Preparation from Alkenes
Formation of haloalkanes via addition reactions of hydrogen halides or halogens to alkenes.
What is Preparation from Alkenes?
A rule stating that in the addition of a protic acid HX to an alkene, the hydrogen atom attaches to the carbon atom of the double bond that has the greater number of hydrogen atoms, and the halide ion attaches to the carbon atom that has fewer hydrogen atoms.
Key points
- Understand the mechanism of electrophilic addition of HX to alkenes.
- Apply Markovnikov's rule to predict the products of addition reactions.
- Explain the peroxide effect in the addition of HBr.
- Describe the mechanism of halogen addition to alkenes.
Common exam trap
Confusing Markovnikov's and anti-Markovnikov's rules.
Definitions
- Term
Markovnikov's Rule
- Meaning
A rule stating that in the addition of a protic acid HX to an alkene, the hydrogen atom attaches to the carbon atom of the double bond that has the greater number of hydrogen atoms, and the halide ion attaches to the carbon atom that has fewer hydrogen atoms.
- Term
Anti-Markovnikov's Rule
- Meaning
A rule stating that in the addition of HBr to an alkene in the presence of peroxides, the hydrogen atom attaches to the carbon atom of the double bond that has fewer hydrogen atoms, and the bromine atom attaches to the carbon atom that has more hydrogen atoms.
- Term
Vicinal Dihalide
- Meaning
A compound containing two halogen atoms on adjacent carbon atoms.
Learning objectives
Understand the mechanism of electrophilic addition of HX to alkenes.
Apply Markovnikov's rule to predict the products of addition reactions.
Explain the peroxide effect in the addition of HBr.
Describe the mechanism of halogen addition to alkenes.
Differentiate between Markovnikov and anti-Markovnikov addition.
Prerequisites
Structure and nomenclature of alkenes.
Basic understanding of reaction mechanisms (electrophilic addition, free radical).
Carbocation stability.
Types of addition reactions.
Common mistakes
Confusing Markovnikov's and anti-Markovnikov's rules.
Not considering the role of peroxides in HBr addition.
Incorrectly predicting the major product for unsymmetrical alkenes.
Assuming syn-addition for halogenation instead of anti-addition.
Keywords
Alkenes
Haloalkanes
Electrophilic Addition
Hydrogen Halides
Halogens
Markovnikov's Rule
Anti-Markovnikov's Rule
Peroxide Effect
Carbocation
Vicinal Dihalide
Regioselectivity
Addition Reaction
Practice preview
Which of the following reactions is used for the preparation of vicinal dihalides from alkenes?…
easy
Which of the following alkenes will yield only one product upon addition of HBr?…
easy
Addition of HBr to propene yields:…
easy