2.9- Alkenes Flashcards
What is the general formula for alkenes?
CnH2n
What are alkenes?
Unsaturated hydrocarbons
They contain a C-C double bond somewhere in their structure
What is the arrangement and bond angles of the bonds around the C-C double bond?
Planar
120°
What bonds do C-C double covalent bonds consist of?
One sigma bond and one pi bond
What are pi bonds?
They are exposed and have high electron density
They are therefore vulnerable to attack by species which like electrons: these species are called electrophiles
What is E-Z isomerism and when do they arise?
Stereoisomers have the same structural formulae but have different spatial arrangements of atoms
They arise when:
- There is restricted rotation around the C-C double bond
- There are two different groups/atoms attached to both ends of the double bond
What is the priority group?
The atom with the biggest Ar (atomic number) is classed as the priority atom
Define electrophile?
An electron pair acceptor
The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions
Reaction of bromine with alkenes?
Change in functional group: alkene –> dihaloalkane
Reagent: bromine
Conditions: room temperature (not in UV light)
Mechanism: Electrophillic addition
Type of reagent: electrophile, Br (delta plus)
How does the bromine molecule become polar?
As the Br2 molecule approached the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This induces a dipole. Br2 becomes polar and Electrophillic (Br delta plus)
What is a carbocation?
The intermediate formed, which has a positive charge on a carbon atom
Reaction of hydrogen bromide with alkenes?
Change in functional group: alkene –> Haloalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophillic addition
Type of reagent: electrophile, H delta plus
Why is HBr polar?
Br is more electronegative than H. The H delta plus is attracted to the electron-rich pi bond
What is the rule for the major product formed?
In Electrophillic addition to alkenes, the major product is formed via the more stable carbocation intermediate
Stability decided by how many alkyl (eg methyl) groups attached to the carbon
The order of stability:
Tertiary > secondary > primary
This is because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it
Stage 1 of reaction of sulphuric acid with alkenes?
Change in functional group: alkene –> alkyl hydrogensulphate
Reagents: concentrated H2SO4
Conditions: Room temperature
Mechanism: Electrophillic addition
Type of Reagent: Electrophile, H2SO4
