Ch 2: Hybridisation

Chapter 2 : Alkanes

Hybridisation

Hybridisation is a concept that allows us to account for certain key structural issues that are not easily accounted for in other bonding theories when starting from the ground state of a carbon atom 2s² 2p².
The concept involves the "cross breeding" of atomic orbitals to create "new" orbitals. Hence the use of the term "hybrid" : think of a hybrid animal which is a cross breed of two species.

STUDY TIP : It may be useful to think of hybridisation like a food blender !

In the kitchen, take 1 orange and 3 apples, put them in a food blender, put the lid on (so nothing comes out), and turn on the blender. When you stop you have a fruit mixture made from 4 pieces of fruit that has characteristics of both apples and oranges.

At the atomic level, take 1 x 2s orbital and 3 x 2p orbitals, put them in a "blender", put the lid on (so nothing comes out), and turn on the "blender". When you stop you have an orbital mixture made from 4 orbitals that has characteristics of both s and p orbitals.

Let's pose the bonding problems in the context of methane, CH₄

despite the fact that C has only 2 unpaired electrons, it forms 4 bonds
all 4 C-H bonds are of equal strength
tetrahedral shape (i.e. H-C-H bond angle = 109.5 degrees)

General method

The number of orbitals in the hybrid set is determined by the number of σ bonds required at that centre.* 4σ bonds => sp³ hybridised (e.g. alkanes) 3σ bonds => sp² hybridised (e.g. alkenes) 2σ bonds => sp hybridised (e.g. alkynes)
σ bonds will be formed by the "end-to-end" interaction of the hybrid orbitals with either H 1s orbitals or other C hybrids
π bonds will be formed by the "side-to-side" interaction between pairs of unhybridised C p orbitals
* This simplification has to be modified if there is a resonance interaction with another group, e.g. in amides .