3.4.1Physical Chemistry
Structure, Bonding and Reactivity of Alkenes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 76 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Sigma and Pi Bonds
Covalent bonds form by the overlap of atomic orbitals, giving two types of bond.
Sigma (σ) bond
- Sigma (σ) bond Formed by head-on overlap of orbitals along the line between the nuclei, with electron density between the atoms.
Pi (π) bond
- Pi (π) bond Formed by sideways overlap of adjacent p orbitals, with electron density above and below the plane of the atoms.
Double bond
- Double bond A C=C double bond consists of one σ bond and one π bond.
Carbon atoms in ethene
- Carbon atoms in ethene Each carbon uses three outer electrons to form σ bonds, and the fourth occupies a p orbital that forms the π bond.
Section 2 of 4Shape Around the Double Bond
Shape Around the Double Bond
Three regions of charge
- Three regions of charge Each carbon has three bonding regions around it, which repel to give 120° and a flat arrangement.
Planar molecule
- Planar molecule All six atoms in ethene lie in the same plane, and the π electron cloud lies above and below.
Shape in alkanes
- Shape in alkanes Each carbon has four σ bonds, so the electron pairs repel to a tetrahedral shape with 109.5° bond angles.
| Feature | Alkane (C–C) | Alkene (C=C) |
|---|---|---|
| Shape around carbon | Tetrahedral | Trigonal planar |
| Bond angle | 109.5° | About 120° |
| Mean bond enthalpy | 347 kJ mol⁻¹ | 614 kJ mol⁻¹ |
| Reactive centre | None (σ bonds only) | π bond |
Section 3 of 4Why Alkenes Are Reactive
1
The C=C bond enthalpy (614 kJ mol⁻¹) is less than twice the C–C value (2 × 347 = 694 kJ mol⁻¹), so the π bond is weaker than the σ bond.
2
The π electrons are exposed above and below the plane, away from the nuclei, so they are readily attacked.
3
Electrophiles, which are electron-pair acceptors, are attracted to the high electron density in the π bond.
4
The π bond breaks and two new σ bonds form, so alkenes undergo addition reactions.
5
Alkanes contain only strong, non-polar σ bonds with no region of high electron density, so they are far less reactive.
Section 4 of 4Exam Checklist & Tips
3.4.1 – Key Points for the Exam
- Sigma bond A covalent bond formed by head-on overlap of orbitals between the nuclei.
- Pi bond A covalent bond formed by sideways overlap of p orbitals, with electron density above and below the bond axis.
- Electrophile An electron-pair acceptor that is attracted to regions of high electron density.
- Unsaturated Containing at least one carbon–carbon double bond.
Common Mistakes
- Saying the π bond is stronger than the σ bond; it is the weaker bond in a double bond.
- Stating that the double bond is twice as strong as a single bond.
- Giving a bond angle of 109.5° around a double bond.
- Describing the π bond as lying between the two nuclei, when the electron density is above and below.
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