C14. Alkenes Flashcards
What is stereoisomerism?
Same structural formula but different arrangement of atoms in space
What are the general structure of the alkene homologous group?
Hydrocarbons, unsaturated, contains C=C
What is the shape of an alkene?
Planar, 120 degree bonds
What are pi-bonds?
reactive part of a double bond formed above and below the plane of bonded atoms by sideways overlap of p-orbitals
How do alkenes use their electrons in the double bond?
3 electrons in formation of sigma-bond, 1 electron for formation of pi-bond
What are the bond enthalpy properties of double and single bonds?
Double bond (+612) is stronger than single bond (+347); Pi-bonds are weaker than sigma-bonds
What happens to the bonds of alkenes when they react?
Pi-bonds break, sigma-bonds stay intact
What are the difference between E/Z isomers?
Cis Isomer - Z; High priority groups are both on the bottom side or top side; Trans isomer - E; High priority groups are positioned diagonal across the double bond
What is the definition of E/Z isomers?
Type of stereoisomerism , different groups attach to each carbon of a C=C bond, may be arranged differently in space due to restricted rotation of C=C bond
What are the rules of the Cahn-Ingold-Prelog nomenclature system?
On each C=C, group with highest atomic number given priority; group with lowest atomic number is given lowest priority;
What are the criterias for E/Z isomers?
C=C double bond must be present; Each carbon must be attached to 2 different groups
What is a structural isomer?
Molecules have same molecular formula but different structural formula
What are the formulas for alkenes?
CnH2n
Why can a double bond not rotate? (restricted rotation)
2 p orbitals that overlap, forming a cloud of electron density above and below C=C bond, (pi-bond),
What are position isomers?
Isomers with the functional group in different positions
What are geometrical isomers?
Form of stereoisomerism due to restricted rotation, Trans, Cis
What are the physical properties of alkenes?
Double bond does not greatly affect boiling of melting points; Physical properties are very similar to alkanes; Not soluble in water
How reactive are alkenes and why?
More reactive than alkanes; C=C forms electron rich areas above and below the double bond, these regions are easily attacked by electrophiles; most alkene reactions are electrophilic addition
What are the mechanisms for electrophilic addition?
Electrophile attracted to double bond; Positively charged electrophiles accept pair of electrons from =; Carbocation formed; negatively charged ion forms bond with carbocation
What is a carbocation?
Positively charged carbon ion (ion that would be attracted to cathode)
What is the positive inductive effect?
Alkyl groups have a tendency to release electrons ( represented by arrow along bonds showing direction)
What is a tertiary carbocation?
Carbocation that draws electron density from 3 alkyl groups; Most stable
What is a secondary carbocation?
Carbocation that draws electron density from 2 alkyl groups; More stable than primary
What is a primary carbocation?
Carbocation that draws electron density form 1 alkyl group; Least stable carbocation
How do stability of carbocations affect produce?
More stable carbocations will create more products
What are electrophiles?
Electron pair acceptor; Atom that is attracted to a region of high electron density
What are nucleophiles?
Electron rich species; Electron pair donors
How do you test for the presence of carbon double bonds?
Addition reaction; bromine solution/bromine water is decolourised in presence of C=C double bonds
What are the 2 steps for addition reactions of an alkene?
Formation of cation by electrophilic addition; Rapid reaction with negative ion (anion)
What are polymers?
Compound comprising very large molecules that are multiples of simpler chemical units called monomers
What is addition polymerisation?
Reaction in which many unsaturated alkene molecules bond together forming a very long saturated polymer chain
What is a repeat unit?
Specific arrangement of atoms that occur repeatedly in a structure
What are the properties of addition polymers?
Unreactive with substance placed inside; Durable; Do not break down naturally
What is a biodegradable material?
Material that is broken down naturally in the environment by living organisms
