alkenes

what are alkenes

• Alkenes are unsaturated hydrocarbons. 

• Have one or more C=C double bond.

when are alkenes produced

• Produced during thermal cracking of crude oil. 

features of alkenes

• Only van der waals forces between alkene molecules.  

• Melting and boiling point increase with chain length. 

• Not soluble in water.

why can’t the double bond rotate

• As well as the c-c single (sigma) bond there is a p-orbital which contains a single electron on each carbon.

• These two p-orbitals overlap and form an orbital with  a cloud of electrons above and below the single bond.

• This is a 𝝅 orbital and means that the double bond cannot rotate. 

  • They show position and E-Z isomerism. 

what makes alkenes hella reactive lol

The double bond makes them more reactive because of the high electron density between the two carbon atoms.

why do alkenes readily undergo addition reactions

• The C=C forms an electron-rich area in the molecule (nucleophile), which can easily be attacked by positively charged reagents.

• These reagents are called electrophiles (electron liking).

• They are electron pair acceptors.

• A good electrophile is the H⁺ ion.

• As alkenes are unsaturated they can undergo addition reactions.

electrophiles used in electrophilic addition with alkenes

• Br₂ 

• HBr 

• H₂SO₄

process of electrophilic addition

1. Electrophile attracted to double bond

2. Electrophiles are positively charged so accept electron pair from double bond

3. A positive ion (carbocation) is formed

4. A negatively charged ion forms a bond with the carbocation. 

major products

• most stable

• on the secondary or tertiary carbocation

• you need to put the electrophile on the carbon closest to the end

minor products

• less stable

• on primary carbocation

with what molecules are major/ minor products formed

• HX

test for double bond

• This addition reaction is used to test for a carbon-carbon double bond.

• When a few drops of bromine solution, sometimes called bromine water (which is reddish-brown) are added to an alkene, the solution is decolourised because the products are colourless.

what is a polymer

• long chain molecules made up of lots of small monomers joined together

• Can be natural (DNA, proteins etc.) or synthetic (polyethene etc.)

what is a monomer

• short chain molecules that when bonded together they form a polymer. 

• They are based on ethene (2 carbons) and should not be reactive as side reactions may occur if they are. 

what is addition polymerisation

• Formation of long chain molecules from lots of smaller molecules joining together with no other products. 

• The double bond opens up and the monomers join together to form a backbone of carbon atoms.

• They are unreactive due to being saturated and having (usually) non-polar side chains. 

uses of polyethene

plastic bags, pens

uses of polypropene

carpets, ropes, chairs

high density polythene

• Less branching - pack together 

• Stronger

• Withstand higher temperatures 

• Strong intermolecular forces 

low density polythene

• Branching - can’t pack as closely 

• weaker

• Packaging

what are plasticisers and what do they do

• Molecules that stick between chains and keep polymers further apart. 

• This weakens the van der waals forces and lets the chain slide more easily

• Make straight chains more flexible.

• Adding branches also makes them more flexible as they pack less tightly.  

what is poly(chloroethene) and what are its uses

• PVC - POLY(VINYLCHLORIDE)

• Used for rigid plastics like drainpipes. 

• Strong dipole-dipole forces due to the chlorine atom. 

• Adding a plasticiser would make it more flexible - flooring tiles.