Edexcel A Level Chemistry Topic 18: Organic Chemistry III Exam Questions

About Edexcel A Level Chemistry Topic 18: Organic Chemistry III

Topic 18 of Edexcel A Level Chemistry, Organic Chemistry III, covers aromatic chemistry, nitrogen compounds, polymers and synthesis. You study the delocalised structure of benzene, evidence from enthalpy of hydrogenation and bond lengths, and electrophilic substitution reactions including nitration, halogenation and Friedel-Crafts, plus the reactions of phenol. Amines, amides and amino acids are covered, including basicity, making azo dyes and peptide formation. Addition and condensation polymers are compared. The synthesis section brings in Grignard reagents, practical techniques such as recrystallisation and melting temperature determination, and planning multi-step routes. It is assessed in Paper 2, and examiners often ask for mechanisms and synthetic routes with reagents and conditions.

What you need to know

• Structure of benzene Benzene has a planar ring of delocalised pi electrons. Its enthalpy of hydrogenation is less exothermic than predicted for a Kekule structure, and all C-C bonds are the same length.
• Electrophilic substitution Nitration with concentrated HNO3 and H2SO4 at about 50 °C forms NO2+. Halogenation uses a halogen carrier such as FeBr3 or AlCl3, and Friedel-Crafts alkylation and acylation use AlCl3.
• Phenol Phenol reacts with bromine water without a catalyst to give a white precipitate of 2,4,6-tribromophenol, because an oxygen lone pair is delocalised into the ring, activating it.
• Amines Amines are bases because of the nitrogen lone pair. Aliphatic amines are stronger bases than ammonia, and phenylamine is weaker. Phenylamine is made from nitrobenzene with tin and concentrated HCl, then NaOH, and forms diazonium salts below 10 °C that couple to give azo dyes.
• Amides and amino acids Amides form from acyl chlorides and ammonia or amines. Amino acids are zwitterions, have isoelectric points and join through peptide (amide) links to form proteins.
• Polymers Addition polymers form from alkenes with no other product. Condensation polymers such as polyesters and polyamides form with loss of a small molecule, and can be hydrolysed.
• Grignard reagents and synthesis Grignard reagents (RMgX) are made from halogenoalkanes and magnesium in dry ether. They react with CO2 to form carboxylic acids and with carbonyls to form alcohols, increasing chain length.

Common mistakes in Topic 18: Organic Chemistry III

• Drawing the intermediate in electrophilic substitution with a complete delocalised ring; the horseshoe must cover only five carbons, with a positive charge.
• Saying benzene does not react with bromine because it is saturated; it is unsaturated but the delocalised ring is stable.
• Forgetting that Grignard reactions need dry conditions because water destroys the reagent.
• Saying phenylamine is a stronger base than ethylamine.
• Missing the small molecule lost when drawing a condensation polymer.

Try three Topic 18: Organic Chemistry III exam-style questions