ALKENES Flashcards
Definition: Pi bond
A bond formed by the sideways overlap of two py-orbitals, with the electrons above and below the plane of the bonding atoms
Definition: Sigma bond
A bond formed by the overlap of one orbital from each bonding atom, consisting of two electrons and with electron density centred round a line going directly between the nuclei of the two atoms
Why is a sigma bond stronger than a pi bond?
Electron density is higher in a sigma bond because it’s directly between the atoms (closer to the nuclei) however in the pi bond, the electrons are spread out so electrostatic attraction is weaker
Definition: Stereoisomerism
Molecules that have the same structural formula but a different arrangement of atoms in space
How does E/Z isomerism arise?
- Has to have a double bond (restricted rotation)
- Has to have two different groups on each atom of the double bond
When is it E isomerism?
When the priority groups are on opposite sides
What is cis/trans isomerism?
A special case of E/Z isomerism when one of the groups on each carbon of the double bond is the same. If the identical groups are on the same side it is cis and trans if they are opposite.
Products and conditions for hydrogenation
Ni catalyst, 423K
Alkene + H2 -> Alkane
Alkene + Halogen
Dihaloalkane
Room temperature
Alkene + HX
Haloalkane
Room temperature
Alkene + H20 (g)
Alcohol
Phosphoric acid catalyst
Definition: Electrophile
An electron pair acceptor
Definition: Addition reaction
The double bond (C=C) breaks and something is added
Two reactants become one product
Definition: Heterolytic fission
The breaking of a covalent bond forming a cation and an anion (both the electrons go to the same atom)
Markownikoff’s rule
When a hydrogen halide is added to an unsymmetrical alkene, the most likely product is the one in which the H atom adds to the carbon that already has the most H atoms attached to it
Definition: Atom economy
Sum of molar masses of desired products/Sum of molar masses of all products x100. Measure of how well atoms were utilised
Definition: Percentage yield
Actual yield/theoretical yield x100. Shows the percentage of how much product was made
Reasons why 100% yield is rarely achieved
- The reaction may not have gone to completion
- Side reactions may have taken place
- Purification of the product may result in some loss of product
How to improve atom economy
Find a use for other products or change the synthesis reaction so it produces fewer products
How to improve % yield
- Use one reactant in excess to ensure completion of reaction
- Use a catalyst that only catalyses the reaction you want
- Recycle the waste product
Problems with vast quantities of plastic used worldwide
Non-biodegradable so hard to dispose of because they don’t break down easily in the environment. Build up in waste disposal sites which means it can be dangerous for wildlife - at sea animals could become tangled or suffocate
How are biodegradable and photodegradable polymers made?
Biodegradable polymers are made of starch or cellulose or contain additives that alter the structure so that micro organisms break them down (they contain ester groups)
Photodegradable polymers are oil based and contain bonds that are weakened by absorbing light or light absorbing additives are used
Is combustion a good method of disposing polymers?
- It can be used to produce energy for other purposes like generating electricity
- Some produce toxic fumes but this can be rectified by installing scrubbers in chimneys that burn polymers
How are addition polymers treated before reusing?
They are sorted, chopped into flakes, washed, dried and melted to then be recasted
How can types of polymer be separated for effective processing?
They are given resin identification codes during manufacturing
