Chapter 13 - Alkenes Flashcards
What type of compound is limonene?
An alkene
What type of alkene is limonene?
Cyclic
What does limonene smell like?
Citrus fruits
What determines the colour of a flamingo?
The diet
What are alkanes?
Unsaturated hydrocarbons containing at least one C=C bond
General formula for alkenes
Cn H2n
What two bonds are found in a double bond?
Sigma and pi bonds
What is a sigma bond?
When two s orbitals overlap and create a region of high electron density between the two nuclei, forming a single covalent bond
How are sigma bonds formed?
When two s orbitals overlap
Why are sigma bonds so strong?
The high electron density between the two nuclei means that there is strong electrostatic attraction between the positively charged nuclei and the electron cloud
What is a pi bond?
A bond formed by the sideways overlap of two p orbitals. It has two parts to it - one above and one below the molecular axis
How is a pi bond formed?
Two p orbitals overlap sideways
Why are pi bonds much weaker than sigma bonds?
The electron density is spread out above and below the nuclei, so the electrostatic attraction is less
What do pi bonds do to a molecule?
They are rigid and hold the molecule in place around the two carbon atoms, meaning it can’t bend (unlike sigma bonds)
Why is the shape of the two atoms in a double bond trigonal planar 2
There are three regions of electrons
The three regions of electrons repel each other as much as possible, so the angle is 120
Where is maleic acid found?
Unripe fruit
Where is fumaric acid found?
Wild flowers
What are stereoisomers?
They have the same structural formulae but with a different arrangement of atoms
Where do E/Z isomers exist?
Molecules with a C=C bond
What two conditions are needed for there to be E/Z isomers?
A double carbon bond
Different groups attached to each carbon atom of the double bond
What is a Z isomer?
Both of the functional groups either below or above the double bond
What is an E isomer?
The same groups are positioned across the double bond
What conditions are needed for there to be cis-trans isomers?
A C=C bond
Each carbon in the double bond must be attached to 2 different group
One of the attached groups on carbon must be hydrogen
What is a cis isomer?
The groups are on the same side of the double bond
What is a trans isomer?
The groups are on opposite sides of the double bond
What does the Cahn-Ingold-Prelog rule state?
If the groups of high priority are on the same side of the C=C bond, it is a Z isomer
If the groups of high priority are on different sides of the C=C bond, it is an E isomer
How do we tell the priority of a group attached to a carbon?
Look at the Mr - the higher the Mr, the higher the priority
If two atoms attached to the carbon atoms have the same priority, what do you do then?
You find the group with the higher atomic number at the first point of difference
Why are alkanes much more reactive than alkanes?
The pi bond
Why does a pi bond make a compound more reactive?
The pi bond has a much lower bond enthalpy than the sigma bond and so breaks more easily
What happens during an addition reaction with alkenes?
A small molecule is added to the C=C bond, causing it to break and form a new molecule
What happens during an alkane hydrogenation reaction?
Hydrogen is added across the double bond. This forms an alkane
What does vegetable oil contain?
Molecules with long, unsaturated hydrocarbon chains
How is solid margarine formed?
Long, unsaturated chains of hydrocarbons are hydrogenated to form saturated chains. These have a lower melting point, so are more solid
What are trans fats?
The pi bond is broken and reforms in the trans orientation
What happens when alkanes are reacted with halogens?
The halogens are added across the double bond, forming dihaloalkanes
How do you test for unsaturated alkenes?
Add bromine water to the solution. If it goes colourless, an alkene is present. If it stays orange, an alkane is present
Why does bromine water go colourless in the presence of an alkene?
Bromine is added across the C=C bond, forming colourless dibromo-alkane
What happens when alkenes are reacted with hydrogen halides?
Haloalkanes are formed
What is important to remember about when alkanes react with hydrogen halides?
There are two possible products
Why will alkenes also react with concentrated hydrochloric or hydrobromic acid?
They are hydrogen halides dissolved in water
What is formed when alkenes react with steam?
An alcohol
When is the hydration of alkenes widely used in industry?
To make ethanol from ethene
What happens when steam reacts with an alkene?
Steam adds across the double bond to make an alcohol
What catalyst is used to make an alcohol from an alkene?
Phosphoric acid
What are carbocations classified by?
The number of alkyl groups attached to the positive carbon atom
What are the most stable carbocations?
Tertiary carbocations
Why are tertiary carbocations more stable than primary or secondary ones?
Each alkyl group pushes electrons towards the positive charge of the carbocation. The more alkyl groups that are present, the more spread out the charge will be, so the more stable the ion
How do you tell what the major product will be when a hydrogen halide is added to an alkene?
The most stable carbocation
What is electrophilic substitution?
1) The double bond in alkenes represents an area of high electron density
2) This attracts electrophiles
3) The electrophiles break open the double bond and are added across the carbon atoms
What are electrophiles?
Electron pair acceptors
Descibe the mechanism for the reaction between but-2-ene and hydrogen bromide
1) Because bromine is much more electronegative than hydrogen, hydrogen bromide contains the polar bond H-Br
2) When the hydrogen bromide gets near the C=C bond, the electrons in the pi bond are attracted to the positive hydrogen atom, causing the bond to break
3) A bond forms between the hydrogen of the H-Br bond and the carbon of the C=C bond
4) A carbocation and bromide ion are formed
5) The two products react together to form the final product
What is a carbocation?
A molecule containing a positively charged carbon atom
Why is the reaction between bromine and propene different to the reaction between hydrogen bromide and but-2-ene?
Hydrogen bromide is polar and bromine isn’t
Describe the mechanism for the reaction between bromine and propene
1) Br-Br is non-polar because the electronegativities are the same. When this comes into contact with the electrons in the pi bond, however, the bond becomes polarised, with one bromine being positively and the other negatively charged
2) The electrons in the pi bond are attracted to the positive bromine atom
3) This forms a bond between one of the carbons and one of the bromines
4) The Br-Br bond breaks by heterolytic fission, with the electrons going to the negatively charged bromine
5) A bromide ion and carbocation are formed
6) These react to form the final products
What is Markownikoff’s rule?
The major product formed when a hydrogen halide adds to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached
What is formed in the first step of electrophilic addition?
A carbocation
What is a primary carbocation?
A carbocation where the positive charge is on a carbon atom at the end of the chain
What is a secondary carbocation?
A carbocation where the positive charge is on a carbon atom in the middle of the chain
What are polymers?
Long chains of monomers
What is addition polymerisation?
The process by which the C=C bond in alkenes is broken and they link together to form a chain
How are polymers named?
Usually by taking the monomer and adding a prefix of ‘poly’
What is the repeat unit in a polymer?
The arrangement of atoms in a polymer molecule that repeats over and over again
How will the repeat unit always be written?
In square brackets
How would you find the monomer of a polymer?
Take the repeat unit and add a double bond
Where can you find polyethene? (3)
Shopping bags, shampoo bottles and toys
Characteristics of high-density polyethene
Linear chains
Strong
Found in water pipes
Characteristics of low-density polyethene
Branched chains
Little strength but flexible
Found in plastic bags
Name 3 uses of polypropene
Children’s toys, guttering and packing crates
Name two uses of polystyrene
Packing material and cups
Name two uses for poly(tetrafluoroethene)
Coating for non-stick pans
Cable insulation
Why are many polymers not biodegradable?
They are very unreactive
Why is recycling good for the environment?
It reduces the amount of waste going into landfill and conserves fossil fuels
How are polymers recycled?
They are first sorted into type. Then, they are chopped into flakes, washed, dried and melted. The product is cut into pellets and sold to manufacturers
Why is recycling PVC hazardous?
The high chlorine content
How is PVC recycled nowadays?
Solvents dissolve the polymer, then high-grade PVC is recovered by precipitation from the solvent
Why are some polymers used as fuels?
If they are derived from natural gas or petroleum, they have a high energy content. This means they can be burnt to produce heat or generate steam to power a turbine
What is feedstock recycling?
The thermal or chemical processes that reclaim monomers from discarded polymers
Why is feedstock recycling good?
The monomers can be used again to make new polymers and it is able to handle unsorted polymers
When will landfill be used?
When you can’t separate the plastics from each other, there is only a small amount of plastic or it’s too difficult to recycle it
Why is landfill bad?
We generate too much waste, we are running out of land
How are waste plastics reused?
They can either be melted and remoulded or cracked into monomers and used as organic feedstock
What are the three ways in which waste plastic can be gotten rid of?
Buried, reused, burned
Why is burning plastic good?
The heat can be used to generate electricity
What are photodegradable polymers?
They contain bonds that are weakened by exposure to light. This starts the degradation process
What are bioplastics?
They are plastics produced from proteins, starch, cellulose and plant oils
Why are bioplastics good?
Organisms can digest them to produce carbon dioxide and water
Why are compostable polymers beneficial?
They degrade and leave no residues behind them
What are the drawbacks to bioplastics?
You need to create the correct conditions for them to break down, e.g. water and oxygen
They’re more expensive than normal plastics
What is replacing polystyrene to make plates, cups and food trays?
Sugar cane fibres
Why would it be good to make bin liners from starch?
So that the bag and the waste food can be composted together
