Study Notes
Aldehydes and ketones are carbonyl compounds that can be produced by the oxidation of primary alcohols using acidified K2Cr2O7 or KMnO4. They can undergo various reactions, including reduction to alcohols and nucleophilic addition with hydrogen cyanide.
- Aldehyde — an organic compound containing a carbonyl group bonded to at least one hydrogen atom. Example: Ethanal (CH3CHO)
- Ketone — an organic compound containing a carbonyl group bonded to two carbon atoms. Example: Propanone (CH3COCH3)
- Reduction — a chemical reaction that involves the gain of electrons or hydrogen. Example: Reduction of propanone to propanol using NaBH4
- Nucleophilic Addition — a reaction where a nucleophile forms a bond with a carbon atom in a carbonyl group. Example: Reaction of ethanal with HCN to form mandelonitrile
- 2,4-DNPH Test — a test for carbonyl compounds that results in an orange or yellow precipitate. Example: Reaction of propanone with 2,4-DNPH
- Tollens' Test — a test to distinguish aldehydes from ketones by forming a silver mirror with aldehydes. Example: Silver mirror formation with ethanal
- Iodoform Reaction — a reaction that indicates the presence of a CH3CO- group by forming a yellow precipitate. Example: Reaction of ethanal with iodine and NaOH
Exam Tips
Key Definitions to Remember
- Aldehyde: An organic compound with a carbonyl group bonded to at least one hydrogen atom.
- Ketone: An organic compound with a carbonyl group bonded to two carbon atoms.
- Reduction: A chemical reaction involving the gain of electrons or hydrogen.
- Nucleophilic Addition: A reaction where a nucleophile forms a bond with a carbon atom in a carbonyl group.
Common Confusions
- Confusing the oxidation of primary alcohols to aldehydes with further oxidation to carboxylic acids.
- Mixing up the reagents used for reduction: NaBH4 is milder than LiAlH4.
Typical Exam Questions
- How can you distinguish between an aldehyde and a ketone using chemical tests? Use Tollens' test or Fehling's test; aldehydes will react, ketones will not.
- What is the product of the reaction between propanone and hydrogen cyanide? 2-hydroxy-2-methylpropanenitrile
- What is the purpose of using 2,4-DNPH in testing carbonyl compounds? To detect the presence of carbonyl groups by forming a precipitate.
What Examiners Usually Test
- Understanding the oxidation and reduction reactions of carbonyl compounds.
- Ability to identify and differentiate aldehydes and ketones using chemical tests.
- Knowledge of reaction mechanisms, such as nucleophilic addition with HCN.