Organic chemistry: oxygen and nitrogen compounds, polymers and analysis

Alcohols

Primary alcohols oxidise (acidified K₂Cr₂O₇, orange → green) to aldehydes (distil off) then carboxylic acids (reflux); secondary to ketones; tertiary resist oxidation. Dehydration with concentrated acid gives alkenes; esterification with acids gives esters.

Carbonyl compounds

Aldehydes and ketones: nucleophilic addition with HCN. Tests: 2,4-DNPH gives an orange precipitate with both; Tollens' reagent (silver mirror) and Fehling's solution (brick-red) are positive for aldehydes only. Iodoform test (yellow CHI₃) for CH₃CO– groups.

Carboxylic acids and derivatives

Weak acids; react with carbonates (CO₂), alcohols (esters) and are made from nitriles by hydrolysis. Acyl chlorides react readily with water, alcohols and amines.

Nitrogen compounds

Amines are bases (lone pair on N); phenylamine is a weaker base because the lone pair is delocalised into the ring. Amino acids are zwitterions; they form peptides and proteins.

Polymers

Addition polymers (from alkenes) and condensation polymers: polyesters (Terylene) and polyamides (nylon, proteins) release water or HCl. Condensation polymers can be hydrolysed, so some are biodegradable.

Analytical techniques

• Infrared spectroscopy: O–H (broad, 2500–3300 cm⁻¹ for acids; 3200–3600 cm⁻¹ for alcohols), C=O (about 1700 cm⁻¹).
• Mass spectrometry: the molecular ion peak gives Mr; fragments indicate structure.
• NMR (¹H): number of environments, chemical shift and splitting (n + 1 rule).
• Chromatography: paper, thin-layer and gas chromatography separate mixtures.

Check yourself

1. What is observed when ethanol is heated with acidified potassium dichromate?
2. Which test distinguishes an aldehyde from a ketone?
3. An IR spectrum shows a strong absorption near 1700 cm⁻¹. Which bond is present?