydrogenation is another method for the reduction of aldehydes and ketones.
  • The reaction is typically carried out in the presence of a metal catalyst such as platinum (Pt), palladium (Pd), or nickel (Ni).
  • Under suitable conditions, the carbonyl compound reacts with hydrogen gas to yield the corresponding alcohol.
  • For example, the reduction of benzaldehyde (an aldehyde) yields benzyl alcohol. Equation:
    C6H5CHO + H2 → C6H5CH2OH
  • Reduction of carbonyl compounds can also be achieved using complex metal hydrides such as lithium triethylborohydride (LiEt3BH) or sodium cyanoborohydride (NaBH3CN).
  • These reagents are milder and more selective compared to NaBH4 or LiAlH4.
  • For example, the reduction of benzophenone (a ketone) using sodium cyanoborohydride yields 2-hydroxy-2-phenylacetophenone. Equation:
    C6H5COC6H5 + NaBH3CN → C6H5CHOHCOC6H5 + NaCNBH3
  • Another important method for the reduction of aldehydes and ketones is the use of metal hydride complexes such as tris(dimethylamino)borane (B(NMe2)3) or hydrosilanes.
  • These reagents are highly selective and can reduce only aldehydes to primary alcohols, leaving ketones unchanged.
  • For example, the reduction of benzaldehyde using B(NMe2)3 yields benzyl alcohol. Equation:
    C6H5CHO + B(NMe2)3 → C6H5CH2OH
  1. Nucleophilic Addition Reactions of Aldehydes and Ketones
  • Aldehydes and ketones are highly reactive carbonyl compounds that undergo a variety of nucleophilic addition reactions.
  • In these reactions, a nucleophile attacks the electrophilic carbon of the carbonyl group, resulting in the formation of a new bond.
  • The reaction can occur at either the carbonyl carbon or the carbon adjacent to the carbonyl group.
  • Some common nucleophiles include water, alcohols, amines, and cyanide ion.
  • Nucleophilic addition of water:
    • Aldehydes and ketones react with water in the presence of an acid or base catalyst to yield hydrates.
    • The reaction, known as hydration, occurs via nucleophilic addition to the carbonyl carbon.
    • For example, the reaction of ethanal with water yields ethanol. Equation:
      CH3CHO + H2O → CH3CH2OH
  • Nucleophilic addition of alcohols:
    • Aldehydes and ketones react with alcohols in the presence of an acid catalyst to form hemiacetals (for aldehydes) or hemiketals (for ketones).
    • The reaction occurs via nucleophilic addition of the alcohol to the carbonyl carbon, followed by protonation of the oxygen atom.
    • For example, the reaction of propanone with methanol yields the hemiketal 1-methoxypropan-2-ol. Equation:
      (CH3)2CO + CH3OH → CH3COCH2OCH3
  • Nucleophilic addition of amines:
    • Aldehydes and ketones react with amines in the presence of an acid catalyst to form imines.
    • The reaction occurs via nucleophilic addition of the amine to the carbonyl carbon, followed by protonation of the nitrogen atom.
    • For example, the reaction of propanal with methylamine yields N-methylprop-1-imine. Equation:
      CH3CH2CHO + CH3NH2 → CH3CH=CHNHCH3
  • Nucleophilic addition of cyanide ion:
    • Aldehydes and ketones react with cyanide ion (CN-) in the presence of a base catalyst to form cyanohydrins.
    • The reaction occurs via nucleophilic addition of the cyanide ion to the carbonyl carbon, followed by protonation of the oxygen atom.
    • For example, the reaction of propanone with sodium cyanide yields 2-hydroxy-2-methylpropanenitrile. Equation:
      (CH3)2CO + NaCN → (CH3)2CHOHCN
  • Nucleophilic addition reactions of aldehydes and ketones are key steps in the synthesis of many organic compounds.
  • These reactions allow for the introduction of new functional groups and the creation of complex molecular architectures.
  1. Addition-Elimination Reactions of Aldehydes and Ketones
  • Aldehydes and ketones can undergo addition-elimination reactions with certain nucleophiles.
  • In these reactions, a nucleophile initially adds to the carbonyl carbon, followed by elimination of a leaving group to regenerate the carbonyl group.
  • The overall transformation results in the substitution of a nucleophile for the carbonyl oxygen.
  • Some common nucleophiles that undergo addition-elimination reactions with aldehydes and ketones include primary amines and hydrazines.
  • Addition-elimination with primary amines:
    • Aldehydes and ketones react with primary amines in the presence of an acid catalyst to yield imines or enamines.
    • The reaction occurs via nucleophilic addition of the primary amine to the carbonyl carbon, followed by elimination of water.
    • For example, the reaction of propanal with methylamine yields N-methylpropan-2-imine. Equation:
      CH3CH2CHO + CH3NH2 → CH3CH=CHNHCH3 + H2O
  • Addition-elimination with hydrazines:
    • Aldehydes and ketones react with hydrazines in the absence of an acid catalyst to form hydrazones.
    • The reaction occurs via nucleophilic addition of the hydrazine to the carbonyl carbon, followed by elimination of water.
    • For example, the reaction of propanone with hydrazine yields 2,4-dinitrophenylhydrazone. Equation:
      (CH3)2CO + H2NNH2 → (CH3)2C=NHNH2 + H2O
  • Addition-elimination reactions of aldehydes and ketones with nucleophiles provide a means for the synthesis of various organic compounds.
  • These reactions are widely used in the pharmaceutical and agrochemical industries for the construction of complex molecules.
  1. Reduction of Carboxylic Acids
  • Carboxylic acids can be reduced to alcohols using a variety of reducing agents.
  • One commonly used reducing agent is lithium aluminum hydride (LiAlH4).
  • In the presence of a non-aqueous solvent such as diethyl ether or tetrahydrofuran (THF), LiAlH4 reacts with the carboxylic acid to yield the corresponding alcohol.
  • For example, the reduction of ethanoic acid yields ethanol. Equation:
    CH3COOH + LiAlH4 → CH3CH2OH + LiAlO2
  • Another commonly used reducing agent for the reduction of carboxylic acids is the combination of phosphorus (P4) and a strong reducing agent such as sodium or sulfur.
  • This reaction, known as the Vandemergel’s reagent, allows for the selective reduction of carboxylic acids to primary alcohols.
  • For example, the reduction of propanoic acid yields propan-1-ol. Equation:
    CH3CH2COOH + P4 + Na → CH3CH2CH2OH + NaH2PO2
  • Catalytic hydrogenation is another method for the reduction of carboxylic acids.
  • The reaction is typically carried out in the presence of a metal catalyst such as platinum (Pt), palladium (Pd), or nickel (Ni).
  • Under suitable conditions, the carboxylic acid reacts with hydrogen gas to yield the corresponding alcohol.
  • For example, the reduction of benzoic acid yields benzyl alcohol. Equation:
    C6H5COOH + H2 → C6H5CH2OH
  • Reduction of carboxylic acids can also be achieved using complex metal hydrides such as lithium triethylborohydride (LiEt3BH).
  • This reagent is milder and more selective compared to other reducing agents.
  • For example, the reduction of formic acid using LiEt3BH yields methanol. Equation:
    HCOOH + LiEt3BH → CH3OH + LiEt3BO2
  • The reduction of carboxylic acids to alcohols is an important transformation in organic synthesis.
  • It allows for the conversion of carboxylic acids, which are typically acidic and less reactive, to the corresponding alcohols, which are more versatile and can undergo a wide range of reactions.
  1. Hydrolysis of Acid Derivatives
  • Acid derivatives such as esters, acid halides, and anhydrides can be hydrolyzed to yield carboxylic acids.
  • The hydrolysis reaction involves the cleavage of a carbonyl carbon-oxygen bond and the addition of water.
  • Different acid derivatives require different reaction conditions and catalysts for hydrolysis.
  • Hydrolysis of esters:
    • Esters can be hydrolyzed to carboxylic acids and alcohols using either acidic or basic conditions.
    • Under acidic conditions, the reaction is known as acid-catalyzed ester hydrolysis.
    • Under basic conditions, the reaction is known as base-catalyzed ester hydrolysis or