3.4 Alkenes
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What is an alkene ?
An unsaturated hydrocarbon with a C=C double bond.
What is the general formula of an alkene ?
CnH2n
Why is there no rotation around a C=C bond ?
Due to the pi bond, there is electronegativity above and below the original bond - this prevents it from rotating.
Are alkenes more or less reactive than alkanes ?
They are more reactive, due to the high electronegativity of the the double bond and the pi bond is slightly easier to break.
What intermolecular forces of attraction do alkenes have ?
Only Van der Waals.
Are alkenes soluble in water ? Why ?
Insoluble as they have no polar bonds.
Name the three types of isomerism alkenes have.
Chain isomerism (branches) Position isomerism (C=C on different carbon molecule) Geometric E/Z isomerism.
Write the formula for the complete conbustion of pent-2-ene.
CH3CHCHCH2CH3 + 7.5O2 -> 5CO2 + 5H2O.
What is an electrophile ?
An electron deficient atom/ion which accept a pair of electrons, such as from a C=C.
What is the most stable kind of carbocation intermediate ? Why ?
Alkyl groups (C+) have a positive inductive effect, so the most stable version would be a tertiary (bonded to three other carbons ).
What conditions are needed for the electrophilic addition of H2O into an alkene ?
An acid catalyst, typically phosphoric acid.
What are the products of the electrophilic addition of H2O to an alkene ?
An alcohol.
Describe the mechanism for the electrophilic addition of H2O to an alkene.
• Arrow from C=C into a H+ ion • This forms a carbocation (C+), draw arrow from lone pair on H2O into cation • C-O bond forms making O+, draw arrow from an O-H bond into the O+ atom.
Draw a mechanism for the electrophilic addition of HBr and an alkene.
• Arrow from C=C to H on HBR, then arrow from H-Br bond to Br • This forms a carbocation, which the new Br- ion joins (arrow from Br- lone pair to C+).
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene ?
Standard conditions and an organic solvent.
How does a hydrogen halide molecule with a non-polar bond react as if it is an electrophile ?
C=C bond with a high electron density induces a temporary dipole in the halogen -> d+ molecule attracted to double bond.
Describe/draw the mechanism for the electrophilic addition of Br2 to an alkene.
• Temporary dipole on Br2, one d+ one d-, induced by high electron density of C=C • Arrow from C=C to d+ Br, then a further arrow from Br-Br to the d- Br • This forms a carbocation, which the Br- ion joins (arrow from lone pair of Br- to C+).
Draw/describe the electrophilic addition reaction of sulphuric acid and an alkene.
• Arrow from C=C to the H of SOH, further arrow from O-H bond to O • This forms O- which has a lone pair and a carbocation • Draw arrow from lone pair of O- to the C+.
What forms if you add water to the product of the electrophilic addition of sulphuric acid to an alkene ?
It forms an alcohol.
What is an addition polymer ?
When many monomers are bonded together via the rearrangement or bonds - without losing any molecules.
What are monomers ? What form can they take ?
Molecules which combine to form a polymer, an example would be an alkene as the C=C breaks to form a repeating pattern.
How would you draw the polymerisation of ethene.
n CH2=CH2 -> n -[-CH-CH-]- .
Give three uses of poly(chloroethene) / PVC.
• Drainpipes • Vinyl • Aprons.
Give two examples of plasticisers.
Esters and phthalates.