Alkenes Flashcards
What is an unsaturated hydrocarbon
- An alkene
- if they have at least one double carbon bond (C=C)
What is the general formula for an alkene
CnH2n
How many sigma and Pi bonds can occur in a double bond (C=C)
one sigma bond and one Pi bond
Where are the sigma and pi bonds located
The sigma bond will be like a fat straight line to the bond for example C=C will have one straight bond which is sigma
*the pi bond is above the double bond and orbits around the bond from left to right on both below and above it
Facts about pi bond you may need to know
π bonds are exposed and have high electron density.
They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles.
Define stereoisomerism
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.
Why might an alkene exhibit stereoisomerism
*There is restricted rotation around the C=C double bond.
*There are two different groups/atoms attached to both ends of the double bond.
How does priority work in E-Z isomers
Priority is given to the atom with the largest atomic mass
If it is the same then go down one chain
Define electrophile
an electron pair acceptor
Why are electrophiles attracted to alkenes
*The double bonds in alkenes are areas with high electron density. *This attracts electrophiles and the alkenes undergo addition reactions.
What is an addition reaction
A reaction where two molecules react together to produce one
What is the change in functional group/Reagent/conditions/mechanism name and type of reagent in the Reaction of bromine with alkenes?
*Change in functional group: alkene –> dihalogenoalkane
*Reagent: Bromine
*Conditions: Room temperature (not in UV light)
*Mechanism: Electrophilic addition
*Type of reagent: Electrophile, Br(delta)+
What is the change in functional group/Reagent/conditions/mechanism name and type of reagent in the Reaction of hydrogen bromide with alkenes
*Change in functional group: alkene–>halogenoalkane
*Reagent: HCl or HBr
*Conditions: Room temperature
*Mechanism: Electrophilic addition
*Type of reagent: Electrophile, H+
‘Markownikoff’s Rule’
In most cases, bromine will be added to the
carbon with the fewest hydrogens attached to it.
What is the The order of stability for carbocation
tertiary > secondary >primary
Spmething you mayneed to know
In exam answers
*Draw out both carbocations and identify as primary, secondary and tertiary
*State which is the more stable carbocation e.g. secondary more stable than primary
*State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion.
*(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved)
What is the change in functional group/Reagent/conditions/mechanism name and type of reagent Reaction of sulfuric acid with alkenes
Stage 1
Change in functional group
alkene –> alkyl hydrogensulfate
Reagents: concentrated H2SO4
Conditions: room temperature
Mechanism: Electrophilic addition
Type of reagent: Electrophile, H2SO4
CH2=CH2 + H2
SO4 CH3CH2OSO2OH
Stage two of sulfuric acid with alkenes
Stage 2
Change in functional groupalkyl hydrogensulfatealcohol
Reagents: water
Conditions: warm mixtureType of reaction: hydrolysis
CH3CH2OSO2OH + H2O –> CH3CH2OH + H2SO
What are the essential conditions for alkenes being hydrated to form alcohols
High temperature 300 to 600°CHigh pressure 70 atm
Catalyst: concentrated H3PO4
(reaction is called hydration)
Testing for alkenes with bromine water
Bromine water decolourises in the presence of a double bond. This can be used as a
test for the presence of an double bond in a molecule. It can be used quantitatively to
show the presence of multiple double bonds in compounds like polyunsaturated oils.
Combustion of alkenes
Alkenes are too valuable to burn
Alkenes burn with a sootier flame
WHat are the conditions required for an elimination reaction in alkenes
High temp and pressure
Concentrated phosphric acid catalyst
what is heterolytic fission
One atom takes both electrons in a covalent bond
Why is more of the major product made?
The major product is always more stable due to the positive inductive effect
What are the three rules when drawing polymers
- Single c-c bond in the middle
- Draw all subsitituents as before
- Add trailing bonds and brackets
What is the use of plasticisers
Makes them more flexible
What is the use of PVCs
Drains pipes
Used for aprons
What is the use of HDPE’s (high desnity polythene)
Crates
Buckets
Bottles
What is the use of LDPE’s (low desnity polythene)
Plastic bags
Electrical insultaion
Stretch
What are the condtions for HDPE’s
Low temp and pressure
Ziegler Natta catalyst
Often gives unbranched alkenes
What are the condtions for LDPE’s
High temp and pressure
Formed using free radicals
often branched
Define biodegradeability
Ability to break down by buological agents
What are examples of features that make soemthing biodegradgavle
Unreactive
Non polar
Saturated
Whata are ways in which non biodegradeable things get depleted
They get burnt or thrown into a landfil
What are the two ways in which enviornmnatally freidndly disposal is achieved
Mechanical
Feedstock
What happens when you repeatedly heat/melt something
It can damage the chains and degrade the plastic properties
What are the steps in Mechanical disposal
- Sort
- Wash
- Ground into pellets
- Melted and remolded
What are the steps in feedstock disposal
- Heat
- Reproduce monomer
- Recombine
