ALKENES ☆*: .｡. o(≧▽≦)o .｡.:*☆

GENERAL FORMULA

C_nH_2n

All alkenes molecules have at least one C=C double covalent bond

this makes them unsaturated

because alkenes are unsaturated

they can make more bonds with extra atoms in addition reactions

there are 2 pairs of electrons in C=C

this gives them a high electron density meaning alkenes are pretty reactive

CYCLIC ALKENE GENERAL FORMULA

C_nH_2n-2

electrophilic addition reactions occur because of alkene high electron density

which means they can be easily attacked by electrophiles

electrophiles

electron pair acceptors, normally positively charged

ethene + X-Y (electrophile) general formula

CH2CH2 + X-Y → CH₂XCH₂Y

electrophilic addition steps:

1. bonds in an electrophile are either already polarised OR polarised by the electrons in the C=C bond

2. the electrons from C=C attack the δ+ atom creating a new bond between carbon 1 and the δ+ atom

   • this breaks the bonds between the electrophile and the electrons are taken by the δ- atom to form a negative ion with a lone pair of electrons

   • carbon 2 is left with a positive charge since the double bond broke and carbon 1 took the electrons to form a bond with δ+ atom

   • this forms an intermediate carbocation

3. the negative ion acts as a nucleophile and attacks the positive carbon 2 to form a bond

test for unsaturation

alkene + bromine water → colour change from orange to colourless

alkene + hydrogen halide ^{(electrophilic addition)} → halogenoalkane

the amount of product formed depends on the stability of the carbocation

the more stable the carbocation the more product formed

the more alkyl groups the more stable the carbocation

BECAUSE alkyl groups feed electrons towards the positive charge and

polymers

a long chain of small repeating units, monomers,

natural polymers

DNA

synthetic polymers

polyethene

addition polymers are made from

the double bonds of alkenes opening up to create a long chain

where n stands for the number of repeating units in the polymer

you can also use substituted alkenes as monomers in addition polymerisation

a substituted alkene is an alkene with a hydrogen atom replaced for something else

To find the monomer used to form an addition polymer

take the repeating unit and remove unnecessary side bonds and add a double bond

nomenclature of addition polymers

poly(X), where X is the name of the monomer

if the monomer has a number in it then you but a brackets around it

poly(but-2-ene)

if the monomer does NOT have a number then you don’t need brackets

polypropene

alkene monomers are unsaturated

but alkene addition polymers are saturated

polyalkenes are chemically inert because

• monomers are unsaturated

• the main carbon chain of polyalkenes is saturated

• the main carbon chain of polyalkenes is non-polar

monomers within a polymer chain have strong covalent bonds

however weak intermolecular forces which affects the polymer’s properties

longer chains with fewer branches have stronger intermolecular forces

making these polymer materials stronger and more ridgid

poly(chloroethene)/ polyvinyl chloride/ PVC

an addition polymer formed from chloroethene monomers

covalent bonds between the chlorine and carbon atoms are polar

because chlorine is more electronegative, creating a permanent dipole-dipole force BETWEEN polymer chains

the permanent dipole-dipole force makes PVC hard but brittle material

PVC is used to make drain pipes and window frames

plasticisers

chemicals added to modify polymer properties

adding a plasticiser makes polymer chains bendier

because they get between the polymer chains and push them apart

plasticisers reduce the strength of the intermolecular forces between the chains

so they can slide around more making them flexible

plasticised PVC is more flexible so has more uses

• electrical cable insulation

• flooring tiles

• clothing