Aldehydes, Ketones and Carboxylic Acids

• After studying this Unit, you will be able to:

  • Write the common and IUPAC names of aldehydes, ketones, and carboxylic acids
  • Write the structures of compounds containing functional groups namely carbonyl and carboxyl groups
  • Describe the important methods of preparation and reactions of these classes of compounds
  • Correlate physical properties and chemical reactions of aldehydes, ketones, and carboxylic acids with their structures
  • Explain the mechanism of a few selected reactions of aldehydes and ketones
  • Understand various factors affecting the acidity of carboxylic acids and their reactions
  • Describe the uses of aldehydes, ketones, and carboxylic acids
• Carbonyl compounds are essential in organic chemistry, found in fabrics, flavorings, plastics, and drugs
• In aldehydes, the carbonyl group is bonded to a carbon and hydrogen, while in ketones, it is bonded to two carbon atoms
• Carbonyl compounds where the carbon of the carbonyl group is bonded to carbon or hydrogen and oxygen of hydroxyl moiety are known as carboxylic acids
• Aldehydes, ketones, and carboxylic acids are widespread in plants and the animal kingdom, playing a crucial role in biochemical processes of life
• Nomenclature of Aldehydes and Ketones:

  • Common names are derived from the common names of corresponding carboxylic acids by replacing the ending -ic of acid with aldehyde
  • IUPAC names are derived from the names of corresponding alkanes by replacing the ending -e with -al for aldehydes and -one for ketones
• The carbonyl carbon atom in aldehydes and ketones is sp2-hybridized and forms three sigma (s) bonds
• The carbon-oxygen double bond in carbonyl compounds is polarized due to the higher electronegativity of oxygen relative to carbon
• Aldehydes and ketones are generally prepared by the oxidation of primary and secondary alcohols, respectively
• Dehydrogenation of alcohols is suitable for volatile alcohols and is of industrial application
• Important methods for the preparation of aldehydes and ketones include:

  • Oxidation of alcohols
  • Dehydrogenation of alcohols
  • From hydrocarbons (ozonolysis of alkenes, hydration of alkynes)
  • From acyl chloride
  • From nitriles and esters
• Ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones, or a mixture of both depending on the substitution pattern of the alkene
• Acyl chloride can be hydrogenated over a catalyst to give aldehydes in a reaction known as Rosenmund reduction
• Hydration of alkynes results in acetaldehyde from ethyne in the presence of H2SO4 and HgSO4, while other alkynes give ketones in this reaction
• Nitriles can be reduced to corresponding imines with stannous chloride, which on hydrolysis give corresponding aldehydes in a reaction known as Stephen reaction
• Aromatic aldehydes like benzaldehyde are prepared from aromatic hydrocarbons through various methods such as oxidation of methylbenzene or Gatterman-Koch reaction
• Aldehydes and ketones have physical properties such as boiling points higher than hydrocarbons and ethers of comparable molecular masses due to weak molecular association from dipole-dipole interactions
• Lower aldehydes and ketones like methanal, ethanal, and propanone are miscible with water due to hydrogen bonding, but solubility decreases with increasing alkyl chain length
• Aldehydes and ketones are used in perfumes and flavoring agents due to their fragrant properties
• Aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, and chloroform
• Aldehydes and ketones undergo similar chemical reactions due to the carbonyl functional group, including nucleophilic addition reactions
• In nucleophilic addition reactions, aldehydes and ketones undergo nucleophilic addition reactions
• Mechanism of nucleophilic addition reactions:

  • A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group
  • The attack occurs from a direction approximately perpendicular to the plane of sp2 hybridized orbitals of the carbonyl carbon
  • The hybridization of carbon changes from sp2 to sp3, producing a tetrahedral alkoxide intermediate
  • The intermediate captures a proton from the reaction medium to give the electrically neutral product
  • The net result is the addition of Nu– and H+ across the carbon-oxygen double bond
• Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons:

  • Sterically, the presence of two relatively large substituents in ketones hinders the approach of the nucleophile to the carbonyl carbon compared to aldehydes
  • Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in ketones
• Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions:

  (a) Addition of hydrogen cyanide (HCN): Aldehydes and ketones react with HCN to yield cyanohydrins, catalyzed by a base

  (b) Addition of sodium hydrogensulphite: Adds to aldehydes and ketones to form addition products, useful for separation and purification

  (c) Addition of Grignard reagents

  (d) Addition of alcohols: Aldehydes react with monohydric alcohol to yield hemiacetals and acetals; ketones react with ethylene glycol to form ethylene glycol ketals

  (e) Addition of ammonia and its derivatives: Nucleophiles like ammonia and its derivatives add to the carbonyl group of aldehydes and ketones
• Reduction:

  • Aldehydes and ketones are reduced to primary and secondary alcohols respectively by NaBH4, LiAlH4, or catalytic hydrogenation
  • Aldehydes and ketones can be reduced to hydrocarbons using zinc-amalgam and concentrated HCl or Wolff-Kishner reduction
• Oxidation:

  • Aldehydes are easily oxidized to carboxylic acids with common oxidizing agents
  • Ketones are generally oxidized under vigorous conditions, involving carbon-carbon bond cleavage to yield a mixture of carboxylic acids with fewer carbon atoms
  • Tollens' test and Fehling's test are used to distinguish aldehydes from ketones
• Since (A) does not reduce Tollens’ or Fehling reagent, it must be a ketone
• Compound (A) forms a 2,4-DNP derivative, indicating it is an aldehyde or a ketone
• (A) responds to the iodoform test, suggesting it is a methyl ketone
• The molecular formula of (A) indicates a high degree of unsaturation, but it does not decolourise bromine water or Baeyer’s reagent, indicating unsaturation due to an aromatic ring
• Compound (B) is an oxidation product of a ketone and should be a carboxylic acid
• The molecular formula of (B) indicates it should be benzoic acid, making compound (A) a monosubstituted aromatic methyl ketone
• The molecular formula of (A) indicates it should be phenyl methyl ketone (acetophenone)
• Carboxylic acids have bonds to the carboxyl carbon lying in one plane and separated by about 120°
• Oxidation of methyl ketones by haloform reaction:

  • Methyl ketones are oxidised by sodium hypohalite to form sodium salts of corresponding carboxylic acids with one carbon atom less than the carbonyl compound
  • The methyl group is converted to haloform
  • This oxidation does not affect a carbon-carbon double bond, if present in the molecule
• The carboxylic carbon is less electrophilic than the carbonyl carbon due to possible resonance structures
• Methods of preparation of carboxylic acids:

  1. From primary alcohols and aldehydes using oxidising agents like potassium permanganate or potassium dichromate

  2. From alkylbenzenes by vigorous oxidation with chromic acid or potassium permanganate

  3. From nitriles and amides, hydrolysed in the presence of catalysts

  4. From Grignard reagents reacting with carbon dioxide to form salts of carboxylic acids
• Carboxylic acids can also be prepared from aldehydes using mild oxidising agents
• Carboxylic acids can be prepared from esters through acidic or basic hydrolysis