chemistry- alkenes

Describe alkenes

• Unsaturated

• Contain carbon-carbon double bond

Describe C=C double bond & arrangement of bonds

Consist of one sigma and one pi bond

Represents a region of higher localised electron density- attacked by electrophiles

The arrangment of bonds around the C=C is planar and has the bond angle 120º

Restricted rotation around the double bond- responsible for geometric isomerism

What are geometric isomers caused by

• Molecules with a C=C with 2 different groups attached to each C of the C=C

What are optical isomers caused by

• E-Z isomers

• C atoms having 4 different groups attached leading to molecules that are non superimposable mirror images of each other

When does geometric isomerism occur

• when there is a carbon-carbon double bond (due to restricted rotation around the double bond)

• When the carbon atoms on both sides of the bond are attached to different groups

What are Z isomers

• Both hydrogen atoms/higher mass groups are on the same side of the molecule

• Zame Zide

What are E isomers

• Both hydrogen atoms/higher mass groups are on opposite sides of the molecule

Why are alkenes non polar

• No dipole moment, no significant difference in electronegativity between carbon and hydrogen

Why are alkenes insoluble in water

Water is a polar substance and alkenes are non polar

Why does melting and boiling points of alkenes increase with chain length

• Mr increases

• more points of contact between molecules

• So vdw forces (IMF) between molecules is stronger

Why do Z isomers have a higher boiling point than E isomers

• Z isomers are polar molecules (net dipole moment)

• E isomers are non polar molecules

• Permanent dipole-dipole interactions (IMF) between the molecules is stronger than the weaker vdw forces

Why do E isomers have a higher melting point than Z isomers

• molecules of the E isomer can ‘pack’ more closely and therefore have more points of contact than the Z isomer

What are addition reactions

A reaction where 2 molecules react together to produce one

What is the intermediate formed in electrophilic addition reactions called

Carbocation intermediate- positive charge on carbon atom

Electrophilic addition reaction of bromine with ethene- description and equation and mechanism

Electrophilic addition reaction of hydrogen bromide with But-2-ene- description and equation and mechanism

What happens if the alkene is unsymmetrical in electrophilic addition of HBr to but-1-ene

The addition of hydrogen bromide can lead to 2 isomeric products

What is Markownikoffs rule

The halogen atom will be added to the carbon with the fewest hydrogens attached to it

Reaction mechanism for propene and hydrogen bromide

How is the major product formed in electrophilic addition to alkenes

Via the more stable carbocation intermediate

Why are carbocation intermediates more stable

• the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it

What is the order of stability for carbocations

Tertiary > secondary > primary

What to do for exam answers relating to stability of carbocations?

Conditions and equations for electrophilic addition of sulfuric acid with alkenes

What is the overall role of sulfuric acid?

How are alkenes converted to alcohols industrially

• Hydration

• reacted with water in the presence of an acid catalyst

Hydration of ethene conditions and equation

Test for alkenes/double bond(s) in a molecule

Bromine water turns colourless

Process where addition polymers are formed from alkenes

Addition polymerisation

Equations for addition polymerisation of ethene and propene

One repeating unit in addition polymerisation

No n

properties of poly(chloroethene) in its pure form and if plasticiser added

• Waterproof, electrical insulator, doesnt react with acids

• Rigid plastic due to strong intermolecular bonding between polymer chains which prevents them moving over each other

• Used to make uPVC window frame coverings and guttering

• If plasticiser is added the intermolecular forces are weakened which allows the chains to move more easily resulting in more flexibility in the polymer

• In this form PVC is used to make insulation on electrical wires and waterproof clothing