Aldehydes, Ketones and Carboxylic Acids
Easy Overview
Carbonyl compounds — that's the C=O group. Aldehydes have it at the end of a chain, ketones in the middle. They're everywhere: formalin (preserves specimens), acetone (nail polish remover), and the stuff that gives vanilla its flavor. This chapter is all about that double bond between carbon and oxygen.
Structure and nomenclature
Aldehydes have a carbonyl carbon bonded to at least one hydrogen. R-CHO. Ketones have two alkyl/aryl groups attached to carbonyl. R-CO-R'. Naming: replace -e of alkane with -al for aldehydes, -one for ketones. Propanal vs propanone. Methanal (HCHO) is formaldehyde — used to preserve dead things.
Preparation methods
Alcohols can be oxidized to aldehydes (with mild oxidizer like PCC) or to ketones. Ozonolysis of alkenes gives aldehydes/ketones. Friedel-Crafts acylation puts a ketone on a benzene ring. Rosenmund reduction: acid chloride + H2 over poisoned Pd catalyst → aldehyde. Board exams love Rosenmund.
Nucleophilic addition
The carbonyl carbon is partially positive (oxygen pulls electrons). So nucleophiles attack it. HCN adds to give cyanohydrins. Grignard reagents add to give alcohols. NaHSO3 gives a crystalline addition product (used for purification). This is the most important reaction of carbonyl compounds.
Alpha-hydrogen reactions
The hydrogen right next to the carbonyl group is slightly acidic. This lets aldehydes/ketones do aldol condensation — two molecules combine to form a β-hydroxy carbonyl compound. Also haloform reaction: methyl ketones + halogen + base → haloform + carboxylate. This is how chloroform was made historically.
Distinction tests
Tollens' test: aldehydes give a silver mirror (ketones don't). Fehling's test: aldehydes give a red precipitate of Cu2O. Iodoform test: methyl ketones and ethanol give yellow precipitate of CHI3. These are the classic board exam 'identify the compound' questions.
Key Points
- •Aldehydes: R-CHO; Ketones: R-CO-R'
- •Carbonyl carbon is electrophilic (δ+) — attacked by nucleophiles
- •Rosenmund reduction: RCOCl + H2/Pd-BaSO4 → RCHO
- •Aldol condensation: 2 molecules of aldehyde/ketone with α-H
- •Tollens' test: aldehyde → silver mirror
- •Fehling's test: aldehyde → red Cu2O precipitate
- •Iodoform test: CH3-CO-R or CH3-CH(OH)-R → yellow CHI3
Practice Questions
- Distinguish between aldehydes and ketones using chemical tests.
- Explain the mechanism of nucleophilic addition in carbonyl compounds.
- What is aldol condensation? Write the mechanism.
- How is benzaldehyde prepared from toluene?