Alkenes: bonding and E/Z isomerism

Sigma and pi bonds

Alkenes are unsaturated hydrocarbons with general formula CnH2n (cycloalkenes CnH2n-2). The C=C bond is:

• a sigma (σ) bond: head-on overlap of orbitals, electron density between the nuclei
• a pi (π) bond: sideways overlap of p orbitals, electron density above and below the plane

The π bond is weaker and exposed, so alkenes react by addition.

Restricted rotation

The π bond prevents rotation about C=C. If each carbon of the double bond carries two different groups, two geometric isomers exist, a type of stereoisomerism (same structural formula, different arrangement in space).

Cis/trans and E/Z

• cis: the same groups on the same side; trans: on opposite sides. This only works when the two carbons share a common group.
• E/Z works for all cases: on each carbon give priority to the atom with the higher atomic number. Z (zusammen): the higher-priority groups are on the same side. E (entgegen): opposite sides.

Example: but-2-ene has cis (Z) and trans (E) isomers; but-1-ene has none because C1 carries two H atoms.

Worked example

Name the isomer of CHCl=CHBr in which Cl and Br are on the same side.

1. On C1, Cl (Z = 17) beats H; on C2, Br (Z = 35) beats H.
2. The higher-priority groups are on the same side.

Answer: (Z)-1-bromo-2-chloroethene

Check yourself

1. How is a π bond formed?
   • Head-on overlap of orbitals
   • Transfer of electrons
   • Sideways overlap of p orbitals
   • Donation of a lone pair
   Show answer

   Sideways overlap of p orbitals. π bonds come from sideways p orbital overlap, above and below the bond axis.

2. Which compound shows E/Z isomerism?
   • Pent-2-ene
   • Propene
   • But-1-ene
   • 2-methylpropene
   Show answer

   Pent-2-ene. In pent-2-ene each C of the double bond has two different groups (H and CH3; H and C2H5).

3. Why do E/Z isomers exist?
   • The molecules have different molecular formulae
   • The C=C bond is very long
   • Rotation about the C=C bond is restricted by the π bond
   • Alkenes have hydrogen bonds
   Show answer

   Rotation about the C=C bond is restricted by the π bond. Without rotation the two arrangements cannot interconvert.

4. In E/Z naming, which group has higher priority on a carbon carrying Br and CH3?
   • CH3
   • Br
   • They are equal
   • It depends on the other carbon
   Show answer

   Br. Br (atomic number 35) beats C (6).

5. What is the general formula of the alkenes?
   • CnH2n+2
   • CnH2n-2
   • CnHn
   • CnH2n
   Show answer

   CnH2n. One C=C means two fewer H than the alkane.

6. Why are alkenes more reactive than alkanes?
   • Alkenes have more hydrogen atoms
   • The π bond is weaker and its electron density is exposed to attack
   • The σ bond is very weak
   • Alkenes are ionic
   Show answer

   The π bond is weaker and its electron density is exposed to attack. Electrophiles are attracted to the high electron density of the π bond.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.