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?
Unriped 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 C=C bonds
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
How is a haloalkane formed?
Reacting alkenes with gaseous hydrogen halides
What is formed when alkenes react with steam with a catalyst (phosphoric acid)?
And alcohol as the steam ads across the double bonds
When is the hydration of alkenes widely used in industry?
When producing ethanol from ethene
What is electrophilic addition?
An addition reaction in which the first step is attack by an electrophile on a region of high electron density
What happens during electrophilic addition?
- double bond in alkenes represents a region of high electron density due to pi-electrons
- Which attracts electrophiles
What is the an electrophile?
An atom or group of atoms that is attracted to an electron – which centre and accepts an electron pair.
Usually a positive ion/partially positive (Delta) charge
What is Markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkenes where the hydrogen attaches to the carbon atom which has a greater number of hydrogen atoms and smaller number of carbon atoms
What is a carbocation?
An ion that contains a positively charged carbon atom
What is formed in the first step of electrophilic addition?
Carbocation
What makes a carbocation stable?
The electron-donating ability of alkyl groups where each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The charge is spread equally over the alkyl group.
The more alkyl groups attached to the positively charged carbon atom, the more charge is spread out, making the ion more stable
What’s a polymer?
A large molecule formed from many thousands of repeated units of smaller molecules known as monomers
What is additional polymerisation?
Formation of a very long molecular chain, by repeated addition reactions of many unsaturated alkenes molecules (monomers)
What is the monomer?
A small molecule that combines with many of the monomers to form a polymer
What’s a repeated unit?
A specific arrangement of atoms that occur in the structure over and over again
Where is the repeated unit seen?
On the outside of brackets with the symbol n
How is industrial polymerisations carried out?
At very high temperatures and high pressures using catalysts
How is poly(ethene) made?
Heating a large number of ethene monomers at high pressure
What objects are made out of poly(ethene)?
Supermarket bags/shampoo bottles/children’s toys
What can poly(chloroethene) be prepared to make?
A polymer which is flexible/rigid
What are the usages for ply(chloroethene)?
Pipes/flooring/fabric treatment/installation/cable sheathing
What can a poly(propane) be used for?
Children’s toys/packing crates/guttering
What can poly(styrene) be used for?
Packaging material/food trays/cups
What is poly(tetrafluoroethene) be used for?
Non-stick pans/shoes/cable insulation
Why is polymers having a lack of reactivity good?
For storing food and chemicals safely
What is bad about polymers?
Most are nonbiodegradable hence killing marine life
How can polymers be recycled?
Dance polymers are sorted, they can be chopped into flakes and washed, dried and melted. These polymers are cut into pellets in use by manufacturers making new products
Why is your recycling PVC hazardous?
High chlorine contents therefore when burning PVC releases hydrogen chloride which is a corrosive gas
Why can use waste polymers as fuel?
They have a high stored energy calue which when incinerated produces heat, generating steam to drive a turbine producing electricity
What is feedstock recycling?
Chemical and thermal processes that can reclaim monomers, gases or oil from waste polymers
What can be done with the materials produced from foodstoke recycling?
Can be used as raw materials for production of new polymers
What is the advantage of food stock recycling?
Is able to handle unsorted and unwashed polymers
How are bioplastics produced?
From plants starch, Celulose, plan oils and proteins
What is good about bioplastics?
Protects the environment/conserve valuable oil reserves
How are biodegradable polymers broken down?
By microorganisms in water, carbon dioxide, and biological compounds
What up biodegradable polymers made out of?
Starch, cellulose
What is good about, possible polymers?
They degrade and leave no visible or toxic residues
