topic 4: alkenes Flashcards
structure of alkenes
-alkenes are unsaturated (double bond) hydrocarbons
-produced during thermal cracking of crude oil
-CnH2n general formula
bonding + reactivity of alkenes
-shape: planar
-the double bond makes them more reactive because of the really high electron density between the carbon atoms
why do electrophillic addition reactions happen to alkenes
alkenes have a double bond which had lots of electrons and is easily attacked by electrophiles
electrophiles definition + examples
-electron pair acceptors, attracted to areas of lots of electrons
examples: HBr (hydrogen bromide), H2SO4 (sulfuric acid) and Br2 (bromine)
-e.g. positively charged ions: H+ and NO2+
and polar molecules, the d+ atom is attracted to places with lots of electrons
ELECTROPHILIC ADDITION MECHANISM
- Electrophile attracted to double bond
- Electrophiles are positively charged so accept electron pair from double bond
- A positive ion (carbocation) is formed
- A negatively charged ion forms a bond with the carbocation.
electrophillic addition with Br2
- There is a temporary induced dipole, delta positives and negatives. The delta+ end of this dipole is attracted to the double bond in the alkene - the bromine molecule has become an electrophile.
- The electrons in the double bond are attracted to the Br delta + (the electrophile). They repel the electrons in the Br-Br bond and this strengthens the dipole of the bromine molecule.
- Two electrons from = bond with Br+, leaving a Br- ion and a positive carbocation, removing the double bond.
- The Br- then bonds to the carbocation.
A dihaloalkane is formed
electrophillic addition with HBr
- Br is more electronegative than hydrogen, so forms a polar bond with partial charges.
- H+ is electrophile, attracted to = due to its high electron density.
- Double bond breaks, H+ ion forms bond and carbocation formed.
- Br- with free electron pair bonds to carbocation
- A haloalkane is formed.
Major and minor products
electrophillic addition with H2SO4
- Electrophile is the H+ on the top of the H2SO4
- Carbocation created, oxygen left with negative charge, free electron pair which bonds to the carbocation.
- HSO4 is now bonded, a halidehydrogensulfate is created.
- Can have major and minor products.
what happens when you add water to product of H2SO4 electrophillic addition?
If you add water to the product, you form an alcohol and the sulfuric acid is reformed. This is because the overall effect is to add water H- OH across the double bond and the sulfuric acid is a catalyst for the process.
what is a major and minor product
major: most stable - On the secondary or tertiary carbocation. You need to put the electrophile on the carbon closest to the end
minor: less stable - on primary carbocation
use of bromine to test for unsaturation
-when you shake an alkene with orange bromine water, the solution quickly decolourises
-positive result: orange to colourless
additional polymerisation - formation + reactivity
-additional polymers are formed from alkenes and substituted alkenes
-Formation of long chain molecules from lots of smaller molecules, monomers joining together with no other products.
-They are unreactive due to being saturated and having (usually) non-polar side chains.
intermolecular forces between molecules of polyalkenes
-polyalkene chains are usually non polar - so the chains are only held together by Van Der Waals
-the longer the polymer chains are and the closer together they can get, the stronger the Van Der Waals forces between the chains will be
-long straight polyalkenes chains tend to be strong and rigid
-short, branched polyalkene chains tend to be weaker and more flexible
poly(chloroethene) - PVC
-formed from chloroethene
-long closely packed polymer chains, making it hard but brittle at room temp
-rigid PVC used for drainpipes and window frames
-plasticised PVC is much more flexible than rigid PVC - used to make electrical cable insulation, flooring tiles and clothin
-strong dipole-dipole forces due to the chlorine atom
properties of PVC can be modified using a plasticiser
-adding a plasticiser to a polymer makes it more flexible, the plasticiser molecules get between the polymer chains and push them apart, this reduces the strength of the intermolecular forces between the chains - so they can slide around more and make the polymer easy to bend
