Alkenes are basic hydrocarbon units made of many polymers and contain C=C bonds, so they are unsaturated.
When naming alkenes, they end in the suffix "-ene"; an alkene with 2 C=C bonds is a diene, and alkenes can be cyclic e.g. hexene.
All bond angles around the C=C are 120 degrees because there are 3 groups of electrons around each carbon atom (2 single bonds, one double).
Alkenes undergo electrophillic addition reactions. There are 4 electrons in the double bond- this creates a negatively charged reigon between the 2 carbon atoms- electrophiles are attracted to this reigon and accept a pair of electrons from the double bond to start the reaction:
When bromine is added, the molecule approaches the alkene and the alkene becomes polarised and the electrons in the bromine are repelled back along the molecule. The electron density is unequally distributed and the bromine atom nearest the alkene becomes slightly positively charged. The bromine now acts as an electrophile- a pair of electrons moves towards the bromine atom to make a C-Br bond.
The carbon species is now positively charged- a carbocation. The other bromine becomes negatively charged and moves in to make another bond.
If Cl - ions are present, they can also attack the carbocation, so usually 2 products are produced one with bromine attached and one with chlorine attached.
Br2 CH2B2CH2Br Room temperature and pressure
Br2 (aq) CH2BrCH2OH Room temperature and pressure
HBr (aq) CH3CH2Br Aqueous solution, room temperature and pressure
H2O CH3CH2OH Phosphoric acid 300 degrees celcius 60 atm OR conc. H2SO4 then H2O at 1 atm
H2 CH3CH3 Pt catalyst, room temp and pressure or Ni catalyst at 150 degrees celcius and 5 atm
Addition & Co polymerisation
Alkenes go through this- you start with small unsaturated monomers and they form together to form a saturated polymer (because no double bonds); no other product is formed e.g.
CH2=CH2 + CH2=CH2 + CH2=CH2 ----> CH2--CH2--CH2--CH2--CH2--CH2
This is when 2 different monomers join together to make a co polymer chain e.g.
CH3--CH2=CH + CH2=CH2 ---> CH2--CH--CH2--CH2
CH3 (Joined on to the CH)
Properties of polymers
Chain length- The longer the chain, the stronger the polymer because long chains are more entangled and they have stronger intermolecular bonds.
Side groups on a polymer chain- The more polar side groups (e.g. Cl) a polymer has, the stronger it's bonds between chains will be.
Branching- Straight unbranched chains are more tightly packed together and so have stronger intermolecular bonds.
Chain flexibility- The more rigid the chain the stronger the polymer.
Cross-linking- More cross-linking makes the polymer harder to melt and stronger too.
Stereoregularity- the more regular orientation of side groups, the closer the packing, and the stronger the polymer.
Thermoplastics and thermosetting plastics
- Polymers without cross-links between the chains, and the intermolecular bonds are much weaker.
- When warmed, the intermolecular bonds are overcome and the chains slide over each other so the polymer becomes deformed.
- On cooling the bonds reform and the new shape of the polymer is held.
- Polymers with extensive cross-links between the chains, and have much stronger intermolecular bonds.
- The covalent bonds can't be overcome when the plastic is heated and so the chains can't slide over each other and change shape.
- If heating is continued the polymer eventually just burns.
Primary alcohol- at the end of a chain.
Secondary alcohol- in the middle of a chain.
Tertiary alcohol- attached to a carbon which carries no H atoms.
OXIDATION OF ALCOHOLS:
Can be oxidised using acidified potassium dichromate (VI)- the OH group is oxidised to a carbonyl group (C--OH to C=O) and the Cr in K2Cr2O7 is reduced so the overall colour change in the reaction is ORANGE TO GREEN. Heat under reflux is the condition the experiment needs to be conducted in.
When oxidising with a primary alcohol, an aldehyde is produced and this can oxidise further to make a carboxylic acid. The aldehyde can be recovered to prevent it oxidising further through distillation; keep heating under reflux with potassium dichromate to make a carboxylic acid.
When a secondary alcohol is used, a ketone is produced and no further oxidation happens.
Tertiary alcohols can't be oxidised as they have no H atoms on the carbon attached to the OH group.
The following groups show what oxidation products look like-
Carboxylic acid R-COOH
Alcohols can also be dehydrated through an elimination reaction. H2O is taken out and a double bond is formed creating an alkene. The reaction is done with Al2O3 catalyst at 300 degrees celcius and 1 atm or refluxing with conc. H2SO4 e.g.
CH2CH2OH ----> CH=CH2 + H2O
It is one type of steroeisomerism, in which the atoms are bonded in the same order but are arranged differently in space in each isomer.
Two naming systems- Cis-trans notation can be used in simple molecules whilst E/Z notation can work for both simple and complex molecules. Butene can be put together in 2 ways:
Isomer with 2 methyl groups on the same side is the Cis isomer (is also the Z isomer)
Isomer with 2 methyl groups on seperate sides is the Trans isomer (is also the E isomer)