Organic Chemistry: Identification of Aromatic Compounds

Study Notes

Molecular Formula and Unsaturation

Compound (A) has a high degree of unsaturation but does not decolourise bromine water or Baeyer's reagent, indicating the presence of unsaturation due to an aromatic ring.
Compound (A) is phenyl methyl ketone (acetophenone) based on its molecular formula.

Reactions due to α-Hydrogen Acidity

Aldehydes and ketones undergo reactions due to the acidic nature of α-hydrogen.
The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

Aldol Condensation

Aldehydes and ketones having at least one α-hydrogen undergo aldol condensation in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol).
The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds.

Identifying Compounds that Undergo Aldol Condensation and Cannizzaro Reaction

Methanal and benzaldehyde undergo Cannizzaro reaction but not aldol condensation.
2-Methylpentanal, benzophenone, cyclohexanone, and 2,2-dimethylbutanal undergo aldol condensation but not Cannizzaro reaction.
1-Phenylpropanone and phenylacetaldehyde undergo both aldol condensation and Cannizzaro reaction.

Converting Ethanal to Other Compounds

Ethanal can be converted to butane-1,3-diol through aldol condensation.
Ethanal can be converted to but-2-enal through aldol condensation followed by dehydration.
Ethanal can be converted to but-2-enoic acid through aldol condensation followed by oxidation.

Aldol Condensation Products from Propanal and Butanal

Four possible aldol condensation products can be formed from propanal and butanal, with each aldehyde acting as both nucleophile and electrophile.

Identifying Compounds

An organic compound with the molecular formula C9H10O forms 2,4-DNP derivative, reduces Tollens' reagent, and undergoes Cannizzaro reaction, and on vigorous oxidation, it gives 1,2-benzenedicarboxylic acid, indicating it is phenylacetaldehyde.

Derivatives of Aldehydes and Ketones

Different derivatives of aldehydes and ketones include cyanohydrin, acetal, semicarbazone, aldol, hemiacetal, oxime, ketal, imine, and 2,4-DNP-derivative.

Naming and Drawing Structures of Compounds

The IUPAC names of various ketones and aldehydes can be determined using the IUPAC system of nomenclature.
The structures of various derivatives, such as 2,4-dinitrophenylhydrazones, oximes, ketals, and hemiacetals, can be drawn.

Description

This quiz tests your knowledge on identifying aromatic compounds based on molecular formulas and chemical reactions. The quiz specifically focuses on distinguishing between compounds containing unsaturation due to aromatic rings and those that do not decolourise bromine water or Baeyer’s reagent.