- Define and explain The Reactive Alkenes in your own words
- Use key terms such as electrophile accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on The Reactive Alkenes: explore the double bond, electrophilic addition and polymers.
Explore the double bond, electrophilic addition and polymers.
Key ideas
The double bond dominates alkene chemistry
Alkenes contain a C=C double bond: one sigma bond plus one pi bond, and the pi bond's exposed electron density attracts electrophiles. In electrophilic addition, the pi bond breaks and both carbons gain new atoms — for example, ethene + HBr → bromoethane. With unsymmetrical alkenes, Markownikoff's rule predicts the major product: the hydrogen adds to the carbon already carrying more hydrogens, giving the more stable carbocation.
Synthesis is planned backwards
Planning a synthesis means working backwards from the target molecule, choosing reactions that build each functional group in turn. Each step must use conditions that do not destroy groups already present — for instance, oxidise an alcohol before introducing an alkene that the oxidiser would attack. Good routes are short, high-yielding and use cheap starting materials.
Key term — electrophile: An electron-pair acceptor, attracted to electron-rich areas such as C=C double bonds.
Explain why the addition of HBr to propene gives mainly 2-bromopropane.
H⁺ adds to C1, giving a secondary carbocation on C2, which is more stable than the primary alternative; Br⁻ then attacks C2 (Markownikoff's rule).
Answer: H⁺ adds to C1, giving a secondary carbocation on C2, which is more stable than the primary alternative; Br⁻ then attacks C2 (Markownikoff's rule).
- Forgetting that alkanes only react by free-radical substitution Alkanes are saturated and unreactive towards electrophiles — with halogens they react only in UV light by free-radical substitution. Writing an electrophilic addition for an alkane is a fundamental error.
- Drawing curly arrows starting at atoms Curly arrows show electron pairs moving: they start at the electron source (a bond or lone pair) and point to where the electrons go. An arrow starting at an atom rather than the bond is meaningless.
Practice
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
The orange bromine water decolourises; the product is 1,2-dibromoethane.
Ethanoic acid; the orange dichromate turns green as it is reduced.
Butane (CH₃CH₂CH₂CH₃) and methylpropane (CH₃CH(CH₃)CH₃).
Quick check
Which of these best defines "electrophile"?
Plan a two-step synthesis of ethanoic acid from ethene.
- The Reactive Alkenes: explore the double bond, electrophilic addition and polymers.
- The double bond dominates alkene chemistry: Alkenes contain a C=C double bond: one sigma bond plus one pi bond, and the pi bond's exposed electron density attracts electrophiles.
- functional group: The atom or group of atoms responsible for a compound's characteristic reactions, e.g.
- Watch out for: forgetting that alkanes only react by free-radical substitution