Alkene Flashcards
What are alkenes
Saturated C=C double bonds
What bond do alkenes have
Pi and sigma bonds
What’s a pi bond
Is a sideway overlap of p orbitals one from each carbon atom of the double bond. The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms
Can atoms of alkenes move freely
No the pi bonds lock the carbon atoms into position and prevent them rotating around the double bonds
What is the shape of alkenes
Trigonal planar at 120 degrees
Three regions of electron density around each of the carbon atoms
The three regions repel each other as far as part as possible
What are stereoisomers
Have same structural formula but different arrangements in space
What is E/Z isomerism
Stereoisomerism around double bonds arises because rotation around a double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
What conditions are required for E/Z isomers
C=C double bond present
And different groups attached to each carbon atom of the double bond
How to identify cis isomer
Has hydrogen atoms and methyl group on the same side of the molecule
How to identify trans isomer
The hydrogen and methyl groups are diagonally opposite each other
What is cis isomer known as
Z isomer
What is trans isomer also known as
E isomer
Why is chan in gold prelog used
To classify stereoisomers which are not cis or trans
How to use chan in gold prelog rules
If the groups of higher priority are on same side of the double it is a isomer
If the group of higher priority is placed diagonally it is E isomer
Are alkenes reactive
More reactive than alkanes because of presence of the pi bond because being on the outside of the double bonds the pi electron are more exposed than electrons in sigma bond.A pi bond readily breaks and alkenes undergo addition reaction more quickly
What reaction ms do alkenes undergo
Hydration
Halogenation of alkenes
Hydrogenation
Hydrogen halides
What happens in the hydrogenation of alkenes
When an alkene is mixed with hydrogen and passed over nickel catalyst at 423K an addition reaction takes place and produces an alkane
What happens in the halogenation of alkenes
Undergo a rapid addition reaction with the halogens chlorine or bromine at room temp
How to test for in saturation
By adding bromine solution to sample of saturated hydrocarbon decolourises
What happens in the addition reaction of alkenes with hydrogen halides
Alkene react with gaseous hydrogen halide at room temperature to form haloalkanes
If the alkene is gas than th reaction takes place when two gasses are mixed
If the alkene is a liquid than hydrogen halid is bubbled through
Alkenes also react with concentrated hydrochloric acid or concentrated hydrbromic acid which are the solutions of hydrogen halides in water
What happens in the hydration reaction of alkenes
Alcohol is formed when alkene. React with steam and phosphoric acid catalyst H3PO4.Steam adds across the double bond
What mechanism is electrophilic addition
Alkenes usually take part in addition reaction to form saturated compounds
What happens in electrophilic addition
Double bond in alkene represents a region of high electron density because of presence of pi electrons
High electron density of the pi electrons attracts elctrophiles
What’s an electrophile
Electron pair acceptor
What type of fission is electophilkic addition
Heterolytic fission
What’s a carbocation
Contains positively charged carbon atom
Additional polymerisation
Have high molecular masses
What conditions does polymerisation occur
High temp and pressure
How can polymers be disposed
Burn them and use the energy released by combustion used to generate electricity
And organic feedstock to synthesise other products
makes plastic
