Reactions of Alkenes Flashcards
Electrophilic addition HX, XX + H2SO4 + Asymmetrical alkenes
What type of reactions do alkenes mainly undergo?
Electrophilic addition reactions
Why are alkenes reactive towards electrophiles?
The C=C double bond has four electrons, creating. a region of high electron density that attracts electrophiles
What is an electrophile?
An electron pair acceptor that is either positively charged or has a partially positive area
What happens in the first step of electrophilic addition?
The electrophile is attracted to the double bond
What happens after the electrophile approaches the double bond?
It accepts a pair of electrons from the double bond to form a new bond
What intermediate forms during electrophilic addition?
A carbocation (a positively charged carbon ion )
What happens to the carbocation in electrophilic addition?
A negatively charged ion bonds to the carbocation, completing the reaction
When alkenes react with hydrogen halides in electrophilic addition, how do they add across?
The hydrogen and the halide ion add straight across the alkene
When alkenes react with halogens in electrophilic addition, how do they add across?
The two halogens add straight across the alkene
What does concentrated sulfuric acid do in reactions with alkenes?
It adds across the double bond in an electrophilic addition reaction
What are the conditions for the reaction betwen an alkene and concentrated sulfuric acid?
Room temperature; the reaction is exothermic
What intermediate forms when concentrated sulfuric acid reacts with an alkene?
A carbocation intermediate
What is the electrophile in the reaction between an alkene and concentrated sulfuric acid?
A partially positively charged hydrogen atom in H-O-SO3H
What happens to the carbocation formed in the reaction with concentrated sulfuric acid?
It reacts rapidly with the negatively charged hydrogen sulfate ion (HSO4-)
Why can asymmetrical alkenes form two products in electrophilic addition with HBr?
This is because the Br can bond to any of the carbons on either side f the C=C double bond
What are the two possible products when propene reacts with HBr?
Which is the major product when propene reacts with HBr, and why?
1-bromopropane and 2-bromopropane
2-bromopropane will be the major product because it forms from the more stable secondary carbocation instead of the less stable primary carbocation
What is the positive inductive effect?
The tendency of alkyl groups to release electrons toward a positively charged carbon atom, stabilising it
How do alkyl groups affect carbocation stability?
More alkyl groups increase stability by releasing electrons (positive inductive effect)
Rank carbocation from most to least stable
a/ Secondary
b/ Tertiary
c/ Primary
- Tertiary
- Secondary
- Primary
