Alkenes Flashcards
Describe how single bonds are formed
Sigma bonds are formed from end on overlap of atomic orbitals.
Thye are free to rotate.
Memorise diagram as well
Describe how double bonds are formed
Pi bonds- the 2nd bond forms is from side to side overlap of p-orbitals.
They are not free to rotate as it would break the overlap.
Memorise diagram
Where do the bonding electrons exist in the pi bond
Above and below the atom in 2 halves.
In a carbon bond is a sigma or pi bond stronger
Sigma bond
Define stereoisomers
They have the same structural formula but a different arrangement of atoms in space. They exist because C=C double bond cannot rotate.`
How can E/Z isomerism arise
- When there is restricted rotation- around a double bond- pi bond
- When each atom of the double bond has two different groups attached- non-symmetrical
- Non-rotation of the double bond leads to groups on either side of the carbon being fixed in poisition
Why do methyl-propene and but-1-ene not show E/Z isomerism
Because they have the same groups coming off them - symmetriacl so swapping then over gives the same molecule.
How do you decide whether a stereoisomer is E or Z
- For each carbon of C=C assign groups as high or low priority- CIP rules
- Higher atomic number=high priority
- Look at bonded atoms first
- If the same look at next atom in prority order
- If high priority is on the same side it is a Z isomer
- If on opposite sides it is an E isomer
Draw E/Z isomers of CH3CBr=CF(OH)
pack page 14 has answers
1-Fluoro-2bromoporp-1-en-1-ol is E
What is cis/trans isomerism
A special case of E/Z isomerism in which 1 of the two groups attained to each carbon of the double bond is the same.
If the same group on the same side of double bond-cis
otherwise-trans
Reaction of ethene with H2
Forms ethane
150 degrees and Ni catalyst
Reaction of ethene with Cl2 or any other halogen
Forms 1,2-dichloroethane
RT and pressure
Reaction of ethene with HBr or any other halogen
Forms 1-bromoethane
RT and pressure
Reaction of ethene with H2O
Forms ethanol
Steam and acid catalyst (H3PO4)
Describe what happens when you add bromine water and shake it with an equal volume of cyclohexane
On initial addition, the clear, colourless cyclohexane would form a separate layer on top of the orange bromine water.
On shaking, the two layers would remain, but the majority of the colour would be transferred into the upper (organic/cyclohexane) layer because bromine is more soluble in organic solvents than water.
Describe what happens when you add bromine water and shake it with an equal volume of cyclohexene
On initial addition, the clear, colourless cyclohexene would form a separate layer on top of the orange bromine water.
On shaking, the two layers would remain, but the orange colour would RAPIDLY disappear (decolourisation), leaving two clear, colourless layers.
How can bromine water be used to test for C=C
When added to an alkene the orange disappears - bromine undergoes an addition reaction and the double bond is broken
When added to an alkane there is no colour change
Define electrophile
An atom which is attracted to an electron rich centre or atom where it accepts a pair of electrons to form a new covalent bond
Defie addition reaction
A reaction in which a reactant molecule is added to an unsaturated molecule to make one saturated molecule.
What does a curly arrow represent
It represents the movement of a pair of electrons
Describes what happens in Electrophilic addition
The HCl (or other reactant) molecule receives a pair of electrons from C=C- acting as an electrophile. The HCl bond breaks with both electrons going to Cl - Heterolytic fission
Draw mechanism between ethene and HBr
answer p25 of study pack
Ensure curly arrows start and finish in the correct places.
You have put the dipoles in- negative dipole goes to most electronegative atom.
You have shown the charges in the middle step.
And the pair of electrons.
Define heterolytic fission
When a bond breaks and all the electrons go to the same atom forming a cation and anion.
What is the intermediate in electrophilic addition called
A carbocation
Describe Markownikoff’s rule
- The delta positive atom will tend to attach to C with fewer C-atoms attached- the edge usually
- Other end the delta negative will tend to attache to C with more C atoms attached.
Major product is the product that follows this rule, minor is when it doesn’t
Why does Markownikoff’s rule happen
Because the carbocation in the formation of the major product is more stable because it is attached to more C atoms.
What is a primary, secondary and tertiary carbocation
Primary means the Carbon is attached to only 1 carbon- least stable
Secondary means it is attached to 2 carbons
Tertiary means it is attached to 3 carbons- most stable
Write equation for percentage yield
Percentage yield= Actual yield/ theoretical yield *100
Write equation for atom economy
Atom economy= Total (molar) mass of useful products/ total (molar) mass of all products *100
Why is 100% yield rarely achieved in organic reactions
- Reaction may not go to completion
- It may be difficult to isolate the desired product from the reaction mixture
- Side products may be formed
How can atom economy be improved
- Find a use for any other products
2. Change the synthesis reaction so that it produces fewer other products
How can percentage yield be improved
Change the conditions so the reaction is more efficient
What is an addition polymer
A very large molecular chain formed b the addition reactions of many unsaturated alkene molecules.
Write equations to show the polymerisation of Hex-2-ene, and cyclohexene
- ensure there are dangly bonds sticking out
2. Brackets around the product with n on the outside
Why are polymers difficult to get rid of
They are non-biodegradeable
Give advantage and disadvantage of recycling polymers
Adv- reduces environmental impact by conserving finite fossil fuels and decreasing amount of waste going to landfill.
Disadv- They have to be sorted-very time consuming
Poblems with the recycling of PVC
It is hazardous due to the high chlorine content present in the polymer.
Dumping PVC in landfill is not sustainable.
When burnt PVC releases hydrogen chloride a corrosive gas and other pollutants.
So it is dissolved using solvents and is recovered by precipitation of the solvent.
Give advantage and disadvantage of using waste polymers as fuel
Adv- can be used to generate electricity when incinerated.
Give advantage and disadvantage of feedstock recycling
Adv- can handle unsorted and unwashed polymers
produces materials that can be used as raw materials form the productions of new polymers
What are biodegradable polymers and their advantages
Are broken down by microorganisms into water, CO2 and biological compounds.
Are made from starch or cellulose or contain additives.
Bioplastics are likely to be more extensively used in packaging, electronics and more fuel-efficient and recyclable vehicles.
What are photodegradable polymers and their advantages
They are oil-based polymers which contain bonds that are weakened by absorbing light to start the degradation.
