Alkenes

-unsaturated hydrocarbons with one or more C=C double bond

-this makes them more reactive than alkanes because of high conc of electrons between the 2 C atoms 

Shapes of alkenes:

• ethene is a planar shape with angles between each bond roughly 120 degrees
• no rotation about C=C bond, so if a H atom is replaced by another atom/group-shape will stay the same flat shape 

why the double bond cant rotate:

• as well as a C-C single bond, there is a P orbital on each C which contains a single electron. These 2 orbitals overlap to form an orbital with a cloud of electron density above and below the double bond- called pi orbital 

Physical properties: 

• the double bond doesn't greatly affect properties like BP's.
• its the size of the VdW forces that affect it 
• as chain length increases, MP/BP increases

How alkenes react:

• alkenes are more reactive than alkanes as the C=C bond forms an electron rich area which can easily be attacked by electrophiles ( electron pair acceptors) eg H+ ion. 
• as alkenes are unsaturated they can undergo electrophilic addition 

Alkenes can combust in air but are not used as fuels as their reactivity makes them useful for other purposes 

Electrophilic addition:

• electrophiles are attracted to the C=C double bond and can form a bond by using 2 of the 4 electrons in the double bond

Mechanism:

• electrophile (+vely charged) attracted to double bond accepts a pair of electrons 
• a +ve ion (carbocation) is formed 
• a -vely charged ion forms a bond with the carbocation 
• Hydrogen halides usually react with alkenes in this way to form halogenoalkanes as the H-X bond is polar, making the H+ the electrophile 

-when the double bond is not exactly in the middle of the chain, there are 2 possible products as the -ve ion can bond to either one of the carbon atoms of the double bond 

-so there is a possibility of isomers

- which isomer is most likely to form depends on alkyl groups eg CH3 or C2H5

-these have a tendency to release electrons. Called the Positive Inductive Effect. Sometimes represented by an arrow on their bonds to show their direction of release

-This electron releasing effect stabilises the +ve charge of the intermediate carbocation

-The more alkyl groups the +vely charged C atom has attached to it, the more stable the carbocation

• tertiary has 3 alkyl groups attached to the C- most stable- products tend to form from these 
• secondary has 2
• primary has 1- least stable 

-example of electrophilic addition where the halogen molecule acts as the electrophile 

• the X-X bond will have an instantaneous dipole and the +ve end receives 2 electrons from the double bond to form a bond and the other halogen atom becomes a halide ion 
• this also leaves a carbocation in which the C atom not bonded to the halogen has a +ve charge
• the -ve halide ion now forms a bond with the carbocation 

-exothermic reaction that occurs at room temperature 

-electrophile is the partially positively charged H atom in the sulfuric acid molecule 

Mechanism:

When water is added to the product, an alcohol is formed and sulfuric acid reforms 

-water adds on across the double bond 

-this reaction is used industrially to make alcohols and is carried out with steam at a sutiable temp, pressure and in presence of an acid catalyst 

polymers are very large molecules that are built up from small molecules called monomers. 

Addition polymers are made from monomers with C=C double bonds (alkenes) 

- they are made from monomers based on ethene (ethene structure with an R group). the R may be an alkyl or aryl group 

-properties of polymers can be modified by the use of additives such as plasticisers. These are small molecules that get between the polymer chains, forcing them apart and allowing them to slide across eachother (more flexible) 

-PVC w/o plasticisers- hard for drainpipes but with- soft for aprons

-Because the alkane backbone of the polymer has strong non polar bonds, they are very unreactive. But this also means they cant be attacked by enzymes so arent biodegradable 

LDPE: made by polymerising ethene at high pressure via a free radical mechanism, producing a polymer with significant branching. 

the branches dont pack together as well- flexible, stretchy and sutiable for plastic bags 

HDPE: made at temps and pressure a little higher than room temp, using a catalyst, results in a polymer with less chains- MP is higher. 

More rigid and used for crates, bottles etc 

Mechanical recycling: seperating different types of plastic, melting and remoulding them

Feedstock recycling: plastics are heated to a temp that will break the polymer bonds and produce monomers- these can then be used to make new plastics.