13. Alkenes
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What is the functional group of alkenes?
CnH2n
Is alkene saturated or unsaturated?
Unsaturated
Why is alkene unsaturated?
Because it contains a double bond/pi bond.
Is alkene reactive? Why?
Yes, because it contains the double bond which contains a pi bond. It is weaker than sigma bonds so that single bonds, so it is easier to break.
What are stereoisomers/geometric isomers?
Isomers that have the same structural formula but different arrangement in space.
How can E/Z isomerism occur?
• there is a C=C double bond • each carbon is bonded to a different atom/group
How does the C=C double bond cause E/Z isomerism to occur?
The pi bond in alkenes restricts the rotation of molecules.
How does E(entgegen)/Trans isomer look like?
Atoms/groups on the opposite sides.
How does Z(zusammen)/Cis isomer look like?
Atoms/groups on the same sides.
How is cis-trans isomer different from E/Z isomer?
In cis-trans isomers, one of the attached group on each carbon atom must be hydrogen.
What does CIP (Cahn, Ingold & Prelog) Rule do?
To assign stereoisomers when the groups are not identical
How to use the CIP (Cahn, Ingold & Prelog) Rule to identify E/Z isomers?
• use the atomic number of the atom that is directly bonded to the C=C bond • if the atomic number is equal, the next atom is taken into account • identify if it is E or Z isomer ☆ halogens are heavier in 99% of situations ☆
Alkene + halogen > ?
Dihaloalkane
Alkene + hydrogen halide > ?
Haloalkane
Alkene + water (with acid catalyst) > ?
Alcohol
Alkene + hydrogen > ?
Alkane
What happens during an electrophile addition reaction?
• an electron from the hydrogen is taken from the halogen, and the bond is broken by heterolytic fission • hydrogen becomes delta positive and halogen becomes delta negative • one of the bonds from the double bond of the alkene is moved to the hydrogen • hydrogen is joined to the alkene • a carbocation is formed, and the carbon from the alkene that lost a bond becomes positively charged • a negatively charged halogen is joined to the carbocation (alkene)
What is heterolytic fission?
The breaking of a covalent bond in which one bonding atom receives both electrons from the bonded pair, forming a positive ion and a negative ion.
What is a carbocation?
An ion that contains a positively charged carbon atom.
What is Markownikoff’s Rule?
Methyl carbocation is the least stable, then primary, then secondary, and tertiary carbocation is the most stable.
How does the number of alkyl affect the stability of carbocations?
The more alkyl groups attached to the positively-charged atom > the more the charge is spread out > ion is more stable
What is addition polymerisation?
Formation of a very long molecular chain by repeated addition reactions of many unsaturated alkene molecules.
Will the polymer be saturated or unsaturated after addition polymerisation?
Saturated
What are the benefits for sustainability of processing waste polymers by?
• combustion for energy production • use as an organic feedstock for the production of plastics and other organic chemicals • removal of toxic waste products eg. HCl