3.4 Alkenes Flashcards
Why does the C=C double covalent bond in alkenes have a high electron density?
∵ they’re 2 pairs of electrons in the C=C double bond
(High conc. of electrons )
Why is there restricted rotation around double bond in alkenes?
- C-C single bond & p-orbital (contains single electron) on each carbon
- These 2 orbitals overlap & form an orbital with a cloud of electron density above and below the single bond
- Called π orbital & its presence = bond can’t rotate
What is the only intermolecular forces between alkene molecules?
Van der Waals forces
Alkenes are ____ in water
insoluble
Alkenes are ____ reactive than alkanes
more
Why are alkenes more reactive than alkanes?
∵ C=C forms electron-rich area = can be easily attacked by electrophiles
Why do electrophilic addition reactions occur?
∵ double bond has lots of electrons = easily attacked by electrophiles
What are electrophiles?
electron-pair acceptors
Where are electrophiles attracted to?
Attracted to areas where there’s lots of electrons
e.g. Positively charged ions (H+ NO2+) and polar molecules (δ+)
When you shake an alkene with orange bromine water, why does the solution decolourises?
∵ Bromine is added across C=C double bond to form colourless dibromoalkane
(happens via electrophilic addition)
Electrophilic Addition
Name the reactant + bonds & the products
- Halogen
- X—X = dihalogenoalkane
- Hydrogen Halides
- H—X = halogenoalkane
- Sulfuric Acid
- H2SO<span>4</span> = alcohol
What happens when you add a hydrogen halide to an unsymmetrical alkene?
You get 2 possible products
Adding Hydrogen Halides to Unsymmetrical Alkenes
What does the amount of each product formed depend on?
How stable carbocation formed is = known as carbocation intermediate
Why are carbocations with more alkyl groups more stable?
∵ alkyl groups feed electrons towards positive charge
Ethene + Br2
Balanced Equation (structural formula)
Ethene + Br2
Mechanism
Propene + HBr
Mechanism
Ethene + H2SO4
Mechanism
What do alkenes form initally when reacted with cold concentrated sulfuric acid?
Alkyl hydrogen sulfates
How can you convert alkyl hydrogen sulfates into alcohols?
By adding water & warming reaction mixture
What are polymers?
Long chain molecules formed when monomers join together
What are addition polymers formed from?
alkenes and substituted alkenes
Why can alkenes make up addition polymers?
Can act as monomers and form polymers ∵ double bond can open up & join together to make long chains
Draw poly(phenylethene) from phenylethene
Why are addition polymers very unreactive?
- Polyalkene chains = saturated molecules
- Main carbon chain of polyalkene = non-polar
How are the polyalkene chains held together?
Van der Waals forces
(∵ polyalkene chains = non-polar)
Longer the polymer chains & closer together they can get = …
stronger Van der Waals forces between chains will be
Name 2 properties of polyalkenes made from long, straight chains
strong & rigid
Name 2 properties of polyalkenes made from short, branched chains
weaker & more flexible
Why are addition polymers not biodegradable?
- ∵ Chain = saturated alkane molecule
- have non-polar C-C bonds
- Means they cannot be attacked by biological agents (e.g. enzymes)/nucleophiles
How can you modify the properties of polymers?
By adding plasticiser to polymer = more them more flexible
Why does adding plasticiser to polymer make it more flexible?
- ∵ plasticiser molecules get between polymer chains & push them apart
- = reduces strength of intermolecular forces between chains
- ∴ chains slide around more = making polymer easy to bend
Draw the repeating unit of the polymer formed from ethene, C2H<span>4</span>
Draw chloroethene becoming poly(chloroethene)
What is epoxyethane used as?
An intermediate chemical from which many useful products can be made
How is epoxyethane made?
- Made by partially oxidising ethene
- Mixture of ethene and oxygen is passed over catalyst = silver
Why is making epoxyethane hazardous?
- Epoxyethane = poisonous and carcinogenic
- It’s highly flammable/explosive when in contact with air
Why is epoxyethane highly reactive?
- Bonding pairs in ring of atoms in molecule = forced very close together
- ∴ bond angles = 60° rather than 109.5° i.e. C–O–C bond angle much smaller than 109.5°
- Bonding pairs repel each other
- = ring tends to spring open (molecule has ‘ring strain’ due to the repulsion between bonding pairs - atomic orbitals overlap) as system becomes more stable if ring is broken
What are the products produced from reacting epoxyethane with alcohol used as?
Plasticisers or detergents
What is formed when epoxyethane reacts with water?
ethane-1,2-diol
What is ethane-1,2-diol used as?
antifreeze
Write the equation for when epoxyethane reacts with water
C2H4O + H2O → C2H6O2
Draw the mechanism for when epoxyethane reacts with water
What is poly(chloroethene), PVC, used for?
Rigid PVC:
- Drainpipes
- Window frames
Plasticised PVC:
- Electrical cable insulation
- Flooring tiles
- Clothing
Electrophilic Addition
(CH3)2C=CHCH3 + HBr → (CH3)2CBrCH2CH3
The minor product in this reaction mixture is 2-bromo-3-methylbutane. Explain why this bromoalkane is formed as a minor product. (3)
- Reaction goes via intermediate carboncations
- Formed via tertiary carboncation which is more stable (than secondary carboncation)
- Due to more alkyl groups release electron density towards the positive carbon
Write equation for how epoxyethane is formed
2 CH2CH2 + O2 → 2 CH2CH2O
2-methyl but-2-ene reacts with concentrated sulfuric acid to form 2 different products. Draw the structure of the minor product of this reaction. (1)
Name the conditions for making expoxyethane
- 250-300°C
- 15 atm
- Silver catalyst
Write the skeletal formula for epoxyethane
Explain why alkenes undergo electrophilic addition reactions with bromine (2)
- C=C is area of high electron density
- (this) induces a dipole in the bromine molecule
