13.3 Shapes of organic molecules; σ and π bonds
- Syllabus
- 9701–2028–2029
- Topic
- 13.3
- Level
- AS
Learning objectives
13.3.1Organic molecules as either straight-chained,• Describe organic molecules as either straight-chained, branched or cyclic13.3.2Shape of, and bond angles in, molecules• Describe/explain: the shape of, and bond angles in, molecules containing sp, sp 2 and sp3 hybridised atoms13.3.3Arrangement of σ and π bonds in molecules• Describe the arrangement of σ and π bonds in molecules containing sp, sp2 and sp3 hybridised atoms13.3.4Terms: planar when describing the arrangement• Use terms: planar when describing the arrangement of atoms in organic molecules, e.g. ethene
Organic molecules may be straight-chain, branched or cyclic
A carbon skeleton can be straight-chain, branched or cyclic. This describes connectivity; it is separate from whether the molecule is saturated and from the functional group it carries.
Trace the carbon–carbon framework before naming a molecule or counting isomers. A ring closes the chain, while a branch creates a carbon substituent attached to the parent chain.
Butane is straight-chain, 2-methylpropane is branched, and cyclohexane is cyclic. All three are hydrocarbons, but their connectivity and physical properties differ.
A cyclic molecule is not automatically aromatic, and a branched molecule does not have fewer carbon atoms than its unbranched isomer.
Hybridisation fixes local shape and ideal bond angle
| Hybridisation at the atom | Hybrid orbitals / bonding directions | Local arrangement | Ideal angle | Carbon example |
|---|---|---|---|---|
| sp | 2 | linear | 180° | each C in HC≡CH |
| sp² | 3 | trigonal planar | 120° | each C in H₂C=CH₂ |
| sp³ | 4 | tetrahedral | 109.5° | C in CH₄ |
Hybrid orbitals point as far apart as possible, which minimises repulsion between bonding electron regions. Two directions lie opposite, three spread in one plane, and four point towards the corners of a tetrahedron.
Assign the geometry around the atom being considered: a carbon in C≡C is sp, a carbon in C=C is sp², and a carbon with four single-bond directions is sp³. The label describes a local bonding environment, not automatically the shape of the whole molecule.
The angles are ideal values. Lone pairs and unequal surrounding groups can alter measured angles, but they do not change the defining sp, sp² and sp³ arrangements used here.
Hybrid orbitals make the σ framework; unhybridised p orbitals make π bonds
A sigma (σ) bond is formed by end-on overlap along the internuclear axis. A pi (π) bond is formed by sideways overlap of parallel unhybridised p orbitals, with electron density on opposite sides of that axis.
| Hybridisation at carbon | σ-bond directions | Unhybridised p orbitals | Possible local multiple-bond contribution |
|---|---|---|---|
| sp³ | four tetrahedral directions | 0 | σ bonds only |
| sp² | three coplanar directions | 1, perpendicular to that plane | one π bond |
| sp | two collinear directions | 2, mutually perpendicular | two π bonds |
Every bonded pair of atoms has one σ bond: a single bond is 1σ, a double bond is 1σ + 1π, and a triple bond is 1σ + 2π. Ethene therefore has five σ bonds and one π bond; ethyne has three σ bonds and two π bonds.
A double bond is not two π bonds. The π overlap accompanies the σ connection and requires the p orbitals on neighbouring atoms to remain parallel.
Planar means the stated atoms lie in one plane
A planar arrangement places the stated atoms in the same geometric plane. In ethene, both carbon atoms are sp² and the two carbon atoms plus their four attached hydrogen atoms are planar.
The three σ-bond directions around each sp² carbon lie in one plane. The remaining p orbitals stand perpendicular to that plane and must stay parallel for sideways overlap, so rotation about C=C would destroy the π overlap.
State which atoms are planar rather than calling an entire large molecule flat. A molecule may contain a planar alkene region alongside sp³ atoms whose bonds point out of that plane.
Planar does not mean aromatic, and a flat drawing on paper does not prove that all represented atoms are coplanar in three dimensions.