Everything from the 4 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
At A Level the organic toolkit grows. An arene contains a benzene ring (C6H5– is the phenyl group); a halogenoarene has a halogen joined directly to the ring; a phenol has –OH joined directly to the ring. Acyl chlorides (RCOCl), amides (RCONH2) and nitriles (RCN) are named from the parent acid or alkane, counting the functional carbon. Amines are classified by the number of carbon groups bonded to the nitrogen: one (primary, RNH2), two (secondary, R2NH) or three (tertiary, R3N). An amino acid has both –NH2 and –COOH, and 2-amino acids carry both on the same carbon. On a benzene ring, substituents get locants numbered to be as low as possible, as in 1,3-dinitrobenzene.
Amines are classified by counting the carbon groups on the nitrogen, not by the type of carbon the nitrogen is on: (CH3)3CNH2 is a primary amine even though it would be a tertiary alcohol if the NH2 were OH. Phenol has –OH directly on the ring; C6H5CH2OH is an alcohol (phenylmethanol). Chlorobenzene is a halogenoarene; C6H5CH2Cl, with Cl on a side chain, behaves like a halogenoalkane.
In electrophilic substitution an electrophile such as NO2+ attacks the delocalised electrons of a benzene ring and replaces an H atom, so the ring survives. In addition-elimination a nucleophile adds to the C=O carbon of an acyl chloride and then Cl− is eliminated, leaving HCl. A condensation reaction joins two molecules with the loss of a small molecule such as H2O or HCl. Hydrolysis breaks a bond by reaction with water, often catalysed by acid or alkali. Reduction adds hydrogen or removes oxygen (written [H]); oxidation adds oxygen or removes hydrogen (written [O]). Diazotisation turns phenylamine into a diazonium salt, which couples with phenol to make an azo dye.
Benzene undergoes electrophilic substitution, not addition: an addition would destroy the stable delocalised ring. Addition-elimination happens at the C=O carbon of an acyl chloride, so it is not the same as nucleophilic substitution of a halogenoalkane. Hydrolysis and condensation are opposites: condensation releases a small molecule, hydrolysis uses water to split a bond.
Each carbon in benzene is sp2 hybridised: it forms three sigma bonds (two to C, one to H) in a plane at 120°, so the ring is planar. The remaining p orbital on each carbon overlaps sideways with both neighbours, forming a delocalised pi system of six electrons in rings above and below the plane. The evidence against the Kekulé structure is threefold: all six C–C bonds are 0.139 nm, between C–C (0.154 nm) and C=C (0.134 nm); the enthalpy of hydrogenation is −208 kJ mol−1, not the −360 kJ mol−1 predicted from three C=C bonds, so benzene is about 152 kJ mol−1 more stable; and benzene resists addition, preferring substitution that keeps the delocalised ring.
The circle does not mean benzene has no double bond character: every C–C bond has some. Stabilisation is 152 kJ mol−1, the gap between −360 and −208, not 208. The pi electrons are above and below the plane of the ring, not in it; the sigma framework lies in the plane.
A chiral centre is a carbon atom bonded to four different groups. A molecule with one chiral centre exists as two enantiomers: non-superimposable mirror images of each other. Enantiomers have identical physical properties and identical reactions with non-chiral reagents, but they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture contains equal amounts of the two and has no net effect on plane-polarised light. Living systems are chiral, so enantiomers can act very differently as drugs: one enantiomer of thalidomide is a sedative, the other caused birth defects. A molecule with n chiral centres has at most 2n stereoisomers.
A chiral centre needs four different groups, not four different atoms: in butan-2-ol the CH3 and C2H5 both join through carbon but are different groups. A carbon in a C=C or C=O is never a chiral centre. Enantiomers have the same boiling point, density and reactions with non-chiral reagents; only the direction of rotation of plane-polarised light (and their behaviour with other chiral molecules) differs. 2n is a maximum: symmetrical molecules can have fewer.