Alkenes Flashcards
(39 cards)
What are alkenes
Unsaturated hydrocarbons
What is the general formula of alkenes
CnH2n
What is the arrangement of the bonds around C=C and what is its bond angle
Planar and 120*
What does the C=C covalent bond consist of
One sigma bond and one pi bond
What are pi bonds vulnerable to and why
Electrophiles (species that like electrons)
And because they are exposed and have a high electron density
What kind of isomerism can alkenes show
E-Z stereoisomerism
When do E-Z stereoisomers arise
- When there’s restricted rotation around the C=C double bond
- When there are 2 different groups/atoms attached both ends of the double bond
Why do E-Z isomers exist
Due to the restricted rotation about the C=C bond
What is a priority group
The atom with the bigger atomic number is classed as the priority atom
What is an electrophile
An electron pair acceptor
Why do alkenes attract electrophiles
Because their double bonds are areas with high electron density
Reaction of bromine with alkenes
Change in functional group:
Alkene-> dihalogenoalkane
Reagent: bromine
Conditions: room temperature
Mechanism: electrophilic addition
Type of reagent: electrophile Br
What is a carbocation
A carbon atom that has a positive charge
Explain the reaction of bromine with alkenes in terms of pi bonds
As the bromine molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond -> this induces a dipole
- bromine becomes polar and electrophilic
Reaction of hydrogen bromide with alkenes
Change in functional group:
Alkene-> dihalogenoalkane
Reagent: HCl or HBr
Conditions: room temperature
Mechanism: electrophilic addition
Type of reagent: electrophile H
Why is HBr polar molecule
Because Br is more eneg than H the H is attracted to the electron rich pi bond
If the alkene is unsymmetrical..
The addition of hydrogen bromide can lead to 2 isomeric products
Markownikoffs rule
In most cases bromine will be added to the carbon with the fewest hydrogens attached to it
The order of stability for carbocations
Tertiary > secondary > primary
Why is a secondary carbocation more stable than a primary one
Because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it
How is the major product formed in electrophilic addition to alkenes
Via the more stable carbocation intermediate
How to answer an exam question about the reaction of hydrogen bromide and alkenes forming a major and minor product
- Draw out both carbocations and indemnify as primary, secondary and tertiary
- State which is the more stable carbocation
- State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion
- If both the carbocations are secondary then both will be equally stable and a 50/50 split will be achieved
Stage 1 of reaction of sulfúrico acid with alkenes
Change in functional group:
Alkene-> alkyl hydrogensulfate
Reagent: concentrated H2SO4
Conditions: room temperature
Mechanism: electrophilic addition
Type of reagent: electrophile H2SO4
Stage 2 of reaction of sulfúrico acid with alkenes
Change in functional group:
Alkyl halogensukfate-> alcohol
Reagent: water
Conditions: warm mixture
Type of reaction: hydrolysis
