Alkenes

What are alkenes?

Unsaturated hydrocarbons containing at least one C=C bond

What does a double bond represent?

• Two shared pair of electrons

• Therefore a double bond is a region of high electron density

What is the functional group of alkenes?

C=C

What is the genera formula of alkenes with only one double bond?

C_nH_2n

What is a molecule with many double bonds classed as?

Polyunsaturated

What are double bonds comprised of and how are they formed?

• Made up of a π bond and a σ (sigma) bond

• The pi bond is formed from a sideways overlap of p orbitals above and below the place of the carbon atoms

• The sigma bond is formed from the overlap of p orbitals between each carbon

Note that there are not two pi bonds, it’s just split into two parts

Which electrons from two carbons in a C=C bond form bonds with hydrogen?

Electrons from the 2s² sub-level

How is E/Z isomerism formed in alkenes?

Rotation around the double bond in alkenes is restricted by the presence of the pi bond

What is stereo isomerism?

When two isomers have the same molecular and structural formula, but differ in the spatial orientation of some atoms

What are the criteria for E/Z isomerism?

• Has to have a C=C bond

• Each carbon in the C=C bond has to be attached to two different groups

How do we decide if a molecule is an E or Z isomer?

• Draw the molecule in crab notation

• Pick the atom on each side that has the highest atomic number and is directly bonded to the carbon

• Assign those atoms priority on each side of the double bond

• If these groups are on the same side (both below or both above the carbon double bond) then it is a Z isomer

• If these groups are on different sides (One above and one below the double carbon bond) then is it a Z isomer

• If both atoms one each branch of one carbon are the same, move to the next atom in the chain attached to the first carbon and see if the atomic number is higher

What are electrophiles?

• Electron deficient species that accept a pair of electrons

• They are attracted to regions of high electron density (double bonds)

What causes alkenes to undergo addition reactions?

• Being attacked by electrophiles

• The C=C bond is a region of high electron density and attracted positive or partially positive species

By what mechanism do alkenes undergo addition reactions?

Electrophilic addition

What happens during electrophilic addition?

• The electrophile molecule is polar either because of a difference in electronegativity or an induced dipole by the C=C bond

• When the electrophile attacks the alkene, the electrons that form the pi bond in the C=C bond move to the slightly positive atom (A) in the molecule

• This in turn breaks the bond between the slightly positive (A) and slightly negative atom (B)

• A carbocation intermediate with a positive charge on the carbon is formed

• The B becomes an negative ion and will act as a nucleophile, attacking the carbon with the positive charge and forming a bond with it

• This forms a molecule with both A and B attached to it by addition

Br₂ is not a polar molecule, so why can it act as a electrophile?

The C=C bond is an area of high electron density, so those electrons can repel the electrons in the bromine molecule, inducing a dipole

How can alkenes be detected?

• Detected using bromine water

• This tests for unsaturated molecules

• Alkanes will stay orange when bromine water is added to them

• Alkenes with turn colourless

• The double bond is broken in order to add bromine to the compound- an addition reaction

What are the three reactions for addition?

• Liquid or aqueous bromine (Br₂)

• Aqueous hydrogen bromide (Hbr)

• Concentrated sulfuric acid (H₂SO₄)

What is the product of alkenes reacting with HBr

A halogenoalkane

How can a mixture of products be formed in electrophilic addition including HBr?

For 3 carbons or more, you can form a mixture of elimination products

Why is electrophilic addition a better way of producing halogenoalkanes than free fradical substitution?

• The atom economy of electrophilic addition is 100%

• Free radical substitution can form other unwanted products or molecules can undergo further substitution

What are the products of the reaction of alkenes with concentrated sulfuric acid?

Alkyl hydrogen sulfate

What happens in the reaction where an alkyl hydrogen sulfate reacts with water?

• Alkyl hydrogen sulfates are readily hydrolysed

• Water can be added to separate the carbon in the carbon chain from the OSO₃H molecule attached to it

• This produces an alcohol and sulfuric acid is regenerated

• This means sulfric acid acts as a catalyst in the reaction

What is the difference between potential products of addition reactions with unsymmetrical alkenes and symmetrical alkenes?

• Unsymmetrical alkenes can have a mixture of products

• Symmetrical alkenes can only have one product

How do we class carbocations as primary, secondary and tertirary?

• If one alkyl group is attached to the positive carbon atom, then it is a primary carbocation

• If there are two attached to the positive carbon atom, it is a secondary carbocation and so on

Why may some electrophilic addition products be produced more than others, and what do we call them?

• Some elimination products are formed from less stable carbocations

• The more alkyl groups that are attached to the positive carbon atom, the more stable the carbocation is so secondary carbocations are more stable than primary

• A product formed from the more stable carbocation are what we call the major product as they are produced the most

• A products formed from the less stable carbocation are called minor products and are produced less

How does positive inductive effect work?

• Alkyl groups release electrons towards the positive carbon and try and stabilise the positive charge in carbocations

• This means the more alkyl groups that are attached to this positive carbon, the more stable the carbocation will be

What is polymerisation?

Formed when thousands of repeating units made from small molecules called monomers join together to make a long chain molecule

What happens during addition polymerisation?

Unsaturated molecules (alkenes) join together to form a polymer by breaking a double bond

How are addition polymerisation equations written?

• Draw the alkene in crab notation then draw the product arrow

• Draw the same molecule but with only a double bond between the two carbons’

• Draw brackets on the outside of the molecule

• Draw two lines from each carbon extending out of the brackets

• Add an n outside of the brackets

What is the product of the addition polymerisation reaction of ethene and what is the product used for?

• Polyethene (polythene)

• Used in plastic bags, food wrappers, straws

What is the product of the addition polymerisation reaction of propene and what is the product used for?

• Polypropene

• Plastic trays, drinks bottles, stronger plastics

What is the product of the addition polymerisation reaction of chloroethene and what is the product used for?

• Polychloroethene or polyvinylchloride (PVC)

• Records, pipes, window frames, clothing (made from plasticised)

• There are two types of PVC- plasticised and unplasticised

What is the product of the addition polymerisation reaction of phenylethene or styrene and what is the product used for?

• Polyphenylethene or polystyrene

• Cups

What is the product of the addition polymerisation reaction of tetrafluoroethene and what is the product used for?

• Polytetrafluoroethene, PTFE or Teflon

• PTFE used for joints in plumbing

• Teflon used for non-stick pans

What is the product of the addition polymerisation reaction of propenenitrile or acrylonitrile and what is the product used for?

• Polypropenenitrile or polyacrylonitrile

• Acrylic nails

What should you remember if asked to draw the repeating unit of a polymer?

Do not draw the n outside of the bracket, only add the n if it is a polymerisation equation

Why are addition polymers unreactive and what are the consequences of this?

• They are saturated molecules and there are no slight positive carbons/polar bonds usually

• In order for them to react you would need to break strong C-C bonds

• This means that these polymers are not biodegradable and remain in the environment forever

What is a more environmentally friendly way polymers are made?

Made by condensation reactions

What kind of structure are addition polymers

The are simple molecules, they don’t exist in a lattice

How are the strength and melting points of addition polymers determined

• By intermolecular forces

• This is normally weak IMF like PDD or van der Waals

How can branching affect the boiling points of addition polymers?

• Branching chains means that polymers cannot pack together as closely and reduces the number of points of contact

• Therefore van der Waals forces are weaker in branched molecules than straight chain ones

• This results in lower bpt

What is the difference between PVC and uPVC (unplasticised PVC)

• uPVC is strong and rigid and can be used for window frames

• PVC is soft and flexible and would be used for clothing that feels like leather

Why is uPVC soft and flexible?

• Plasticiser molecules allow chains of PVC to roll over each other

• This makes the material flexible