Study Notes
Alkenes are hydrocarbons with at least one carbon-carbon double bond, which makes them unsaturated and reactive. They can be produced through methods like dehydrohalogenation, dehydration of alcohols, and thermal cracking.
- Dehydrohalogenation — a reaction to produce alkenes by eliminating HX from a halogenoalkane using ethanolic NaOH and heat. Example: R-X + NaOH → R-OH + NaX
- Dehydration of Alcohols — a reaction to produce alkenes by removing water from an alcohol using a heated catalyst or concentrated acid. Example: ROH → R + H2O
- Thermal Cracking — a process to produce alkenes by breaking down longer chain alkanes at high temperatures. Example: CnH2n+2 → C2H4 + Cn-2H2n
- Electrophilic Addition — a reaction where an electrophile adds to the electron-rich double bond of an alkene. Example: Bromine adds to ethene to form 1,2-dibromoethane
- Markovnikov's Rule — in the addition of a protic acid to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the most hydrogen substituents. Example: In propene, HBr adds to form 2-bromopropane
Exam Tips
Key Definitions to Remember
- Dehydrohalogenation: Elimination of HX from a halogenoalkane to form an alkene.
- Electrophilic Addition: Reaction where an electrophile adds to a double bond.
- Markovnikov's Rule: Hydrogen adds to the carbon with more hydrogen atoms in unsymmetrical alkenes.
Common Confusions
- Confusing dehydrohalogenation with dehydration of alcohols.
- Misunderstanding the role of catalysts in hydrogenation reactions.
Typical Exam Questions
- What is dehydrohalogenation? It is the elimination of HX from a halogenoalkane to form an alkene.
- How does Markovnikov's Rule apply to the addition of HBr to propene? The hydrogen adds to the carbon with more hydrogen atoms, forming 2-bromopropane.
- What happens when bromine is added to ethene? Ethene undergoes electrophilic addition to form 1,2-dibromoethane.
What Examiners Usually Test
- Understanding of alkene production methods.
- Ability to apply Markovnikov's Rule in reactions.
- Knowledge of electrophilic addition mechanisms.