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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

4.1.3 Alkenes

Addition reactions of alkenesPolymers from alkenesProperties of alkenesStereoisomerism in alkenes

Bonding in Alkenes

Specification Reference 4.1.3 (a)–(b)

Quick Notes

  • Alkenes are unsaturated hydrocarbons containing a C=C double bond made of a σ-bond (direct orbital overlap) and a π-bond (sideways p-orbital overlap).
    • OCR (A) A-Level Chemistry diagram showing sigma and pi bonding in an alkene C=C double bond.
    • The π-bond restricts rotation around the C=C bond.
  • The C=C bond in alkenes gives rise to a trigonal planar shape.
    • Bond angle approximately 120° around each carbon in the double bond.

Full Notes

Structure of Alkenes

Alkenes contain a C=C double bond made up of a σ-bond and π-bond:

OCR (A) A-Level Chemistry diagram showing sigma and pi bonding in an alkene C=C double bond.

The σ-bond is formed by end-to-end overlap of orbitals.

OCR (A) A-Level Chemistry diagram showing sigma bond formed by end-to-end orbital overlap in an alkene.

The π-bond is formed by sideways overlap of adjacent p-orbitals, located above and below the plane of the bonded atoms.

OCR (A) A-Level Chemistry diagram showing pi bond in an alkene from sideways overlap of p-orbitals.

This π-bond prevents free rotation around the double bond, a key feature responsible for stereoisomerism in alkenes (see here).

Shape Around C=C

Each carbon in the double bond is bonded to three other atoms.

The bonding pairs repel each other and adopt a trigonal planar arrangement, with bond angles of approximately 120°.

OCR (A) A-Level Chemistry diagram showing trigonal planar shape around carbon-carbon double bond in alkenes.

This flat shape arises from the electron pair repulsion in the region around the C=C bond. If you are unsure why this is the case, see Shapes of Molecules.

OCR (A) A-Level Chemistry dot-and-cross diagram showing electron distribution in alkene bonding.

Summary