Shapes of organic molecules
Shapes of organic molecules
- The shape around a carbon depends on its hybridisation.
- Single bonds are sigma bonds; double bonds add a pi bond.
- This is why ethene is flat.
Hybridisation and shape
| Hybridisation | Bonds | Shape | Angle |
|---|---|---|---|
| $\text{sp}^3$ | 4 single | tetrahedral | $109.5°$ |
| $\text{sp}^2$ | 1 double + 2 single | planar (flat) | $120°$ |
| $\text{sp}$ | 1 triple | linear | $180°$ |
Practice
An sp³ carbon (four single bonds) is:
Four single bonds (sp³) give a tetrahedral shape at 109.5°.
Practice
An sp² carbon (one double + two single bonds), as in ethene, is:
sp² hybridisation gives a flat (planar) arrangement at 120°, so ethene is planar.
Practice
An sp carbon (a triple bond) is:
sp hybridisation gives a linear shape at 180° (e.g. ethyne).
Sigma and pi bonds
- Every single bond is a sigma (σ) bond, made by direct overlap.
- A double bond is one σ + one π bond (pi from sideways p-orbital overlap).
- Ethene is planar because its carbons are $\text{sp}^2$.
Practice
A C=C double bond consists of:
A single bond is one σ; a double bond is one σ plus one π (sideways p-orbital overlap).
You've got it
Key idea
- shape follows hybridisation: sp³ tetrahedral ($109.5°$), sp² planar ($120°$), sp linear ($180°$)
- single bond = σ; double bond = σ + π (π from sideways p-orbital overlap)
- ethene is planar (sp² carbons)