13.3 Shapes of organic molecules; σ and π bonds

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.

An sp-hybridised atom has two electron domains and a linear arrangement near 180°. sp² gives three domains and a trigonal-planar arrangement near 120°, while sp³ gives four domains and a tetrahedral arrangement near 109.5°.

These are idealised local geometries. Lone pairs and substituents can distort angles, and the hybridisation label refers to the atom’s bonding environment rather than the whole molecule’s shape.

The carbon atoms in ethyne are sp and linear; each alkene carbon in ethene is sp² and trigonal planar; methane carbon is sp³ and tetrahedral.

Do not call every molecule with one sp³ atom tetrahedral overall. Apply the geometry around the specific atom being considered.

A sigma (σ) bond forms by end-on overlap along the internuclear axis. A pi (π) bond forms by sideways overlap of parallel p orbitals and exists in addition to a σ bond.

A single bond is one σ bond; a double bond is one σ plus one π; a triple bond is one σ plus two π bonds. Hybridisation describes which orbitals make the σ framework and which unhybridised p orbitals make π bonds.

Ethene has five σ bonds and one π bond. Ethyne has three σ bonds and two π bonds, so rotation around the C≡C axis is not a simple single-bond rotation.

A double bond is not “two π bonds”, and a π bond is not a second atom-to-atom connection independent of the σ bond.

A planar arrangement places the stated atoms in the same geometric plane. In ethene, each carbon is sp² and the carbon atoms and attached substituents are approximately planar.

Planarity follows from the local orbital arrangement and restricted rotation around a C=C bond. It is a structural description, not a claim that every atom in a large molecule lies in one plane.

The two carbon atoms and four hydrogens of ethene form a planar group. Replacing the C=C by a single bond changes the rotational freedom and may remove that planar constraint.

Planar does not mean flat in every dimension of the whole molecule, and it does not by itself prove aromaticity.