Cambridge International AS & A Level Chemistry (9701)
Introduction to A Level Organic Chemistry
The big idea: A Level organic chemistry adds new families of compounds (arenes, phenols, acyl chlorides, amines, amides, amino acids) to the AS ones. Each family is defined by its functional group, the benzene ring is held together by a delocalised π system, and molecules with a chiral centre come as a pair of mirror-image isomers.
Summary — What This Chapter Covers
- The functional group decides the physical and chemical properties of a compound.
- General, structural, displayed and skeletal formulas for arenes, halogenoarenes, phenols, acyl chlorides, amines, amides and amino acids.
- Systematic names for aliphatic compounds (up to six carbons) and for aromatic compounds with one benzene ring.
- Two new mechanism types: electrophilic substitution and addition–elimination.
- Benzene is planar with sp² carbons, a σ-bond framework and a delocalised π system.
- Enantiomers rotate plane-polarised light in opposite directions; chirality matters in drug synthesis.
1. Formulas, Functional Groups and Naming
29.1
The A Level functional groups
A functional group is the atom or group of atoms that gives a homologous series its characteristic reactions. Members of a series share the same functional group, so they react in the same way; their physical properties change gradually with chain length.
| Homologous series | Functional group | Structural formula | Example |
|---|
| Arene | Benzene ring | C6H5– | benzene, C6H6 |
| Halogenoarene | Halogen on a ring | C6H5X | chlorobenzene, C6H5Cl |
| Phenol | –OH on a ring | C6H5OH | phenol |
| Acyl chloride | –COCl | RCOCl | propanoyl chloride, CH3CH2COCl |
| Amine (primary, secondary, tertiary) | –NH2, –NH–, –N< | RNH2, R2NH, R3N | (CH3)2NH, (CH3)3N |
| Amide (primary, secondary, tertiary) | –CONH2, –CONH–, –CON< | RCONH2 | propanamide, CH3CH2CONH2 |
| Amino acid | Amine and carboxyl | HOOCCHRNH2 | 2-aminoethanoic acid, H2NCH2COOH |
Naming secondary and tertiary amines is not required. Where a benzene ring is part of a molecule, you are not expected to draw a displayed formula of the ring: draw the hexagon with a circle inside (or alternating double bonds).
Types of formula
| Formula | Propanoyl chloride |
|---|
| General | CnH2n+1COCl |
| Structural | CH3CH2COCl |
| Displayed | Every atom and every bond shown, including C=O and C–Cl |
| Skeletal | Zig-zag of two lines (carbon chain) ending in a carbon with =O and –Cl drawn on |
Systematic names
Aliphatic: you need up to six carbon atoms in the main chain (six plus six for esters and amides), including cyclic compounds with one ring of up to six carbons. Esters and nitriles are straight-chain only.
- CH3COCl: ethanoyl chloride
- CH3CH2CONH2: propanamide
- CH3CH(NH2)COOH: 2-aminopropanoic acid
- CH3CONHCH3: N-methylethanamide (a secondary amide)
Aromatic: one benzene ring with one or more simple substituents. Number the ring carbons 1 to 6 so that the substituents get the lowest numbers. If the name is based on a parent such as phenol, benzoic acid or phenylamine, the carbon carrying that group is C1.
- C6H5NO2: nitrobenzene; C6H5CH3: methylbenzene
- C6H5COOH: benzoic acid; C6H5NH2: phenylamine
- 3-nitrobenzoic acid: COOH on C1, NO2 on C3
- 2,4,6-tribromophenol: OH on C1, Br on C2, C4 and C6
- 3
4. Optical Isomerism