Alkenes Flashcards
Why does the bonding in alkenes cause stereoisomerism
pi bond due to overlap of orbitals above and below the plane of the molecule
leads to restricted rotation around the C=C bond
What is stereoisomerism
when two double bonded carbon atoms have two different groups attatched to them
- consequence of restricted rotation around the C=C bond
What is E/Z isomerism
Z isomers have groups on the same side of the double bond
E isomers have groups on opposite sides of the double bond
How do you describe isomers as E/Z when they are comprised of four different groups
look at the atomic numbers of the first atom attatched to the carbon in all branches
the higher atomic number receives priority
What is an addition reaction
adding two smaller molecules together to make one larger one
What is the reagent in a hydrogenation / reduction reaction
H2 (g)
What are the conditions for hydrogenation / reduction
150*C
nickel catalyst
What is the product of hydrogenation / reduction
an alkane
What is the reagent in halogenation
Br2 (l) or Cl2(g)
What are the conditions for halogenation
room temperature
What is the product of a halogenation reaction
a dihaloalkane
Why is a halogenation reaction with bromine useful
can be used to test for the presence of a double bond
What is the colour change for a reaction between alkenes and bromine water
orange -> colourless
What are the reagents in a reaction with hydrogen halides
HBr, HCl, HI
What are the conditions of a reaction with a hydrogen halide
room temperature
What is the product in a reaction with a halogen halide
a haloalkane
What is the reagent in a hydration reaction
H2o (g) - steam
What are the conditions in a hydration reaction
300* - 600* C
acid catalyst ( concentrated H3PO4 )
70 atm
What is the product in a hydration reaction
an alcohol
What is the reagent in an oxidation-addition reaction
KMnO4 / H+ (aq)
- oxidising agent - source of O atoms
What are the conditions for an oxidation-addition reaction
acid catalyst ( concentrated H3PO4 )
room temperature
How would you test for alkenes using potassium manganate (VII)
add KMnO4 to excess alkene
- the purple colour of the potassium manganate disappears
What mechanism occurs between alkenes and halogens / hydrogen halides
electrophilic addition
Why are electrophiles prone to attack alkenes
they are attracted to the electron density of the pi bond which lies above and below the plane of the molecule
What is an electrophile
a species which is attracted to negative region of charge
- something with positive or delta positive charge
Draw the electrophilic addition mechanism for the addition of HBr
produces a bromoalkane
Draw the electrophilic addition mechanism for the addition of Br2
produces a dibromoalkane
Draw the electrophilic addition mechanism for the addition of water
produces an alcohol and a H+
What is the use of Markovnikov’s rule
predicts the major and minor products in an addition reaction with unsymmetrical alkenes
What does Markovnikov’s rule state
When a hydrogen halide is added to an unsymmetrical alkene, the hydrogen adds to the carbon with the most hydrogens bonded to it already
Why does Markovnikov’s rule happen
due to the hydrogen adding to an end carbon, this means the carbocation is more stable as it is bonded to more carbons
- label primary, secondary etc
Why are tertiary carbocations the most stable
the 3 carbons around the carbocation are electron releasing
the sigma bonds joining each of these to the carbocation will be polarised and draw electron density towards the carbocation
- has a stabilising effect
Give three uses for waste polymers being separated into specific types of polymer
1) recycling
2) incineration to release energy
3) use as a feedstock for cracking
Give two ways how chemists limit the problems caused by polymer disposal
1) developing biodegradable polymers
2) removing toxic waste gases caused by incineration of plastics
