Carbonyl Compounds

Carbonyl compounds, which include aldehydes and ketones, are defined by the presence of a carbonyl group ($C=O$), where a carbon atom is double-bonded to an oxygen atom.

1. Structure and Bonding

• The carbonyl carbon atom is $sp^2$ hybridized.
• The $C=O$ bond is shorter ($1.23 \text{ \AA}$), stronger ($178 \text{ kcal/mol}$), and more polar than the $C=C$ bond found in alkenes.
• The bond angles around the carbonyl carbon are approximately $120^\circ$.
• The polarity of the $C=O$ bond makes the carbon atom electron-deficient, rendering it susceptible to attack by nucleophiles.

2. Classification and Nomenclature

• Aldehydes: The carbonyl group appears at the end of a hydrocarbon chain and is bonded to at least one hydrogen atom.
  • IUPAC Naming: Replace the final –e of the alkane name with –al.
  • Common Names: Formaldehyde, acetaldehyde, and benzaldehyde are frequently used.
• Ketones: The carbonyl group is bonded to two carbon atoms.
  • IUPAC Naming: Replace the final –e of the alkane name with –one.
  • Numbering: The chain is numbered starting from the end nearest the carbonyl group to indicate its position.

3. Synthesis and Preparation

• Oxidation of Alcohols (Laboratory): This is the most common method. Primary alcohols are oxidized to aldehydes, while secondary alcohols are oxidized to ketones.
  • To stop at the aldehyde stage and avoid further oxidation to carboxylic acids, pyridinium chlorochromate (PCC) in dichloromethane is used.
  • Sodium dichromate ($Na_2Cr_2O_7$) in acetic acid is an effective, inexpensive reagent for preparing ketones.
• Industrial Methods:
  • Fischer-Tropsch Syntheses: Heating $CO$ and $H_2$ with coke (as a reducing agent).
  • Air Oxidation: Methanol can be oxidized to methanal using air, heat, and a copper catalyst.
  • Hydration of Alkynes: Ethanal is manufactured by adding water to ethyne using a mercury catalyst.
• Other Methods:
  • Reduction of Acid Chlorides: Yields aldehydes using mild reducing agents like lithium aluminium hydrogen tritertiary butoxide.
  • Friedel-Crafts Acylation: Reaction of an acid chloride with aromatic hydrocarbons in the presence of a Lewis acid (e.g., $AlCl_3$) to form ketones.

4. Physical Properties

• Polarity: They are moderately polar due to the carbonyl group.
• Hydrogen Bonding: They cannot form hydrogen bonds with each other, but they can form them with water using the lone pairs on the oxygen.
• Boiling Points: Their boiling points are higher than alkanes or ethers of similar mass but lower than alcohols or carboxylic acids because they lack intermolecular hydrogen bonding.
• State and Odor:
  • Methanal is a gas at room temperature; simple aliphatic versions are generally colorless liquids.
  • Lower molecular mass aldehydes have unpleasant, pungent smells, while ketones and benzaldehyde have pleasant, sweet odors.

5. Chemical Reactions

• Nucleophilic Addition:
  • Hydrogen Cyanide ($HCN$): Forms cyanohydrins.
  • Grignard Reagents ($RMgX$): Methanal produces primary alcohols, other aldehydes produce secondary alcohols, and ketones produce tertiary alcohols.
  • Alcohols: Aldehydes form acetals (via hemiacetals), and ketones form ketals (via hemiketals).
• Condensation with Ammonia Derivatives: Aldehydes and ketones react with derivatives like hydroxylamine (forming oximes) and hydrazine (forming hydrazones). The reaction with 2,4-dinitrophenylhydrazine (Brady’s reagent) produces characteristic yellow crystals.
• Reduction:
  • To Alcohols: Using $LiAlH_4$ or $NaBH_4$.
  • To Hydrocarbons: Clemmensen reduction (using amalgamated zinc and $HCl$) or Wolff-Kishner reduction (using hydrazine and a strong base).
• Haloform Reaction: Compounds containing a methyl carbonyl group react with $I_2/NaOH$ to form yellow iodoform ($CHI_3$) crystals with a distinctive smell.

6. Distinguishing Aldehydes from Ketones

• Oxidation: Aldehydes are easily oxidized to carboxylic acids by mild agents (acidified $KMnO_4$ or $K_2Cr_2O_7$), whereas ketones are resistant and only oxidize under harsh conditions to form a mixture of acids with fewer carbons.
• Tollen’s Test (Silver Mirror Test): Aldehydes reduce ammoniacal silver nitrate to metallic silver, forming a mirror on the test tube wall; ketones give a negative result.
• Fehling’s Test: Aldehydes reduce a deep blue copper(II) complex to a red/brown precipitate of $Cu_2O$; ketones do not react.

7. Aromatic Carbonyls

• In aromatic aldehydes and ketones, the carbonyl group deactivates the ring and is meta-directing for electrophilic aromatic substitution reactions.
• These reactions are generally slower than those of benzene and require more stringent conditions.