3.4 Alkenes Flashcards
what are alkenes ?
- unsaturated hydrocarbons with general formula CnH2n
why are alkenes vulnerable to attacks from species?
- they contain a high electron density C=C bond area
what are stereoisomers
- they have the same structural formulae but different spatial arrangement of atoms
how do E-Z isomers arise?
- when there is restricted rotation about the C=C bond
- two different groups attatched at both ends of the double bond
what is an electrophile?
- an electron pair acceptor
what happens if there are two identical groups on one side of the bond?
- E/Z isomers do not occur
give the reagent and conditions and type of reaction for reaction of bromine with alkenes
reagent - bromine
conditions - room temperature
reaction - electrophilic addition
why is bromine used as an electrophile?
- as bromine approaches the alkene, the high density electron area repels the electron pair in Br-Br bond
- this induces a dipole
- Br2 becomes polar and electrophilic
on a sheet of paper, draw the mechanism for the reaction of ethene and bromine
- arrow coming from the double bond onto the Br
- arrow from the Br-Br bond to the other Br
C+ formed, Br lone pair attacks the C+
give the reagent, conditions and type of reaction for reaction of hydrogen bromide with alkenes
reagent - HCl or HBr
conditions - room temperature
reaction - electrophilic addition
why is HBr polar?
- Br more electronegative than H
- H+ attracted to electron rich pi bond
what is Markownikoffs Rule?
- bromine will be added to the carbon with the least hydrogens attached to it
how does the stability of carbocations work?
- methyl groups on either side of the positive carbon are electron releaasing and reduce the charge on it, stabilising it
give the order of stability of carbocations
tertiary > secondary > primary
how is the major product formed?
- via the more stable carbocation
give the reagents, conditions and type of reaction for reaction of sulfuric acid with alkenes
stage 1
reagents - concentrated H2SO4
conditions - room temperature
mechanism - electrophilic addition
give the reagents, conditions and type of reaction for reaction of sulfuric acid with alkenes
stage 2
reagents - water
conditions - warm mixture
type of reaction - hydrolysis
what is the overall role of sulfuric acid in reactions of sulfuric acid with alkenes
it acts as a catalyst
what is meant by hydrolysis
a reaction where the molecule is split by the addition of water
what are the essential conditions for the industrial hydration of alkenes to form alcohols
high temperature 300 to 600 degrees
high pressure 70atm
catalyst : concentrated H3PO4
what is meant by hydration reaction?
- a reaction where water is added to a molecule
how can we test for alkenes?
- bromine water
- decolourises in the presence of a double bond
