Topic 6 - OC Alkanes Flashcards
General Formula of Alkanes
CnH2n+2
Saturated
Alkanes and cycloalkanes are saturated because they do not contain a double bond
Where are alkane fuels obtained from?
Fractional Distillation, Cracking and Reforming
What is Fractional distillation of Alkanes
Crude oil is a complex mixture of hydrocarbons which can be seperated into different fractions. Crude oil is first heated in a furnace to convert it into vapour which is passed into a distillation columb via the bottom. There is a temperature gradient along the column and therefore as the vapor passes through different fractions of molecules will condense at different heights depending on their melting points
–> The bottom of the column is hotter and therefore larger molecules with longer chains and higher melting points will condense
–> The top of the column is cooler and therefore smaller molecules with shorter chains and lower melting points will condense
What is Cracking of Alkanes
There is a surplus of longer chain hydrocarbons because the world has fewer uses for them. However, demand for shorter chain hydrocarbons is high because they are better fuels and can be used to make other substances such as polymers. There are not enough to satisfy demand and therefore we convert longer chain hydrocarbons into shorter chains via the process of cracking
–> Cracking is completed by passing hydrocarbons throuh a heated catalyst (usually zeolite) which causes larger molecules to break into smaller ones
What is Reforming of Alkanes
Converts striaght chain alkanes into branched chain alkanes and cyclic hydrocarbons by heating with a catalyst (usually platinum)
- because straight chained hydrocarbons burn less efficiently than branched or those with rings (cyclic compound)
What are the products of complete Combustion of Alkane Fuels
Produces water and carbon dioxide
Incomplete combustion
When there is insufficient oxygen present, all the hydrogen atoms in the alkane molecule are converted into water, but some of the carbon atoms can form gaseous carbon monoxide ot solid carbon
Carbon formed as a byproduct of incomplete combustion
Can often be seen as smoke in the air or soot on the burner
- tiny particles of carbon in the air can be harmful
Carbon monoxide formed as a byproduct of incomplete combustion
Toxic gas that can cause death. Prevents the transport of oxygen around the body since it binds to haemoglobin and takes the place of oxygen. It is colourless and odourless
Unburned hydrocarbons form as a byproduct of incomplete combustion
Hydrocarbon does not burn at all and are released into the atmopshere unchanged
Oxides of sulfur formed as a byproduct of incomplete combustion
Some of the molecules in crude oil contain atoms of sulfur and are not removed via fractional distillation, cracking or reforming
- during the combustion of alkanes, these atoms of sulfur form sulfur dioxide which reacts with the atmopshere to form sulfur trioxide
- These gases are both acidic oxides and therefore when they dissolve in the atmosphere, they form sulfurous acid and sulfuric acid
- These acids contribute to the formation of acid rain which is responsible for damage to aquatic life in lakes and rives, and damage to crops and forests
Oxides of nitrogen formed as a byproduct of incomplete combustion
Few molecules contain atoms of nitrogen - however at high temperatures, like in an engine, nitrgoen atoms can react with oxygen in the air to form oxides of nirtogen
- Nitrogen dioxide is acidic and can dissolve in water ion the atmopshere, forming nitrous acid and nitric acid
Catalytic converters
Designed to reduce pollution emissions - can monitor level of oxygen in exhaust stream via an oxygen sensor
Contains both a reduction and oxidation catalyst - both consist of a ceramic structure coated with a metal catalyst - usually platinum, rhodium and palladium - usually formed in a honeycomb arranged to allow for maximum surface area
reduction catalyst causes NO bond to break to form an 02 molecule
oxidation catalyst oxidises the unburnt hydrocarbons and CO
Why do we need alternative fuels?
Relying on combustion of fossil fuels for energy causes many cocnerns. It causes pollution, global warming and climate change, while also reducing natural resources.
Carbon neutrality
Carbon neutrality means having a balance between emitting carbon and absorbing carbon from the atmosphere in carbon sinks.
Biofuels
Biofuels are clsoer to being carbon neutral than fossil fuels - however they are not compeltely carbon neutral because plants have to be harvested, transported and processed in a factory which uses up more carbon dioxide than it releases
Biodiesel vs Bioalcohol
Biodiesels can be made from vegetable oils such as sunflower or rapeseed
Bioalcohol (most common one is bioethanol) - can be produced via the fermentation of sugars to produce alcoholic drinks - ethanol then has to be seperated from large amounts of water which requires lots of energy
- Now bioethanol refers to ethanol produced via bacteria rather than enzymes.
Natural Gas vs Biodiesel and bioethanol
Natural Gas
1. no land is needed because it comes from undergground sources, however may destroy land
2. very high yield
3. exploration and drilling costs are very high, processing and transport costs are low
4. not carbon neutral
Biodiesel
1. A lot of land is needed - may replace land which would otherwise be used to grow food
2. Low but gradually increasing yield
3. No exploration or drilling costs
4. Closer to being carbon neutral
Substitution reaction of alkanes with Halogens
Alkanes are non polar and very stable so do not undergo many reactions,
no curly arrows
One atom or a group is replaced by another atom of group
- Initiation - one molecule becomes two radicals
When alkanes and halogens (diatomic molecules) are mixed and heated with UV light. the mixture will get very energetic and The halogen molecules will turn into individual halogeno atoms (this bond breaking is known as homolytic fission)
Cl2 –> CL + Cl* - Propagation - a molecule and a radical become a different radical and molecule and there are two reactions in this step
Chlorine radicals are very reactive species and when they collide with the hydrocarbon they react by breaking the bonds between the C-H and replace the H - Termination - two radicals become one molecule
Radical
- A species with an unpaired electron and is represented in mechanisms by a single dot
- Formed by homolytic fission of a covalent bond (eahc of the bonding electron leaves with one species)
Understand the limitations of the use of radical substitution reactions in the synthesis of organic molecules, in terms of further substitution reactions and the formation of a mixture of products
Since the collisions are uncontrollable you can’t use this method to make one particular product.
Exam questions often ask you to draw the first two of these propagation reactions to form CH3Cl.
But once that has been successfully done more collisions are inevitable because the remaining Chlorine radicals can’t be stopped from colliding with this desired product.
So further substitutions will happen and dichloro-, trichloro and tetrachloromethane will form, creating a mixture of products.
This would mean that messy and expensive separation would be required.
