13.1 -13.5 Flashcards
What are alkenes
- unsaturated hydrocarbon
What are unsaturated compounds
- compounds that have at least 1 double covalent bonds
What is the general formula for alkenes
- CnH2n
What type of reactions do alkenes undergo
- Electrophilic addition reactions
What makes alkenes fairly reactive
- the fact that they have a high electron density due to the double bond
What are the two bonds that alkenes have
1) sigma bonds
2) pi bonds
What type of bonds do all covalent molecule have
- sigma bonds
Complete the sentence:
Sigma bonds have a high…
- bond enthalpy
What are sigmas bonds
- a horizontal overlap of 2s orbitals
What are pi bonds
- the parallel overlap of 2p orbitals
Why are pi bonds weaker than sigma bonds
- as the electron density in pi bonds is spread out above and below the nuclei so there is a weaker pi bond
Why do pi bonds have a lower bond enthalpy
- because the electrostatic attraction is weaker
Are alkanes or alkenes more reactive and why
- alkenes because they have 4 electrons being shared in the double bond and as the pi bond has a low bond enthalpy
Why are alkenes open to attack from electrophiles
- as it has a high electron density
Define the term stereoisomers
- when they have the same structural formula but a different arrangement of atoms in space
What is the shape and bond angle of the C=CH2
- Trigonal planer (120)
Fill in the blanks:
Atoms can’t _______ around the C=C double bond and it is ____
1) rotate
2) rigid
When do we get E/Z isomerism
- where you have 2 different atoms or groups of atoms on the same carbon
What do you do when deciding if an isomer is E or Z when we have 4 different groups around the double bond
- use CIP rules
1) number carbons 1 & 2
2) find which atom has priority on each side by working out the atomic number
3) they look at where the two high priorities are and distinguish whether it is E or Z isomerism
What are electrophiles
- an electron pair acceptor which forms covalent bonds
What is the charge of electrophiles
- positive
What are examples of electrophiles
- NO2 +
- H+
- H-Br
- H2SO4
Where does the curly arrow start and go towards in electrophilic addition reactions
- from the double bond to the electrophile
How do alkenes become alkanes
- reagent: H2
- catalyst: Ni
How do we test for alkenes
- the discolouration of bromine water
(browny-orange to colourless)
What is the electrophiles in the test from alkenes
- bromine
What is formed when testing for alkenes
- dibromoalkane
How do alkenes from alcohols
- hydration of alkenes
Reagent: steam
Catalyst: phosphoric acid (H3PO4)
How do alkenes form halogenoalkanes
- alkene + hydrogen haldies
- in an electrophilic reaction
What does the stability of carbocation intermediate determine
- the amount of the 2 products
What determines the stability of the intermediate and why
- the amount of alkyl groups bonded to it
- because the alkyl push electrons towards the positive carbocation stabilising it.
Complete the sentence:
The more stable the carbocation the…
- more likely it will form
Complete the sentence:
Minor products are…
- less stable and formed less often
Complete the sentence:
Major products are…
- more stable and formed more often
What is Markownikoff’s rule
- that the major product will be formed when the hydrogen adds to the carbon with the most number of hydrogens already attached
What can we use alkenes to make
- polymers
- petrochemicals
Fill in the blank:
Alkenes are _____ which join to form addition polymers
- monomers
What are the two types of polymers
1) Natural- proteins
2) Synthetic- poly(ethene) etc
Why do we dispose of plastics
- as they are polymers and most of them are not biodegradable and so need to be disposed carefully
What are the 3 ways we dispose of non-biodegradable plastics
1) landfill
2) recycling
3) incineration(burning)
What type of plastics are landfill useful for disposing
- plastics too difficult to recycle
- plastics too difficult to separate from other materials
- not enough plastics to extract to make it economically viable
What is the problem with disposing plastics in landfill
- isn’t sustainable as large amounts of land is needed
- becoming increasingly expensive to use land for waste
Why do we recycle plastics
- as plastics are made from crude oil which is non-renewable so recycling means we are reducing our dependency on crude oil for making plastics
What are the two things we can do with recycled plastics
1) some plastics can be RE-MOULDED into new objects
2) other plastics are cracked into monomers to be used to make new plastics
When do we use incineration
- when the plastic can’t be recycled
What do we use the energy from burning plastics to do
- to generate electricity
What is the problem with burning plastics
- they can release toxic fumes
- especially PVC which produces HCl
How do we overcome the problem of toxic gases being released in incineration
- use fuel gas scrubbers
- which neutralise the acidic gases by firing bases
How do biodegradable polymers decompose
- naturally under certain conditions by organisms
How are biodegradable polymers made
- from oil fractions and renewable sources
What is the problem with biodegradable polymers compared to non-biodegradable polymers
- they are more expensive
What are the certain conditions that biodegradable plastics need to decompose
- good oxygen supply
- moisture
What are biodegradable plastics used for
- protective sheets for plants
