Aldehydes, Ketones & Carboxylic Acid Study Notes

VJETA - 2026 Class 12th OC: Aldehydes, Ketones & Carboxylic Acid

By: Shourya Grover Ma'am

Introduction

  • Carbonyl Group: Definition
    • Organic compounds containing the functional group (>C=O) are classified under carbonyl groups.
Structure of Carbonyl Group:
  • Aldehydes representation: RCHO
  • Ketones representation: R2C=O
  • Aldehyde Structure: R-C-H [or RCHO]
  • Ketone Structure: R-C-R' (where R and R' are carbon-containing groups)
  • Exceptions:
    • Formaldehyde (methanal) HCHO
    • Formic Acid (methanoic acid) HCOOH
Importance of Carbonyl Groups
  • Aldehydes, ketones, and carboxylic acids are prevalent in both plant and animal kingdom.
  • They are involved in several biochemical processes, adding fragrance (e.g., vanillin) and flavor to nature.
  • Common applications include their use in food products, solvents (e.g., acetone), adhesives, paint, resins, perfumes, plastics, and fabrics.

Nomenclature

Common Names
  • HCHO - Formaldehyde
  • CH3CHO - Acetaldehyde
  • C6H5CHO - Benzaldehyde
  • Example: 3-Bromobutyraldehyde
IUPAC Nomenclature for Aldehydes and Ketones
  • Structure: 2 Prefix + 1 Prefix + Word Root + 1 Suffix + 2 Suffix
  • Aldehyde endings: -al; Ketone endings: -one
  • Functional group priority: Carboxylic > Aldehyde > Ketone > Alcohol
  • Example: 2-Methylpropanal, 1-Pentanone, Benzene-1,2-dicarbaldehyde
  • Variations in Naming:
    • Aldehyde + Formyl
    • Ketone + -one

Physical Properties

  1. State:
    • Methanal is a gas at room temperature, while ethanal is a volatile liquid. Other aldehydes and ketones exist as solids or liquids.
  2. Boiling Points:
    • The boiling point order: Carboxylic Acid > Alcohol > Aldehyde > Ether > Hydrocarbons.
  3. Solubility:
    • Lower members of aldehydes and ketones (methanal and ethanal) are miscible in water due to hydrogen bonding, but solubility decreases with increased alkyl chain length.
  4. Fragrance:
    • Traditionally, lower aldehydes have strong odors, which become less intense and more pleasant with increasing molecular weight.

Chemical Properties

  1. Nucleophilic Addition Reaction
    • An electrophile forms new bonds with hypocenter carbon, which shifts hybridization from sp² to sp³, forming a tetrahedral alkoxide.
  2. Reactivity:
    • Aldehydes are more reactive than ketones in nucleophilic addition due to steric hindrance and electronic effects.
  3. Addition of Hydrogen Cyanide:
    • Both aldehydes and ketones can form cyanohydrins through their reaction with HCN.
  4. Addition of Alcohols:
    • Formation of gem-dialkoxy compounds (acetals).
  5. Addition of Ammonia:
    • Carbonyl compounds react with NH3 to form imines, while reactions with hydroxylamine and hydrazine yield oximes and hydrazones, respectively.
  6. Clemmensen Reduction and Wolff-Kishner Reduction:
    • Both reduce carbonyl compounds to respective hydrocarbons.
  7. Oxidation
    • Aldehydes are easily oxidized to carboxylic acids; ketones require more vigorous conditions.
  • For aldehydes:
    • Tollens’ test: Produces a silver mirror.
    • Fehling’s test: Produces a reddish brown precipitate (Cu2O).
  1. Aldol Condensation
    • Occurs under the presence of a base to form β-hydroxy aldehydes/ketones, resulting in self-aldol or cross-aldol products.
  2. Cannizzaro Reaction:
    • Aldehydes without α-hydrogens undergo self-oxidation-reduction, forming alcohols and carboxylic acids.

Carboxylic Acids

  1. General Formula: RCOOH
  2. Physical Properties:
    • Carboxylic acids typically have higher boiling points than aldehydes and ketones due to dimerization through hydrogen bonding.
    • Soluble in water (up to 4 carbon atoms) before becoming hydrophobic with long chains.
  3. Chemical Properties:
    • Reactivity includes cleavage reactions forming carboxylate anions and ionic salts with bases (acidity).
    • Formation of anhydrides and esters through reactions with alcohols and acid chlorides.

Preparation Methods

Aldehydes and Ketones
  1. Oxidation of Alcohols:
    • Primary alcohols yield aldehydes and secondary alcohols yield ketones upon treatment with oxidizing agents.
  2. Ozonolysis of Alkenes:
    • Generates aldehydes and ketones from unsaturated hydrocarbons.
  3. Hydrocarbon Reactions:
    • Etard Reaction, Friedel-Crafts acylation provide means to synthesize both functional groups.
  4. Grignard Reagents:
    • Use of Grignard reagents can yield aldehydes or ketones.
Preparation of Only Aldehydes
  1. From Acyl Chlorides:
    • Hydrogenation under specific conditions yields aldehydes (Rosenmund Reduction).
  2. From Nitriles:
    • Reduction to imines, subsequently hydrolyzed yields aldehydes.
  3. From Hydrocarbons:
    • Side chain chlorination and hydrolysis or through Gatterman-Koch reactions.
Preparation of Only Ketones
  1. From Acyl Chlorides:
    • Ketones are formed through reactions with organometallic reagents (Dialkyl cadmium).
  2. From Nitriles and Benzene Derivatives:
    • Friedel-Crafts acylation also provides ketones.

Miscellaneous Topics

  • Conversion Questions: Various reaction steps enabling the transformation of one functional group to another e.g., from carboxylic to aldehydes, etc., through specific reactions or reagents.

  • Further exploration of the properties, reactivities, and synthesis methods of aldehydes, ketones, and carboxylic acids, including comparative reactivity studies.

The carbonyl group is defined as organic compounds containing the functional group (>C=O) and includes aldehydes and ketones. Aldehydes are represented by the formula RCHO, while ketones are represented as R2C=O. The structure of aldehydes can be depicted as R-C-H (or RCHO), and ketones as R-C-R' (where R and R' are carbon-containing groups). Notable exceptions in this classification are formaldehyde (methanal, HCHO) and formic acid (methanoic acid, HCOOH).

Aldehydes, ketones, and carboxylic acids are prevalent in both the plant and animal kingdoms. They play vital roles in various biochemical processes and contribute to the fragrances and flavors found in nature, such as vanillin. Common applications for these compounds are seen in food products, solvents like acetone, adhesives, paints, resins, perfumes, plastics, and fabrics.

In terms of nomenclature, common names include formaldehyde (HCHO), acetaldehyde (CH3CHO), and benzaldehyde (C6H5CHO), with an example being 3-bromobutyraldehyde. For IUPAC nomenclature, aldehydes and ketones follow a structured naming rule that consists of two prefixes, a word root, and two suffixes, specifically using -al for aldehydes and -one for ketones. The functional group priority is ranked as carboxylic > aldehyde > ketone > alcohol, with examples including 2-methylpropanal, 1-pentanone, and benzene-1,2-dicarbaldehyde.

Regarding physical properties, methanal exists as a gas at room temperature, while ethanal is a volatile liquid; other aldehydes and ketones can be solid or liquid. The boiling points of these compounds are ordered as follows: carboxylic acid > alcohol > aldehyde > ether > hydrocarbons. Lower members of aldehydes and ketones, such as methanal and ethanal, are miscible in water due to hydrogen bonding, but this solubility decreases as the alkyl chain length increases. Lower aldehydes are known for having strong odors, though these tend to diminish and become more pleasant as the molecular weight increases.

Chemical properties highlight the nucleophilic addition reaction, where an electrophile forms new bonds with the carbon, shifting the hybridization from sp² to sp³ and thus forming a tetrahedral alkoxide. Aldehydes exhibit greater reactivity than ketones during nucleophilic addition due to the steric hindrance and electronic effects. Both aldehydes and ketones can form cyanohydrins when reacted with hydrogen cyanide (HCN), while reaction with alcohols allows for the formation of gem-dialkoxy compounds (acetals). Additionally, carbonyl compounds react with NH3 to form imines, whereas reactions with hydroxylamine and hydrazine create oximes and hydrazones, respectively. The Clemmensen and Wolff-Kishner reductions can reduce carbonyl compounds into hydrocarbons. Aldehydes can be easily oxidized to carboxylic acids, whereas ketones require more rigorous conditions; common tests include Tollens’ and Fehling’s tests.

Aldol condensation occurs in the presence of a base, resulting in the formation of β-hydroxy aldehydes or ketones, leading to either self-aldol or cross-aldol products. The Cannizzaro reaction describes a process where aldehydes without α-hydrogens undergo self-oxidation-reduction to yield alcohols and carboxylic acids.

Carboxylic acids are generally represented by the formula RCOOH and typically have higher boiling points than aldehydes and ketones due to dimerization through hydrogen bonding. These acids are soluble in water for up to four carbon atoms, after which they become hydrophobic with longer chains. Chemical properties include reactivity that involves cleavage reactions that form carboxylate anions and ionic salts with bases, and they can form anhydrides and esters through reactions with alcohols and acid chlorides.

For the preparation methods, aldehydes and ketones can be synthesized through the oxidation of alcohols where primary alcohols yield aldehydes and secondary alcohols yield ketones when treated with oxidizing agents. Ozonolysis of alkenes can generate aldehydes and ketones from unsaturated hydrocarbons. Other reactions with hydrocarbons, such as the Etard reaction and Friedel-Crafts acylation, are used to synthesize both functional groups, and Grignard reagents can also yield aldehydes or ketones. Aldehydes can be specifically prepared from acyl chlorides through hydrogenation (Rosenmund reduction), nitriles (by reduction to imines followed by hydrolysis), or hydrocarbons (through side-chain chlorination and hydrolysis or Gatterman-Koch reactions). Ketones, conversely, can be formed from acyl chlorides via reactions with organometallic reagents (like dialkyl cadmium) and from nitriles or benzene derivatives through Friedel-Crafts acylation. Lastly, the topic of conversion questions pertains to various reaction steps allowing the transformation of one functional group to another, such as from carboxylic to aldehydes through specific reactions or reagents, as well as the comparative reactivity studies of these compounds.