9 - Alkenes Flashcards
What is an alkene?
An unsaturated hydrocarbon
it has a double bond
What is general formula of an alkene?
C~n H~2n
What is the functional group of alkenes?
C=C
What type of reactions do alkenes take part in?
Addition reactions
How can you test to see if a substance is an alkene?
You react the substance in bromine water
If it is an alkene, the bromine water will turn from brown to colourless
What is the general formula of cycloalkenes?
C~n H~(2n-2)
What is the general formula of arenes?
C~n H~n
What is an example of an arene?
Benzene
How do you need to draw alkenes as displayed formulas?
All bond angles have to be at their maximum
No matter what it ends up looking like
A double bond has the same repulsion as a single bond
What does the number in the middle of an alkene mean?
The carbon which the double bond is on
E.g. but-2-ene:
Double bond is between carbon 2 and carbon 3
What does diene mean?
When there are 2 double bonds
There will be 2 numbers before
What is a double bond made from?
The first C-C bond is a sigma-bond
The second part is a pi-bond
What are valence electrons?
Outer electrons used in bonding
In what 3 ways are sigma-bonds formed?
When 2 S-orbitals overlap
When an S and a P overlap
When 2 P-orbitals overlap
Generally, when do sigma-bonds form?
When there is a direct overlap of atomic orbitals
How is the pi-bond of a double bond formed?
The P-orbitals around the carbons which have a sigma-bond overlap
This overlap is the pi-bond
in short: The pi-bond is caused by sideways overlap of P-orbitals
What effect does the pi-bond have on alkenes?
The pi-bond prevents rotation of atoms around the double bonded carbons
What are stereoisomers?
Molecules with the same structural formula but different arrangements of atoms in space
What are the 2 types of stereoisomers?
E-isomers (trans)
Z-isomers (cis)
What are E-isomers and Z-isomers?
E-isomers:
When the 2 largest groups which are bonded to a central carbon are on the opposite sides of the molecule
Z-isomers:
When the largest groups are on the same side
How do you show if an alkene etc is an E or Z isomer in the name?
Just put E- or Z- before the name
Like E-but-2-ene
What 2 things have to be true for a substance to have stereoisomers?
Double bond (so that there is no rotation)
Each carbon in the double bond is bonded to 2 different atoms or groups
What are Cis and Trans isomers?
A Cis isomer is the Z isomer with a hydrogen on each double bonded carbon
A Trans isomer is the E isomer with a hydrogen in each double bonded carbon
What do you do when classifying stereoisomers in very complex organics?
use the C-I-P rules:
If the 2 highest priority are on the same side, it’s Z and the opposite for E
The priority of an atom is higher if it’s atomic mass is greater
You have to compare the different parts of the structural formulas until one has a higher priority part than another
How reactive are alkenes compared to alkanes and why?
alkenes are more reactive
The pi-bond can break and more sigma-bonds can form
How do pi and sigma bonds compare in terms of strength?
Sigma-bonds are stronger (higher bond enthalpy) than pi-bonds
How do you test for alkenes and cycloalkenes?
Add bromine water
If it is an alkene or cycloalkene
The bromine water will turn from orange to colourless
What is the reaction for bromine water added to alkenes?
Example:
Ethene + Bromine –> 1,2-dibromoethane
What type of reaction is the one between ethene and bromine and why?
An addition reaction
Because the bromine is added across the double bond with one bromine bonding to each carbon
Which bond breaks in the reaction of ethene and chlorine?
The pi-bond between the double bonded carbon
What is the reaction between cyclohexene and fluorine?
They make 1,2-difluorocyclohexane
Define electrophile
Electron pair acceptor
What are electrophiles attracted to and why?
The double bond
There is a high electron density there
What is the full name of reactions between alkenes and halogens etc?
Electrophilic addition reactions
What are the electrophiles in the electrophilic addition reactions?
The halogen or hydrogen which adds across the double bond
What is the name of the reaction where hydrogen is added?
Hydrogenation
What are the conditions for hydrogenation?
Nickel catalyst at 423K
How is margarine made?
Hydrogenation of plant oils
What is the addition of halogens called?
Halogenation
What is the charge of electrophiles and why?
Positive
Because they are attracted to electron pairs
What is Markownikoff’s rule?
The isomer with the tertiary carbocation is favoured over the isomer with the secondary carbocation
Secondary > primary as well
How can you tell which carbocation the isomer had?
You look at the branches off the main chain
How do you denote a carbocation?
The number then a circle as a superscript
Secondary carbocation is 2^o
Explain why hexane is saturated
There is only C-C bonds
No C double bond C
(You have to say C-C instead of single bonds)
Define polymer
A long molecular chain built up from monomer units
Define monomer
A small molecule that combines with many other monomers to form a polymer
What are the 2 requirements for a monomer?
Based on alkenes
Unsaturated
What is the name of the reaction used to make polymers?
Addition polymerisation
How does addition polymerisation work?
Double bonds are opened up
A long chain of carbons forms
How do you draw the addition polymerisation of poly(propene)?
Do the whole thing but only draw the double bond part as displayed and draw the CH~3 coming off a bond
Do the same for the polymer side of the equation
What is the repeat unit?
The thing before the n on the polymer side
What is the advantage and disadvantage of the lack of reactivity of polymers?
They are suitable for storing food
They often take a very long time to biodegrade
What are the 2 advantages of recycling polymers?
It reduces the amount of polymers going to landfill
There is only a finite amount of polymers that can be made and recycling reuses the same polymers over and over again
Describe the polymer recycling process
The polymers are first sorted by type
Mixed polymers can’t be recycled
The polymers are then chopped into flakes
They are then washed and dried
Then they are melted and reformed into small pellets
What are the 2 disadvantages of recycling PVC?
Hazardous due to:
High chlorine content
Range of additives present in the polymers
What are the 2 disadvantages of putting PVC into landfill?
When it is burnt:
The corrosive gas hydrogen chloride is released
Along with other pollutants like toxic dioxins
Describe the old and new way to recycle PVC
old and new way to recycle PVC
We used to grind it and reuse it
We now use solvents to dissolve the PVC and high grade PVC is recovered by precipitation from the solvent
The solvent can now be reused
What do we do with some polymers instead of recycling and why?
We use them as fuels by burning them to generate heat
Because they have a high stored energy value
What is feedstock recycling?
The chemical and thermal processes which can reclaim monomers, gases or oil from waste polymers
What is a big advantage of feedstock recycling?
It can handle unsorted or unwashed polymers
What are bioplastics?
Plastics produced from starch, cellulose, plant oils and proteins
What are the 2 advantages of bioplastics?
Protect our environment
Help conserve our finite oil reserves
What are biodegradable polymers made from normally?
Starch or cellulose
Or a normal polymer contains additives so it can be broken down
How are biodegradable polymers broken down?
Broken down by microorganisms into water, carbon dioxide and biological compounds
What are compostable polymers and what are they usually based on?
Degrade and leave no visible or toxic residues
Based on poly(lactic acid)
How can supermarket bags be changed so that they are compostable?
Made from plant starch
Used as bin liners so that the food and the bag can be composted together
What are compostable plates, cups etc made from?
Sugar cane fibre to replace polystyrene
Sugar cane fibre to replace polystyrene
Oil based polymers which contains bonds which are weakened by light to start the degradation
Or light absorbing additives can be used
What are the reagents and conditions for electrophilic addition of alkenes?
Steam
Acid catalyst
