Alkenes Flashcards
Explain the trigonal planar shale and bond angle around each carbon in the C=C in terms of electron pair repulsion
When two electron pairs are shared in a pi bond, they cannot be concentrated directly between the two nuclei and so one forms a sigma bond and the other a pi bond. Due to the electron repulsion by the lone pairs in the pi bond, alkanes are rectangular shaped, flat molecule with a trigonal planar shape. This has a 121.7 degree angle between the carbon and hydrogen and a 116.6 degree angle between the hydrogens.
Explain the benefits of biodegradable and photodegradable polymers
Chemists have selections biodegradable polymers to solve the disposable problem
Photodegradable polymers are broken down by the action of UV light which causes bond breakage due to rapid bond vibration
They reduce the dependency of finite resources and alleviate disposal problems
Explain the formation of pi bonds
Spare 2p orbitals overlap laterally to form two regions of electron density above and below the molecular plane. Because the electron density is not directly between the two nuclei, the pi bond is more polarisable as it is more easily distorted
Explain all addition reactions of alkanes
1) Hydrogenation:
- Transition metals catalyst (Nickel) at 150 degrees
- Forms alkanes
2) Halogenation
- electrophilic addition
- halogen / halide added
3) Hydration
- conc. H3PO4 catalyst at 300 degrees and 65atm
- addition of water
Define a repeat unit
Smallest section of the chain which generates the whole structure
Define an electrophile
A species which can accept an electron pair
What does it mean for alkanes to be unsaturated hydrocarbons
Unsaturated means that it has a C=C double bond which allows them to undergo addition reactions
Explain the difference, in terms of electrons shared in a covalent bond, between sigma bonds and pi bonds
The pair of electrons in a sigma bond is a region in space (described as a lone) between the nuclei of the two atoms involved.
The pi bond has two regions of electron density above and below the molecular plane.
Explain why a pi bond is more polarisable than a sigma bond
Because the greater electron density (of the delocalised pi electrons) between the two barons are not directly between the two nuclei, the pi bond is more easily distorted and so it is more polarisable
Define addition polymerisation
The building up of long chain molecules from small molecules called monomers
Explain 3 methods of processing waste polymers
1) combustion for energy production
2) organic feedstock / recycling
- some plastics can be reshaped, melted down or cracked into other compounds
3) Removal of toxic waste
- removal of HCL formed during combustion of halogenated plastics
Explain the difference E/Z isomerism and cis-trans isomerism?
E/Z is most common in alkanes as the C=C bond causes a restriction in rotation and requires 2 different substituents to be bonded to a different carbon atom that is part of the pi bond
Cis-trans isomerism requires 2 or substituents where at least 2 are the same.
Explain why alkanes are relatively more reactive than alkanes
1) Do not have the most amount of hydrogens and so can be targeted by electrophiles in addition reactions
2) Have a high electron density region of delocalised pi electrons and so can easily polarise electrophiles
3) Lower bond enthalpy
Explain the CIP priority rules for E/Z isomerism
Applied when the substituents attaches to a carbon atom (that is part of the C=C bond) are all different.
Here “the highest priority group is determined by the highest atomic number” and so is used to identify whether it is the E or the Z isomer
Define stereoisomerism
Compounds with the same structural formula but different arrangement in space
