Topic 5 · page 1 of 15
Alkenes and the double bond
Alkenes are unsaturated hydrocarbons with general formula CₙH₂ₙ. Their C=C double bond is one σ bond plus one π bond, and the exposed π electrons make alkenes reactive.
Alkenes and cycloalkenes 5.1
A hydrocarbon is a compound of carbon and hydrogen only. In the alkanes (Topic 4) every carbon–carbon bond is a single bond. The alkenes are the next family: hydrocarbons with at least one carbon–carbon double bond, C=C.
An alkane with n carbon atoms is CₙH₂ₙ₊₂. To make a C=C, take one hydrogen atom from each of two neighbouring carbons and let those carbons share a second pair of electrons. Two hydrogen atoms are lost, so the general formula of the alkenes is CₙH₂ₙ.
| Alkene | Molecular formula | Check with CₙH₂ₙ |
|---|---|---|
| ethene | C₂H₄ | n = 2, 2n = 4 |
| propene | C₃H₆ | n = 3, 2n = 6 |
| but-1-ene | C₄H₈ | n = 4, 2n = 8 |
| pent-1-ene | C₅H₁₀ | n = 5, 2n = 10 |
Naming and cycloalkenes
The name comes from the alkane stem with -ene in place of -ane. The number in the name is the lower-numbered carbon of the C=C, counting from the end that gives the lowest number: but-1-ene is CH₂=CHCH₂CH₃, and but-2-ene is CH₃CH=CHCH₃. The rules for branches are the ones from Topic 4: 2-methylpropene is CH₂=C(CH₃)₂.
A cycloalkene has a C=C inside a ring. Its ring costs two hydrogen atoms and its double bond costs two more, so cyclohexene is C₆H₁₀, not C₆H₁₂. The general formula for a cycloalkene with one C=C is CₙH₂ₙ₋₂. In a ring, the C=C carbons are numbered 1 and 2, so no number is needed: the name is cyclohexene.
The C=C bond: one σ bond and one π bond 5.1
Each carbon of a C=C is bonded to three other atoms. Its three bonds spread out as far as possible, so they lie in one plane at about 120°. The fourth electron of each carbon sits in a p orbital, which points above and below that plane. The two bonds between the carbons are different kinds.
So a C=C is one σ bond plus one π bond. A C–C single bond is a σ bond only, and so is every C–H bond.
Check yourself
- Give the general formula of the alkenes, and write the molecular formula of the alkene with seven carbon atoms.
- Explain why propene is described as an unsaturated hydrocarbon.
- A hydrocarbon has molecular formula C₆H₁₂. Give two different compounds it could be, and say how you could tell which one you have.
- Describe the bonding in a C=C double bond in terms of σ and π bonds.
- Explain why alkenes are more reactive than alkanes.
- An alkene has a relative molecular mass of 70.0. Work out its molecular formula, and give the formula of the cycloalkene with the same number of carbon atoms.
Answers cover these until you have tried the questions
- CₙH₂ₙ. For n = 7 the formula is C₇H₁₄ (2 × 7 = 14).
- Propene is a hydrocarbon (carbon and hydrogen only) and it contains a C=C double bond, so its carbon atoms are not each bonded to the maximum number of other atoms: it is unsaturated.
- It could be an alkene, such as hex-1-ene (C₆H₁₂), or a cycloalkane, cyclohexane (C₆H₁₂). Both fit CₙH₂ₙ. Shake with bromine water: the alkene turns it from orange to colourless; the cycloalkane leaves it orange.
- A C=C is one σ bond and one π bond. The σ bond is formed by head-on overlap of orbitals, with its electron density on the axis between the nuclei. The π bond is formed by sideways overlap of two p orbitals, with its electron density above and below the axis.
- The π bond is weaker than the σ bond and its electrons lie outside the axis between the nuclei, so they are exposed. They are easily attacked by electrophiles, and the π bond breaks easily. An alkane has only σ bonds, which are stronger and have no exposed region of electron density.
- of CₙH₂ₙ = 12.0n + 2.0n = 14.0n. 14.0n = 70.0, so n = 5. The alkene is C₅H₁₀ (pentene). Check: 5 × 12.0 + 10 × 1.0 = 70.0. The cycloalkene with five carbons is C₅H₈ (cyclopentene): CₙH₂ₙ₋₂ with n = 5.