13) Alkenes Flashcards
Where are the 4 electrons of each carbon of the double bond in an alkene?
3 are used in 3 sigma bonds
1 is in a p-orbital
Describe a pi bond (3 points)
formed by the sideways overlap of 2 p-orbitals, one from each carbon atom of the double bond
each carbon atom contributes 1 e- to the e- pair in the pi bond
pi-electron density is concentrated about and below the plane
What effect does the pi-bond have on the two carbon atoms of a double bond?
locks the carbon atoms in position and prevents them from rotating around the double bond
What is the shape and bond angle of carbon atoms of the double bond?
trigonal planar shape
120 degrees
Define stereoisomers
compounds with the same structural formula but with a different arrangement of atoms in space
Define E/Z isomerism
a type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the double bond
Define cis-trans isomerism
a special type of E/Z isomerism in which there are two non-hydrogen groups and two hydrogen atoms around the C=C double bond
What saying can you use to remember E/Z and cis-trans isomerism?
ET phone home
In Cahn-Ingold-Prelog nomenclature, what 2 things should you remember when assigning priority?
the higher the atomic number, the higher the priority
if two atoms attached to a carbon atom in the double bond are the same, use the first point of difference
Why are alkenes more reactive than alkanes?
pi bond
being on the outside of the double bond pi-electrons are more exposed than e- in the sigma bond
alkene -> alkane
+ H2
Ni catalyst at 423K
addition / hydrogenation
alkene -> haloalkane
+Cl2/Br2/hydrogen halide
addition/halogenation
What should you remember when an unsymmetrical alkene reacts with an unsymmetrical compound?
two products are possible
How can you test for unsaturation?
bromine water
turns from orange to colourless
akene -> alcohol
+H20(g)
H3PO4
addition/hydration
Name the mechanism used for the addition reactions of alkenes
electrophilic addition
Define electrophile
an atom (or group of atoms) which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
In an electrophilic addition reaction of a hydrogen halide with an alkene, how does the bond break and where are the 3 curly arrows?
by heterolytic fission
from the C=C bond to the partially positive H of the hydrogen halide
from the H-halide bond to the partially negative halide atom
from an e- pair of the negative halide ion to the positive carbon of the carbocation
State Markownikoff’s rule
addition of a hydrogen halide to an unsymmetrical alkene forms the major product via the most stable carbocation
What does a carbocation contain?
a positively charged carbon atom
Explain why a tertiary carbocation is the most stable in comparison to a primary or secondary carbocation
each akyl group donate and pushes e- towards the positive charge of the carbocation
the positive charge is spread over the alkyl group
the more alkyl groups, the more charge is spread out - therefore, the carbocation is more stable
Define addition polymerisation
formation of a very large molecular chain, by repeated addition reactions of many monomers (unsaturated alkene molecules)
Give a key feature of an addition polymer
high molecular mass
How are synthetic polymers usually named?
after the monomer that reacts to form their giant molecules, prefixed by ‘poly’
Define repeat unit
the specific arrangement of atoms in the polymer molecule that is repeated
What 2 conditions are needed for polymerisation?
high temperature
high pressure
Why are alkene-based polymers of environmental concern?
their lack of reactivity makes many non-biodegradable
Explain how polymers are recycled and the advantages and disadvantages of this are
polymers are sorted by type, chopped into pellets, washed, dried and melted. Then cut into pellets to be made into new products
advantages: conserves finite fossil fuels; reduces landfill
disadvantages: doesn’t work for mixed polymers
Why is PVC recycling hazardous?
high chlorine content and additives
Describe the effect of incineration of polymers
produces heat, generating steam to drive a turbine producing electricity
Describe feedstock recycling and its advantages
chemical/thermal processes to reclaim monomers, gases, or oil from waste polymers which can then be used as raw materials for the production of new polymers
advantages: able to handle unsorted/unwashed polymers
What are biodegradable polymers usually made from?
starch or cellulose
How are biodegradable polymers broken down?
by microorganisms into water, carbon dioxide and biological compounds
What is special about photodegradable polymers?
contain bonds that are weakened by absorbing light to start degradation - or light-absorbing additives
Suggest why a polymer with an -OH group present can dissolve in water
can form hydrogen bonds with water
The combustion of waste polymers can be used for energy production. What problem is caused by disposing of PVC in this way?
hydrogen chloride gas is produced
