Electrophilic addition in alkenes Flashcards
What is an electrophilic addition reaction?
A reaction mechanism where alkenes participate in addition reactions to form saturated compounds
Electrophilic addition involves breaking the double bond in alkenes.
What does the double bond in an alkene represent?
A region of high electron density due to the presence of π-electrons
This high electron density attracts electrophiles.
Define an electrophile.
An atom or group of atoms that is attracted to an electron-rich center and accepts an electron pair
Electrophiles are usually positive ions or molecules with a partial positive charge.
What is the role of hydrogen bromide in electrophilic addition?
Acts as an electrophile when added to alkenes, forming a single addition product
Example: The reaction between but-2-ene and hydrogen bromide.
What is Markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogen atoms
This rule helps predict the major product in reactions involving unsymmetrical alkenes.
Describe the mechanism of electrophilic addition using hydrogen bromide.
- The electron pair in the double bond is attracted to the partially positive hydrogen atom.
- A bond forms between hydrogen and a carbon atom.
- The H—Br bond breaks, forming a bromide ion and a carbocation.
- The Br- ion reacts with the carbocation to form the addition product
Curly arrows are used to show the movement of electron pairs.
What is a carbocation?
A positively charged carbon atom
Carbocations are intermediates in many reaction mechanisms.
What is the difference between primary and secondary carbocations?
Primary carbocations have the positive charge on a carbon at the end of a chain, while secondary carbocations have it on a carbon with two carbon chains attached
Secondary carbocations are generally more stable than primary ones.
What factors affect carbocation stability?
The number of alkyl groups attached to the positively charged carbon
Tertiary carbocations (three alkyl groups) are the most stable, while primary carbocations (one alkyl group) are the least stable.
True or False: The reaction between propene and hydrogen bromide produces both 1-bromopropane and 2-bromopropane.
True
2-bromopropane is the major product due to the stability of the secondary carbocation.
Fill in the blank: The addition of a hydrogen halide to an unsymmetrical alkene forms the major product via the most stable ______.
carbocation
How does bromine react with alkenes?
Bromine can react with alkenes through electrophilic addition by inducing a dipole
The interaction with π-electrons causes the Br—Br bond to polarize.
What is an induced dipole?
A temporary dipole that occurs when the electron distribution in a molecule is distorted
This can happen when a non-polar molecule approaches an electron-rich center.
What happens during the heterolytic fission of the Br—Br bond?
The bond breaks, with the electron pair going to one of the bromine atoms, forming a bromide ion
This step is crucial in the mechanism of electrophilic addition.
