what are alkenes
unsaturated hydrocarbons that contain a C=C double bond
describe the bonding in alkenes
- the C=C consists of a σ bond - the direct overlap of s orbitals between bonding atoms
- also contains a π bond - the sideways overlap of of adjacent p-orbitals above and below the plane of the molecules
why are pi bonds much weaker than sigma bonds
- e- density is delocalised above and below plane of molecule
- this means that the electrostatic attraction between the nuclei and a shared pair of e- is weaker, so pi bonds have relatively low bond enthalpy
why do sigma bonds (strongest type of covalent bond) have high bond enthalpy
- high e- density between the nuclei means there is a strong electrostatic attraction between the nuclei of the bonded atoms and shared pair of e-
what shape does the C of the double bond have, and what’s the angle
- trigonal planar
- 120 degrees
why does C in the double bond have its shape
- 3 areas of e- density (bonding pairs) around C=C atoms repel equally
- 0 lone pairs
- lone pairs repel more than bonding pairs
why are alkenes relatively reactive
- carbon carbon double bond has high electron density and so pi bond can be attacked by an electrophile
- π bond has low bond enthalpy
why can alkenes exhibit stereoisomerism
C=C restricts rotation
define stereoisomerism
compounds with the same molecular and structural formula, but a different arrangement of atoms in space
what’s E-Z isomerism
stereoisomerism that arises due to the restricted rotation about the C=C bond when there are 2 different groups attached to each carbon of the C=C
how to use E-Z system
- assign priority to each group attached to the first C (atom with the highest atomic number)
- assign priority to each group attached to second C
- two priority groups on same side of double bond = Z isomer (zame zide)
opposite sides = E isomer
what’s cis-trans isomerism
special case of E-Z isomerism where 2 groups attached to each C of the C=C bond are the same
how can alkenes readily undergo addition reactions
- π bond e- can be used to form new bonds with an attacking molecule
general equation for hydrogenation and condition
alkene + H2 –(Ni)–> alkane
- reactants heated
what can hydrogenation be used for
- in manufacture of margarine from vegetable oils
- vegetable oils are unsaturated and contain C=C
- hydrogenation converts these into C-C and changes the physical properties, so oil converted into solid margarine
define electrophile
e- pair acceptors attracted to areas of high e- density
what’s the reaction of alkenes with electrophiles
- electrophilic addition
- electrophiles are attracted to e- in C=C
general equation for the bromination of alkenes
alkene + bromine —-> dibromoalkane
how does bromination of alkenes occur
alkene polarises bromine molecule, which then acts as an electrophile and adds to the alkene
what’s bromination of alkenes used to test for
- unsaturation
- bromine water will go from orange to colourless if Br is added to alkene
how does the Br-Br bond break
- via heterolytic fission
- both bonding e- go to same Br atom, forming Br- ion
general equation for addition reaction of alkenes with steam
- alkene + H2O (g) <–(H3PO4)–> alcohol
conditions: 300C, 60 atm - reversible
addition reaction alkenes with steam is also an example
hydration reactions
with any unsymmetrical molecule there is possibility of forming …. products in addition reactions
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