A-LEVEL CHEMISTRY: 3.3.4: Alkenes (ChemRevise) Flashcards
Alkenes:
Alkenes are unsaturated hydrocarbons
(contain C=C double bond)
Alkene general formula:
CnH2n
C=C double bond consists of: (2)
-1 sigma bond (σ)
-1 pi bond (π)
π bonds are ___ and have…
they are therefore…
exposed, high electron density
vulnerable to attack by electrophiles
electrophiles:
electron pair acceptor
electron pair acceptor:
electrophiles
why are π bonds vulnerable to attack by electrophiles? (2)
-exposed
-high electron density
Stereoisomerism:
-same molecular formula
-same structural formula
-different spatial arrangement of atoms
Alkenes can exhibit a type of isomerism called…
E-Z stereoisomerism
why do E-Z isomers exist?
due to restricted rotation about the C=C double bond
E-Z stereoisomers arise when: (2)
-restricted rotation around C=C double bond
-2 different groups/atoms attached to both ends of the double bond
to name E-Z isomers, you must determine the…
priority groups on both sides of the C=C double bond
Priority group in E-Z isomers:
the group/atom with the bigger atomic number
the C=C double bonds in alkenes are areas with high ___ ___. this attracts ___, & the alkenes undergo…
electron density, electrophiles, addition reactions
addition reaction:
reaction where 2 molecules react together to produce 1
describe the reaction of bromine (Br2) with alkenes (mechanism: electrophilic addition)
(4)
-change in functional group: alkene -> dihalogenoalkane
-reagent: Bromine
-conditions: room temperature
-type of reagent: electrophile, Brδ+
describe the reaction of hydrogen halide with alkenes (mechanism: electrophilic addition)
(3)
-change in functional group: alkene -> halogenoalkane
-reagent: HCl or HBr
-type of reagent: electrophile, Hδ+
explain what happens as the Br2 molecule approaches the alkene (electrophilic addition): (3)
-the π bond electrons repel the electron pair in the Br-Br bond
-this induces a dipole
-Br2 becomes polar & electrophilic (Brδ+)
why is HBr a polar molecule?
because Br is more electronegative than H
Markownkikoff’s rule:
in most cases, Br will be added to the carbon with the fewest hydrogens attached to it
if the alkene is unsymmetrical, the addition of HBr (electrophilic addition) can lead to…
2 isomeric products
in electrophilic addition to alkenes, the major product is formed via the…
more stable carbocation intermediate
in electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
why?
it is more stable, because the methyl groups on either side of the positive carbocation are electron releasing and reduce the charge on the ion which stabilises it.
the order of stability for carbocations is:
tertiary > secondary > primary
