C13 Alkenes Flashcards
Are alkenes unsaturated or saturated
Unsaturated
General formula of alkenes
CnH2n
What’s the bonding like for each of the C of C=C in alkenes
3 of 4 electrons used in 3 sigma bonds ( 1 to other C atom of C=C & other 2 electrons to other atoms)
What happens to 4th electron in each C of C=C in alkenes
1 electron (in p-orbital) on each C of C=C not involved in sigma bonds
Define pi bond
Sideways overlap of 2 p-orbitals, 1 from each C of C=C
Where is pi electron density concentrated in an alkene
Pi electron density concentrated above and below line joining nuclei of bonding atoms
Shape and bond angle in alkene
Trigonal planar
120*
3 regions of electron density around each of C atoms
- 3 regions repel each other as far apart as possible
- all of atoms are in same plane
Define stereoisomer
Compounds with same structural formula but with different arrangent of atoms in space
Conditions needed for E/Z isomerism
C=C
Diff groups attached to each C of C=C
Z isomer
Groups of higher priority on same side of C=C
E isomer
Groups of higher priority are diagonally opposite across C=C
What is the Cahn-Ingold-Prelog rule
Atoms attached to each C atom in C=C are given priority based on their atomic number
When can Cis-trans isomerism only be used
When each C atom in C=C is attached to a single H atom
Why’s there stereoisomerism around C=C
Rotation about C=C restricted, groups attached to each C fixed relative to each other
Reason fro rigidly - position of pi-bond electron density above + below plane of sigma bonds
2 types of isomerism
E/Z isomerism
Optical isomerism
Conditions needed for Cis-trans isomerism
C=C
1 of attached groups on each C of C=C must be the same
How to assign priority if 2 atoms attached to C=C are same
Find 1st point of difference
Group with higher atomic number at 1st point of difference is given higher priority
use enthalpy data explain why pi bonds break more readily than sigma bonds
In Ethene:
C-C (sigma bond) has BE = 347 kJ mol-1
C=C (pi and sigma) BE = 612 kJ mol-1
BE of pi bond : 612 - 347 = 265 kJ mol-1
Pi bonds weaker than sigma bond, so is broken more easily due to lower BE
Hydrogenation of alkene
Hydrogen
Ni at 423K
Form alkane
Are alkenes more/less reactive than alkanes
More reactive
Why are alkenes reactive
C=C made of sigma + pi bonds
Pi electron density conc above + below plane of sigma bond
Pi electrons outside of C=C —> so are more exposed than electrons in sigma bond
Pi bond breaks readily & undergo addition reactions easily
Hydration of alkenes
Steam/H2O(g)
H3PO4
Form alcohol
Reaction of alkene with hydrogen halide
Hydrogen halide (g)
Room temp
Form haloalkane
Test for unsaturation
Br2
Colour change = orange to colourless (bromine water decolourises)
Halogenation of alkenes
Room temp
Br2/Cl2
Form haloalkane
Type of bond fission in electrophilic addition
Heterolytic fission
What’s the most stable carbocation
Tertiary carbocation
( stability increases from primary to tertiary carbocation)
Explanation for electrophilic addition of but-2-ene with HBr
(Delta neg) Br more EN than H (delta pos)
Electron pair in pi bond attracted to delta pos H
C=C break
Bond form between H of HBr and C of C=C
H-Br break by heterolytic fission, electron pair go to Br
Br- and carbocation form
Br- react with carbocation to form addition product
Explanation of Br2 and propene
Pi electrons interact with electrons in Br2
Induces dipole on Br-Br
Electron pairs in pi-bond attracted to delta pos Br, cause C=C to break
Bond form between C and Br
Br-Br break by heterolytic fission, electron pair for to delta neg Br
Br- and carbocation form
Br- react with carbocation to form product
define ectrophile
Atom/group of atoms that’s attracted to a electron rich centre & accepts an electron pair
Markownikoffs rule
When a H-halide reacts with an unsymmetrical alkene the H of H-halide attaches itself to C of alkene with greater number of H atoms and smaller number of C atoms
Why’s tertiary carbocation most stable
carbocation stability linked to e^- donating ability of alkyl groups
each alkyl group donates + pushes e^-s towards pos charge of carbocation
pos charge spread over alkyl groups
more alkyl groups attached to pos-Charged C atom, more charge is spread out,making ion more stable
What’s the major product
more stable carbocation
Using wast polymer as fuel
some polymers made from natural gas and hard to recycle, they have high energy store
incinerate waste polymer, produce heat, generate steam, drive turbine producing elelctrcity
Pros of polymers
Readily available
Cheap
Convenient for throwaway society
Lack reactivity - suitable for storing food
Cons of polymers
Lack reactivity
Enviro effect - killing marine life
Bioplastics
produced from plant starch, cellulose, plant oils, and proteins offer a renewable and sustainable alternative to oil-based products
- protects enviro and conserves valuable oil refineries
PVC recycling
disposing PVC is hazardous due to high Cl content and additives it contains
- when burnt releases Hydrogen chloride (corrosive gas) and pollutants
Compostable polymers
degrade and leave no visible/toxic residues
- based on poly(lactic acid) becoming more common
photodegradable polymers
polymers that contain bonds that are weakened by absorbing light to start the degradation.
alternatively, light absorbing additives are used
Feedstock recycling
chemical and thermal processes that can reclaim monomers, gases or oil from waste products
- products made from this resemble the ones made from crude oil, so can be used as raw materials for production of new polymers
- advantage = can handle unsorted and unwashed polymers
Recycling polymers
pros:
- reduce enviro impact
- conserve finite fossil fuels
- decrease waste going to landfill
.cons:
- if polymer is mixed, it renders the product unusable so it can’t be reused
Biodegradable polymers
- broken down by microorganisms into H20, CO2 and biological compounds
- made from starch/cellulose or contain additives that alter structure of traditional polymers so microorganisms can break them down.
What are alkenes
Unsaturated HC that contain at least 1 C=C made up of a pi bond and sigma bond
What’s the general formula of alkenes
CnH2n
How is a pi bond formed
Electrons in the adjacent p orbitals overlap above and below the C atoms. They can only be made after a sigma bond is formed
What bond restricts rotation of C atoms
Pi bond
What’s the angle and shape of a double bond
Trigonal planar
120*
Are they more or less reactive than alkanes? Why?
More reactive due to high electron density of double bond and the fact the pi-bond is slightly easier to break
What IMFs do they have
Only LFs due to non-polar bonds
Are they soluble In water
No
Non-polar bonds
What are the types of isomers that can be formed using alkenes
E/Z isomers - due to restricted rotation
Cis-trans isomers - if 2 of the same substituents are attached to each C
What is an electrophile
Species that are electron pair acceptors
What is the most stable type of carbocation intermediate? Why?
Alkyl groups have a pos inductive effect, so the most stable carbocation is the one bonded to the most other C atoms, i.e. a tertiary carbocation
Major products will be formed from which kinds of carbocations
Tertiary (or the most stable available)
What conditions are needed for the electrophilic addition of H2O to an alkane? What is this type of reaction called?
Steam in the presence of an acid catalyst, usually phosphoric acid
Reaction is called hydration
What are the products of the hydration reaction
An alcohol
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene
Hydrogen halide gases must be at room temp
What is the reaction called when a halogen is added to alkene
Halogenation
How does a molecule with non-polar bond react as if it is an electrophile
C=C with a high electron density induces a temporary dipole in the halogen molecule —> delta pos atom is attracted to double bond
How can an alkene be converted into alkane? What is the reaction called and what are the required conditions
Alkene + hydrogen = alkane
Hydrogenation
Conditions —> 150*C, nickel catalyst
What is an addition polymer
Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
What are monomers? What form do they usually take?
Molecules which combine to form a polymer
Usually have a C=C which breaks to leave a repeating pattern
What are the ways in which plastics can be disposed
Landfill
Combustion
Electricity generation
Reuse
Recycle
Organic feedstock
What are the disadvantages of recycling
Plastic must be sorted into different types
Expensive
Labour intensive
Requires high technology
How does photodegradable polymers break down
They are broken down chemically using energy with wavelengths similar to light.
Once the break down begins it is not possible to stop the process
Explain what happens in organic feedstock
Plastics are separated and broken down into small organic molecules through a series of reaction.
The molecules can then be used produce plastics and in other industries
Give a disadvantage of photodegradable polymers
May not be exposed to sufficient light
