Everything from the 6 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.
A molecular formula gives the actual number of each atom (C4H10O); a structural formula shows the arrangement unambiguously (CH3CH2CH2CH2OH); a displayed formula shows every bond; a skeletal formula shows only the carbon skeleton and functional groups, with carbon atoms at corners and line ends and hydrogens on carbon left out. A homologous series shares a general formula and functional group, with each member differing by CH2. The functional group is the atom or group responsible for the characteristic reactions.
In a skeletal formula hydrogens on carbon are not drawn, but they are still there: work out how many each carbon needs to make four bonds. Hydrogens on O or N are shown (–OH, –NH2). A structural formula must be unambiguous: C3H7OH is not acceptable because it could be propan-1-ol or propan-2-ol.
Find the longest carbon chain containing the functional group; its length gives the stem (meth-, eth-, prop-, but-, pent-, hex-). The functional group gives the suffix (-ane, -ene, -ol, -al, -one, -oic acid, -nitrile, -amine) or a prefix (chloro-, bromo-, methyl-). Number the chain from the end that gives the lowest locants, list prefixes alphabetically, and use di-, tri- for repeats.
Always find the longest chain, which may bend round a corner in a drawn structure; a “branch” at the end of a chain is just part of the chain. Number from the end that gives the lowest locants, and in nitriles, acids and aldehydes the functional-group carbon is C1 and is included in the stem count.
In homolytic fission a covalent bond breaks with one electron going to each atom, giving two free radicals (species with an unpaired electron). In heterolytic fission both electrons go to one atom, giving ions. A nucleophile is an electron-pair donor that attacks a δ+ carbon (OH−, CN−, NH3, H2O); an electrophile is an electron-pair acceptor attracted to regions of high electron density such as a C=C (H+, Brδ+, NO2+). A curly arrow shows the movement of a pair of electrons, starting from a lone pair or bond.
A nucleophile donates an electron pair; an electrophile accepts one. Curly arrows start from a lone pair or a bond, never from an atom symbol or a charge sign. Homolytic fission gives radicals, not ions.
A carbon bonded to four atoms (single bonds only) is sp3, tetrahedral, 109.5°. A carbon in a double bond is bonded to three atoms: sp2, trigonal planar, 120°. A carbon in a triple bond (or with two double bonds) is bonded to two: sp, linear, 180°. Every single bond is a σ bond; a double bond is σ + π. The π bond forms by sideways overlap of p orbitals above and below the plane, so ethene is planar and there is no rotation about C=C, which is why cis-trans isomers exist.
Count the C–H bonds; they are σ bonds too. A double bond is one σ and one π, not two π. In propene only the two C=C carbons are planar; the CH3 carbon is tetrahedral.
Structural isomers have the same molecular formula but a different structural formula. Chain isomers differ in the carbon skeleton (butane and methylpropane). Positional isomers have the same functional group in a different position (propan-1-ol and propan-2-ol). Functional group isomers belong to different homologous series (propanal and propanone; ethanoic acid and methyl methanoate). Branching lowers boiling point because branched molecules have less surface contact.
Drawing the same molecule twice is the commonest error: a chain drawn bent, or numbered from the other end, is not a new isomer. Name each structure; if two names are identical, the structures are the same. 3-bromobutane is not a new isomer; it is 2-bromobutane.
Stereoisomers have the same structural formula but a different arrangement of atoms in space. Cis-trans isomerism arises because rotation about a C=C bond is restricted; it needs two different groups on each carbon of the double bond. Optical isomerism arises when a carbon atom carries four different groups (a chiral centre); the two forms, enantiomers, are non-superimposable mirror images that rotate plane-polarised light equally in opposite directions. A 50 : 50 mixture is a racemic mixture and has no overall effect on the light.
Cis-trans isomerism needs two different groups on each carbon of the C=C, so but-1-ene does not show it. A chiral centre needs four different groups; a carbon with two identical groups, such as the central carbon in propan-2-ol, is not chiral.