Shapes of organic molecules
Introduced| English | Meaning |
|---|---|
| hybridisation/ˌhaɪbrɪdaɪˈzeɪʃn/ | The mathematical combination of atomic orbitals on the same atom to form new orbitals used to describe bonding. |
| sigma/ˈsɪɡmə/ | sigma |
| pi/paɪ/ | pi |
| tetrahedral/ˌtetrəˈhiːdrəl/ | Having four positions arranged towards the corners of a tetrahedron around a centre. |
| planar/ˈpleɪnə/ | Flat: all atoms lying in one plane, as in the benzene ring. |
The 3-D shape of carbon
- The shape around a carbon depends on its hybridisation 杂化.
- Single bonds are sigma σ键 bonds; double bonds add a pi π键 bond.
- This is why ethene is flat.
A carbon atom forming four single bonds is tetrahedral with bond angles of about 109.5°.
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Four bonding pairs repel equally.
Hybridisation and shape
| Hybridisation | Bonds | Shape | Angle |
|---|---|---|---|
| $\text{sp}^3$ | 4 single | tetrahedral 四面体形 | $109.5°$ |
| $\text{sp}^2$ | 1 double + 2 single | planar 平面 (flat) | $120°$ |
| $\text{sp}$ | 1 triple | linear | $180°$ |
Organic molecules have definite three-dimensional shapes
Shapes of organic molecules
VSEPR around each carbon
Around a single-bonded carbon the four pairs are tetrahedral (109.5°).
An sp³ carbon (four single bonds) is:
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Four single bonds (sp³) give a tetrahedral shape at 109.5°.
An sp² carbon (one double + two single bonds), as in ethene, is:
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sp² hybridisation gives a flat (planar) arrangement at 120°, so ethene is planar.
An sp carbon (a triple bond) is:
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sp hybridisation gives a linear shape at 180° (e.g. ethyne).
Match each bonding feature to its geometry.
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Each item links the term to its correct meaning.
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 $\text{sp}^2$.
Carbon shape from hybridisation: sp3 tetrahedral, sp2 planar, sp linear
A C=C double bond consists of:
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A single bond is one σ; a double bond is one σ plus one π (sideways p-orbital overlap).
Shape around carbon
- A carbon with four single bonds is tetrahedral (~109.5°); around a C=C it is trigonal planar (~120°).
- The shape follows from electron-pair repulsion.
You've got it
- 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)