4. Alkenes Flashcards
(20 cards)
Define “sigma-bond”.
Covalent bond caused by linear overlap of atomic orbitals resulting in free rotation along the bond axis.
Stronger than a pi-bond. Present in all double bonds.
Define “pi-orbital”.
Covalent bond caused by sideways overlap of p-orbitals, resulting in restricted rotation along the bond axis.
Weaker than sigma-bond. Found in double bonds.
Define “bond length”.
The distance between the nuclei of two covalently bonded atoms.
What can the C=C bond in alkenes be described as?
A region of high electron density.
What does this mean?
It is open to attack by electron-deficient species (electrophiles), explaining why alkenes are much more reactive than alkanes.
Define “addition reaction”.
When the pi-bond of a double covalent bond is broken and two species add on across the double bond.
Complete combustion of alkenes?
Requires a source of ignition and a plentiful supply of air
Produces carbon dioxide and water
Fractional
Incomplete combustion of alkenes
Requires a source of ignition and a limited air supply
Produces water and carbon monoxide (and maybe soot)
Fractions
Define “electrophile”.
An electron-deficient species that attacks a region of high electron density and can accept a lone pair of electrons.
Bromination of ethene using HBr?
You get bromoethane, and it’s electrophilic addition.
You have ethene and polar HBr (show deltas).
Then the double bond broken in ethene and an extra H on one of the C atoms, with a slightly positive other C and a negative Br just chilling.
Then the C and Br join and you get bromoethane.
What is the name of the intermediate formed during the bromination of ethene?
A carbocation.
It causes a rapid reaction between itself and the bromide ion, due to ionic attraction.
Define “heterolytic fission”.
Where a covalent bond breaks and the shared electrons go to one atom.
Hydrogenation of an alkene?
Requires nickel catalyst, 180 degrees, and 4 atm
Hydrogenation: a hydrogen molecule is added across a C=C bond
Used to convert oils into solid fats. Also called hardening.
You get an alkane
Hydration of ethene using steam?
Requires concentrated phosphoric acid (as a catalyst, I am supposing?), 300 degrees, 60-70 atm
You get ethanol by adding water
Bromination using bromine water?
Mix bromine water with liquid alkene or bubble gaseous alkene through.
You get a dibromoalkane.
Test for a C=C bond?
Yellow/orange/brown bromine water changes to colourless when there’s a C=C bond present
Define “polymerisation”.
The joining together of many small molecules to form a large molecule.
Addition polymerisation of alkenes?
Polymerisation of ethene produces polythene.
Alkene with straight up and down Hs, and an n to the left to say “loads”. Arrow. Double bond split, brackets around, n on bottom right now.
Use of ‘excess reagent’?
If there’s two double bonds and excess reagent is added, both double bonds will split.
Disposal of polymers?
Addition polymers are non-biodegradable and are disposed of by landfill or incineration.
Recycling is better.
