Organic reagents and the reactions
Homolytic fission-where each bonded atom takes one of the shared pair of electrons, to form two free radicals.
Heterolytic Fission-where one atom takes both shared electrons, forming a cation and a anion.
Nucleophile- Attacks areas of electron-deficiency (a.k.a positively charged) Donates electron pairs. Most nucleophiles have lone pairs.
Electrophile- attacks electron rich areas, and accepts electron pairs. They are often positive ions.
Addition reactions- Two reactants come together to make one product e.g. a molecule is added to an alkene (double bond) to make an alkane (single bond).
Subsitution reactions- When an atom of group of atoms are replaced by a different atom/group of atoms. e.g. bromoethane + OH- > ethanol and Br-
Elimination reactions-When one reactant becomes two products e.g. ethanol become ethene and water.
Crude oil contains over 150 hyrdocarbons, which must be separated out for different uses.
Fractional distillation separates crude oil by the hydrocarbons boiling points, and takes place in a fractioning column.
Short chained hydrocarbons with low boiling points will condense at the top.
Longer chain hydrocarbons with higher boiling points will condense nearer the bottom.
Those that don't condense are gases that are collected as 'petroleum gas'.
Once condensed they collect into baths and are tapped off.
Boiling points of Alkanes
More points of contact between molecules, and larger surface area, so more Van der Waal forces at work. These forces have to be overcome,with energy, so the longer the chain length, the higher the boiling point.
Isomers have the same molecular formula, but branched isomers have lower boiling points. There are less points of contracts and smaller surface area between branched molecules, and they can't get as close together. This means there are less van der waals forces to overcome, and a lower boiling point.
Hydrocarbons as fuels
Fuels and how they are used
- Short chain alkanes are useful as clean fuels; with plentiful oxygen they burn to make CO2 and H2O.
- Incomplete combustion leads to carbon monoxide production. This is a odorless, colourless gas that is poisonous to humans.
- Cracking is used to turn longer chain hydrocarbons into shorter ones, for fuels or polymer production. It also produces alkenes.
- Catalytic cracking uses a zeolite catalyst and temperatures of 450 degrees celcius.
- Isomerisation- turns unbranched chains into branched chains.
- Reforming- turning aliphatic hydrocarbons into alicyclic hydrocarbons and hydrogen gas.
- Fuels with high octane ratings close to 100 burn effieiciently.
- Branched and cyclic hydrocarbons promote efficient combustion.
- The hyrdogen produced from reforming is used fro other processes such as margarine production.
Fossil fuels and future fuels
Crude oil is not a sustainable fuel, and its sources are finite.
A key problem is its environmental effects. When burnt hydrocarbons produce:
- Carbon Monoxide- a potentially fatal poisonous gas
- Carbon dioxide- a greenhouse gas and therefore major contributor to global warming.
- Nitrogen oxides- contributor to acid rain and forest depletion.
- sulfur dioxide- contributor to acid rain
Global warming would mean more dangerous weather, and the melting of ice caps. This will lead to the rise of sea levels and destruction of environments.
Sugar cane and **** seed oil are used more and more for energy. Fermenting sugar creates ethanol, which can be blended with petrol to make it burn more efficiently.
Biodiesel is produced from **** seed oil.
Both have a low carbon footprint.
Halogenation of alkanes
- Alkanes react with Halogens in the presence of UV light or termperatures of 300 degrees.
- This is called radical subsitution.
- The halogen is split by homolytic fission, into two radicals with an unpaired electron. A hydrogen atom in the alkane is subsituted by a halogen atom.
Chlorination- THREE STEPS
Initiation- This is when the chlorine is split from its diatomic state into two radicals, under UV light. Cl2 >> 2Cl
Propagation- This has two steps
- Alkane reacts with a chlorine radical, forming HCL (e.g. a hydrogen is pulled away from alkane by the radical, leaving an alkane radical.)
- The second step is when the alkane radical reacts with another chlorine diatomic molecule, pulling away one atom of chlorine to from a chloroalkane and a chlorine radical.
Termination- three ways
Cl radical plus alkane radical = chloroalkane, 2Cl radicals = Cl2, two alkane radicals= alkane
Alkenes: double bond, shape and cyclic
Made up from two parts: the sigma bond, and the pi bond.
- The sigma bond is formed directly between two carbon atoms by the overlap of orbitals.
- The pi bond is formed above and below the plane of the carbon atoms by sideways overlap of p-orbitals. Each atom contributes one electron from a p orbital to the electron pair on the pi bond.
The pi bond fixes the carbon atoms in position, stopping the rotation of either atom, and allowing the existence of E/Z isomerisation.
SHAPE OF ALKENE
Three regions of electron density surround each carbon atom. The pairs repel eachother to the furthest distance to minimise repulsion. Therefore, a trigonal planar shape forms around each carbon atom. This creates an overall flat planar molecule, with all atoms on the same plane.
- Closed rings of carbon atoms with one or more double bonds.
- They do not follow the same general formula as aliphatic alkenes.
Reactions of Alkenes
Alkenes are more reactive than alkanes, because the pi bond is broken more easily than the plain sigma bond. So the pi bond breaks, allowing a reaction, but the sigma bond remains intact.
These cause the double carbon bond to break, adding new atoms and creating a staurated molecule.
- catalyst= nickel.
- temperature= 150 degrees.
- Hydrogen gas is added agross the bond and breaks it.
- Halogen is added across the bond forming a di-subsituted halogenalkane.
- When bromine is added to an alkane, it turns colourless, showing a reaction.
ALL BY ELECTROPHILIC ADDITION
Further reactions of Alkenes
Addition of hydrogen halides
- e.g. HBr, HCl and HI
- They are bubbled as gases through liquid alkenes.
- Creates halogenoalkanes
Addition of steam/hydration
- catalyst= phosphoric acid
- high temperatures
- creates an alcohol
Addtition to unsymmetrical alkenes
- can form two products
- e.g. propene + HBr > 1-bromopropane OR 2-bromopropane
ALL BY ELECTROPHILIC ADDITION
Addition of a Hydrogen halide e.g. HBr
- HBr is polar, so Hydrogen in positively charged and is attracted to the carbon-carbon double bond.
- The double bond breaks, and the hydrogen attaches to the new-alkane, forming a carbocation. The HBr breaks by heterolytic fission.
- The negatively charged bromine then attaches to the carbocation.
- Bromoalkane is formed.
The same process is used in hydration, as one one of the hydrogens in the H2O is pulled away.
Addition of a halogen
- Diatomic halogen is non-polar, but the carbon-carbon double bond induces a dipole in it, repelling electrons in the nearest halogen atom, making it positive.
- This halogen atom breaks the double bond, and attached forming a carbocation.
- The other halogen atom attaches to the carbocation.
Industrial importance of Alkenes
These are made from repeating units called monomers, which are unsaturated ta first. They are all added together in a process called addition polymerisation to create a long saturated molecule.
Polymers and industry
Radical polymerisation- requires high pressures and 200 degrees celcius. Makes branched polymers and mixtures of polymers.
Ziegler-Natta Process- uses titanium based catalysts and temperature of 60 degree celcius, which the alkane is passed over. Any which isn't reacted is passed over again.
Other uses for alkenes makes important chemicals used for degreasers, antifreeze and even vinegar.
Vegetable oil needed to be hardened, so undergoes hydrogenation. However some cis and trans fats can form , and these are meant to be bad for health.
Polymer- dealing with waste
They pose a problem because they are not biodegradable.
Ways to use it:
Fuels- burning polymers under controlled ocnditions can produce heat energy which can be used to make electrical energy.
Feedstock recycling- converts polymers into a synthesis containg hydrogen and carbon monoxide, which is then converted into useful products.
Recycling PVC- its high chlorine content make it problematic. It has to be dissolved to be removed from other plastics.
Biodegradable plastics- made from sustainable resources, and can be broken down by bacteria to make CO2 and H2O.