Alkenes Flashcards
What is an alkene
An unsaturated hydrocarbon
What is the general formula for alkenes
CₙH₂ₙ
What mechanism can alkenes undergo when they react with an electrophile
Electrophilic addition
Why are alkenes attacked by electrophiles
They contain a double bond with a high electron density
What is an electrophile
An electron pair acceptor
They are deficient in electrons and are therefore attracted to the double bond in alkenes
Give two categories which can be electrophiles
Positively charged ions
Polar molecules
What is the test for alkenes
When we add bromine water, if alkenes are present, a colour change will occur from brown/orange to colourless
What do we form when we add bromine to an alkene
A dibromoalkane
By what mechanism does bromine react with an alkene
Electrophilic addition
Draw the mechanism for the reaction between bromine and ethene
Explain how Br2 is polar when reacting with an alkene
When a bromine molecule approaches the double bond in an alkene, the electrons in the nearest Br atom repel the high electron density in the double bond, causing a temporary induced dipole, and the nearest bromine atom to be be delta positive, and the bromine molecule to be polar
What is the intermediate called which is formed when bromine reacts with an alkene
A carbocation
What colour are dibromoalkanes
Colourless
What is formed when an alkene reacts with a hydrogen halide
Halogenoalkanes
Draw the mechanism for the reaction between hydrogen bromide and ethene
How many products do we form when a hydrogen halide reacts with an unsymmetrical alkene
2
What is the amount of a specific product that is produced dependent on when an unsymmetrical alkene reacts with a hydrogen halide
The stability of the carbocation intermediate
What determines the stability of a carbocation and explain why this is the case
The number of alkyl groups bonded to the posotove carbon
The more alkyl groups bonded, the more stable the carbocation is
This is because alkyl groups push electrons towards the positive carbocation, stabilising it
What is a primary, secondary and tertiary carbocation
Primary - 1 alkyl group bonded to the positive carbon
Secondary - 2 alkyl groups bonded to the positive carbon
Tertiary - 3 alkyl groups bonded to the positive carbon
What do we call the product formed from a less stable carbocation when an unsymmetrical alkene reacts with a hydrogen halide
Minor product
What do we call the product formed from a more stable carbocation when an unsymmetrical alkene reacts with a hydrogen halide
Major product
Draw the mechanism for the reaction between prop-1-ene and hydrogen bromide. Identify which is the major product
What are the conditions for the reaction of bromine with an alkene
Room temperature (not in UV light)
What are the conditions for the reaction of hydrogen halides with an alkene
Room temperature
Draw the structure of sulfuric acid
What is the role of sulfuric acid when reacting with an alkene
Used as a catalyst in making alcohol from an alkene
What do we form when we react sulfuric acid with an alkene
Alkyl hydrogen sulfate intermediates
And alcohol as product
What are the conditions for the reaction between an alkene and sulfuric acid
Cold, concentrated sulfuric acid
Draw the mechanism for the reaction between ethene and sulfuric acid to produce an alcohol, stating the intermediate formed
What can we add to an alkyl hydrogen sulfate to reform sulfuric acid, what is this process called
Cold water
Hydrolysis
Write the equation for the two steps in forming an alcohol from ethene on addition of cold, concentrated sulfuric acid
What are the conditions required for the hydrolysis of an alkyl hydrogen sulfate
Add cold water to warm a warm alkyl hydrogen sulfate
How are polymers made from alkenes
Alkenes are monomers which can join together to form addition polymers
What are the 2 types of polymers
Natural (Proteins & natural rubber)
Synthetic (e.g. Polypropene)
Give an example of how polymers have developed over time
Charles Goodyear in 1844 discovered vulcanised rubber which is now still used in car tyres (goodyear tyres)
In the last 100 years polymers such as nylon and Teflon have improved our standard of living
New polymers continue to be synthesised to this day
Draw the polymer of prop-1-ene
Don’t put the n in the corner when showing a specific number of repeating units
Why are poly(alkenes) normally unreactive and what effect can this have on the environment
Poly(alkenes) are saturated molecules which are normally non polar meaning they are unreactive. Thus, they don’t degrade well in landfill
What affects the properties of a polymer
Intermolecular forces
What type of attraction is present between poly(alkenes)
Most poly(alkene) chains are non polar so they only have van der waals forces between the chains
What affects the strength of the van der waals forces between poly(alkene) chains
The longer the chain and the closer they are to each other, the more VDW forces there are (and the stronger they are)
Give 2 properties of flexible and weak polymers
Shorter chain length
A lot of branching
Why does branching affect the strength of a polymer
The more branches there are, the less close the polymer chains can be packed next to each other, meaning less and weaker VDW forces
Give 2 properties of strong and rigid polymers
Long chain length
Little or no branching
Explain why polyalkenes with halogens bonded to them have different properties to than those without
They are polar, and so they can form permanent dipole-dipole forces which are stronger than VDW forces. Thus, they will have different properties to other non polar polyalkenes
Give an example of a polyalkene with halogen bonded to it, and give one use of it
PVC (polyvinyl chloride)
Drain pipes (strong polymer so weather resistant)
What is the role of plasticisers
Plasticisers make polymers more flexible
Explain how plasticisers make a polymer more flexible
_ Plasticisers slide between the polymer chains, pushing them apart
- This weakens the IMF between the chains
- The chains can now slide over each other more, which makes the polymer easier to bend
State how the properties of PVC change when it is modified using a plasticiser.
Give an example of a use of PVC with and without modification using a plasticiser
PVC is hard and brittle which can be used in drain pipes
PVC with plasticiser added to it is more flexible which can be used for electrical cable insulation and clothing
Give an example of a use of PVC with modification using a plasticiser
Electrical cable insulation and clothing
