Organic Compounds Containing Oxygen – JEE Mains Chemistry
1. General Introduction to Organic Compounds Containing Oxygen
Organic compounds containing oxygen include alcohols, phenols, ethers, aldehydes, ketones, and carboxylic acids. These compounds are characterized by the presence of oxygen in various functional groups, which influence their chemical properties and reactivity.
2. Alcohols
Alcohols contain a hydroxyl group (-OH) attached to a saturated carbon atom. They can be classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the carbon bearing the -OH group.
- Identification of Alcohols: Primary alcohols have the -OH group attached to a carbon that is attached to only one other carbon, secondary alcohols to two, and tertiary alcohols to three.
- Mechanism of Dehydration: Dehydration of alcohols results in the formation of alkenes. The reaction proceeds via the formation of a carbocation intermediate, with the loss of water.
3. Phenols
Phenols are compounds where the hydroxyl group is attached to a benzene ring. They are more acidic than alcohols due to the stabilization of the phenoxide ion after deprotonation.
- Acidic Nature: Phenols exhibit acidic behavior due to the ability of the phenoxide ion to stabilize the negative charge via resonance.
- Electrophilic Substitution Reactions: Phenols undergo electrophilic substitution reactions, such as halogenation, nitration, and sulphonation.
- Reimer-Tiemann Reaction: The reaction of phenols with chloroform and a base leads to the formation of ortho- and para-hydroxybenzaldehydes.
4. Ethers
Ethers consist of an oxygen atom bonded to two alkyl or aryl groups. The general formula for ethers is R–O–R'.
- Structure of Ethers: The oxygen in ethers is sp³ hybridized, and the bond angle around the oxygen atom is about 110°, making ethers relatively non-polar and less reactive compared to alcohols and phenols.
5. Aldehydes and Ketones
Aldehydes and ketones both contain a carbonyl group (C=O), but aldehydes have the carbonyl group attached to at least one hydrogen atom, while ketones have two carbon groups attached to the carbonyl.
- Nature of Carbonyl Group: The carbonyl group is polar, with a partial positive charge on the carbon and a partial negative charge on the oxygen.
- Nucleophilic Addition: Aldehydes and ketones undergo nucleophilic addition reactions, such as addition of HCN, NH₃ and its derivatives, and Grignard reagents.
- Oxidation and Reduction: Aldehydes can be oxidized to carboxylic acids, while ketones are resistant to oxidation. Aldehydes and ketones can be reduced to alcohols via the Clemmensen and Wolf-Kishner reductions.
- Aldol Condensation: A reaction in which aldehydes or ketones with an α-hydrogen undergo condensation in the presence of a base to form β-hydroxy aldehydes or ketones.
- Cannizzaro Reaction: A redox reaction in which non-enolizable aldehydes undergo disproportionation in the presence of a strong base to give alcohol and carboxylate salt.
- Haloform Reaction: Aldehydes and methyl ketones react with halogens in the presence of a base to form a haloform (CHX₃).
- Tests to Distinguish Aldehydes and Ketones: Aldehydes can be distinguished from ketones by using Tollens' or Fehling's test, which give positive results for aldehydes but not for ketones.
6. Carboxylic Acids
Carboxylic acids contain the carboxyl group (-COOH) and exhibit acidic properties due to the ability of the carboxylate anion to stabilize the negative charge through resonance.
- Acidic Strength: The acidity of carboxylic acids is influenced by the electron-withdrawing or electron-donating effects of substituents attached to the aromatic ring or alkyl chain. Electron-withdrawing groups increase acidity, while electron-donating groups decrease acidity.