Overview
This lecture covers Edexcel A-level Chemistry Topic 18: Organic Chemistry 3, focusing on aromatic compounds (benzene, phenol), amines, polymers, organic synthesis, and essential practical techniques.
Aromatic Compounds: Benzene & Phenol
- Benzene has the formula C₆H₆, a planar ring with delocalized π electrons above and below the ring.
- All C–C bonds in benzene are the same length (139 pm), intermediate between single and double bonds.
- Benzene is more stable than the Kekulé structure due to delocalization energy ("resonance energy").
- Benzene burns with a smoky flame due to incomplete combustion.
- Benzene undergoes electrophilic substitution, not addition, to preserve stability.
- Main benzene reactions: nitration, halogenation, Friedel–Crafts acylation/alkylation (all require strong electrophiles).
- Phenol is more reactive than benzene because the –OH group increases electron density in the ring.
Amines and Their Reactions
- Amines are derivatives of ammonia; classified as primary, secondary, tertiary, or quaternary.
- Can be synthesized by: (1) heating haloalkanes with excess ammonia, or (2) reducing nitriles (for pure primary amines).
- Aromatic amines made by reducing nitrobenzene with Sn/HCl (then NaOH).
- Amines are weak bases; base strength depends on electron-donating/withdrawing groups.
- Amines act as nucleophiles and can form complex ions with transition metals.
Polymers: Polyamides, Polyesters, Amino Acids, and Proteins
- Condensation polymers form when monomers with two functional groups (e.g., diacid + diamine) link, eliminating water.
- Polyamides (e.g., nylon, Kevlar) contain amide bonds; polyesters include ester bonds.
- Amino acids have both –NH₂ and –COOH groups; can form zwitterions and are chiral (except glycine).
- Proteins are natural polyamides (polypeptides).
Organic Synthesis & Reaction Pathways
- Functional group transformations require knowledge of reagents, conditions, and types of reactions (oxidation, reduction, condensation, substitution, addition, elimination, hydrolysis).
- Grignard reagents (RMgX) are used to form C–C bonds with CO₂ or carbonyl compounds.
- Identifying synthetic routes and recognizing functional groups are essential skills.
Practical Techniques
- Reflux: used to heat volatile mixtures without loss of product.
- Distillation: separates liquids by boiling point.
- Steam distillation: separates high-boiling or heat-sensitive compounds.
- Separation with a separating funnel, washing, and drying agents to purify products.
- Filtration (gravity or vacuum) and recrystallization purify solids.
- Purity assessed by measuring boiling/melting points.
Key Terms & Definitions
- Electrophile — a species attracted to electron-rich areas, accepts an electron pair.
- Electrophilic substitution — a reaction where an atom is replaced by an electrophile, common in benzene chemistry.
- Zwitterion — a molecule with both positive and negative charges but overall neutral (e.g., amino acids at their isoelectric point).
- Condensation polymerization — monomer linkage with elimination of a small molecule (usually water).
- Grignard reagent — organomagnesium halide (RMgX), used for forming C–C bonds.
Action Items / Next Steps
- Revise key reactions, mechanisms, and required conditions for all major functional groups.
- Practice synthesis pathways and reaction route maps.
- Complete assigned homework or exam-style questions on Topic 18.
- Review practical techniques and ensure understanding of purification methods.