13.4 - Electrophilic addition of alkenes Flashcards
What is an electrophile?
Electrophiles are attracted to negativity.
Electron-pair acceptors.
What is an addition reaction?
Reaction where only one product is created.
Why do alkenes attract electrophiles?
Electrophile attracted to C=C double bond, as it is electron dense area.
What are the common electrophiles?
- Br2
- HBr
- H2SO4
- What does the C=C double bond cause in the Br2 molecule?
It causes an induced dipole.
Electrons are moved from top Br to bottom Br.
- What happens to the electrons in the C=C bond when the delta positive bromine is near?
One of the C=C electrons are donated to the bromine atom. Now a single C-C bond.
Bromine donates both electrons in its bond to the other bromine.
- What forms after the first bromine attaches to one of the carbon atoms?
A carbocation, (+ carbon) because the electron in the C=C bond has been donated to the Br.
- What happens to the remaining bromine negative ion?
It has a lone pair, and donates the electrons to the carbocation to form a covalent bond.
What is the test for an alkene?
Bromine water goes from orange → colourless.
Because the bromine added across the double bond.
Why does adding HBr across the double bond cause an intermediate? (if propane)
There are two different places (C atoms) the H can bond to.
Forms different carbocations.
Positional isomerism.
Which carbocation is produced as the intermediate (for HBr added across double bond)?
The more stable carbocation is produced.
This is the one with the more alkyl groups attached.
Product would be the major product.
What is the name of a carbocation with only one alkyl group attached?
It is a primary carbocation.
Why are carbocations with more alkyl groups more stable?
Alkyl groups push electrons onto carbocation, spreading electrons around molecule more evenly - makes it more stable.
Why is it the hydrogen atom that has a delta positive charge in HBr?
Halogen (Br) is more electronegative than H.
Forms permanent dipole.
What is Markownikoff’s rule?
When a hydrogen halide reacts with an asymmetrical alkene, the hydrogen atom is more likely to bond to the carbon atom which is attached to the greatest number of hydrogen atoms.
(This means that the carbocation formed is not the carbon atom with the most H atoms attached - means it it more stable).
The major product is therefore the positional isomer with the Br not at the end.
