Every carbon atom in an organic molecule has one of three shapes, decided by how many atoms it is bonded to. The π bond in an alkene locks two of those shapes into a flat, rigid unit.
A carbon bonded to four atoms (single bonds only) is sp3, tetrahedral, 109.5°. A carbon in a double bond is bonded to three atoms: sp2, trigonal planar, 120°. A carbon in a triple bond (or with two double bonds) is bonded to two: sp, linear, 180°. Every single bond is a σ bond; a double bond is σ + π. The π bond forms by sideways overlap of p orbitals above and below the plane, so ethene is planar and there is no rotation about C=C, which is why cis-trans isomers exist.
Overlap the orbitals in the sigma and pi simulator and watch the shape around each carbon change from ethane to ethene to ethyne.
| Atoms bonded to C | Hybridisation | Shape, angle | Example |
|---|---|---|---|
| 4 | sp3 | tetrahedral, 109.5° | every C in ethane; the CH3 of propene |
| 3 | sp2 | trigonal planar, 120° | both C=C carbons in ethene; the C of C=O |
| 2 | sp | linear, 180° | ethyne; the C of C≡N |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the H–C–H bond angle in ethene?
How many π bonds are there in ethene?
What is the shape around each carbon atom in ethane?
Why is there no free rotation about a C=C bond?
Count every bond as one σ, then add one π for each double bond and two for each triple bond. Check your answers in the VSEPR simulator.
| Molecule | σ | π |
|---|---|---|
| ethene, CH2=CH2 | 5 | 1 |
| propene, CH2=CHCH3 | 8 | 1 |
| ethanal, CH3CHO | 6 | 1 |
| propanenitrile, CH3CH2CN | 8 | 2 |
Count the C–H bonds; they are σ bonds too. A double bond is one σ and one π, not two π. In propene only the two C=C carbons are planar; the CH3 carbon is tetrahedral.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
How many σ bonds and π bonds are there in propene, CH2=CHCH3?
What is the O=C–H bond angle in methanal, HCHO?
Which molecule contains carbon atoms with two different shapes?
What is the hybridisation of the carbon atom in the C≡N group of ethanenitrile, CH3CN?
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