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what are alkenes

unsaturated hydrocarbons where the molecule contains at least one carbon to carbon double bond in their structure


alkenes general formula

CnH2n but note cyclic alkenes and alkenes with more than one double bond do not follow this general formula


How are pi bonds made

, the sideways overlap of the 2 p-orbitals result in the formation of a pi bond.


why is rotation possible around every atom in alkanes and not alkenes?

the pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond


what is the shape of an alkene

trigonal planar, 3 regions repel each other as far aprt as possible so bond angle is 120.


give one example of alkenes in the natural world

Alkene Limonene C10H16 is found naturally in the rinds of citrus fruits and is responsible for the smell and flavour of oranges and lemons


define stereoisomers

compounds that have the same structural formula but different arrangements of the atoms in space
2 types; E/Z and cis/trans


give a reason for how stereoisomerism is possible in alkenes.

The restricted rotation of groups attached to each c atom around the double bond by position of pi- bonds electron density below the plane of the sigma bond allows for this


what are the two conditions necessary for a molecule to have E/Z isomerism?

- a c to c double b
- different groups attached t oeach carbon atom of the double bond.


how is E/Z isomerism different to cis/trans isomerism ?

Cid/trans isomerism requires one of the attached groups on each c atom of the double bond to be a h atom


Using the cahn-ingold-prelog nomenclature, how are groups prioritised?

if the groups of higehr priority are on the same size then= z isomer
if grops of higher P are diagonally placed across double b then = E isomer
The higher the atomic numebr the higehr the priority


Difference between E/Z isomerism and cis/trans isomerism

E/Z - isomers with different arrangments in space and locked rotation around double bond
cis/trans - a type of E/Z isomerism that arises when there are 2 H's and Two non-hydrogen functional groups attached to each carbon.


why are alkenes unsaturated hydrocarbons?

contains 1 or more carbon to carbon double bonds



electron pair acceptor


addition reaction

2 or more molecules bonded to make one product


what is hydrogenation of an alkene and what are the conditions?

- hydrogen added across double bond
- over a nickel catalyst at 423K in a c
- ADDITION REACTION where alkene becomes saturated = alkane


give an example of an everyday use of alkanes and the effects of this on health?

vegetable and animal oil goes under hydrogenation t o form margarine - forming trans double bonds - trans- fats
raises cholesterol levels


hydration and the conditions

- water molecule in form of steam added across the C-C double bond - producing alcohol
- high pressure and a phosphoric/sulfuric acid catalyst


halogenation and conditions

- halogen molecule added across C-C double bond
- room temp .
- alkene becomes saturated and a dihaloalkane is formed .


describe the test for saturation?

mix alkene with BR2 water = bromine molecules add across double bond = change of colour from orange brwn to colourless


Through what mechanism does halogenation take place?

electrophilic addition


reaction of alkene with hydrogen halides = what and under what conditions?

- hydrogen halide is added across the C-C double bond .
- room temperature
- if alkene is a gas (ethene) then two gases are mixed and reaction takes place
- if alkene is a liquid then the hydrogen halide is bubbled through it.
- forms dihaloalkanes


what is Markownikoff's rule?

states that when a hydrogen halide reacts with an unsymmetrical alkene , the H of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms.



repeating units of monomers


conditions required to form addition polymers ?

high temperature
high pressure


pro's and cons of recycling polymers ;

- conserving finite fossil fuels can be conserved and the amount that goes into landfill is reduced
• Polymers can be reused by a manufacturer to make another product
- con; time and money intensive


pro's and con's of stability of polymers (lack of reactivity )

- pro ; makes it useful for storing food and chemicals safely
- con ; challenge of disposal , alkene based polymers are non-biodegradable so polymer waste can kill marine life


pro's and cons of burning polymers?

- can be used as a fuel due to high stored energy value releasing carbon dioxide
- CO2 gas contributes to global warming
- HCL may be produced because addition polymers usually consist of hydrogen and cl attached to C


feedstock recycling ?

the monomers of a waste polymer are collected, and can then be reused to make other polymers or new organic molecules.


what are the two types of biodegradable polymers?

biodegradable polymers
photodegradable polymers


biodegradable polymers pros?

Polymers that are affected by the action of
microorganisms and environmental conditions,
leading to decomposition
- used as bioplastics for supermarket bags and bin liners


photodegradable polymers pros?

- used when plant-based materials are not available. Polymers that chemically break down when supplied with sunlight.


define hydrocarbon

hydrocarbons that have double or triple covalent bonds between the adjacent carbon atoms.


How does boiling point vary amongst alkenes?

- BP increases with molecular mass (increased induced dipole-dipole interactions)
- greater degree branching lowers the BP- less surface area of contact = weaker London forces. (induced D-D interactions)


solubility of alkenes?

- alkenes are non-polar molecules so tehy are insoluble in water but soluble in organic solvents


why are alkenes non-polar molecules ?

- made of carbons and Hydrogens and C-H bonds so little diff. in EN = non- polar.


what is the shape around the double bond of an alkene?