Alkanes Flashcards
Where do alkanes come from?
Oil and natural gas
Oil is a mixture of liquid alkanes and other hydrocarbons
Higher solid alkanes are residues from oil distillation so tar
Alkanes are obtained and isolated from petroleum and natural gas
How are alkanes with unbranched hydrocarbons named?
By the number of carbons in the chain
First 4 are:
Methane
Ethane
Propane
Butane
1 then 2 hydrogen saturated carbons and so on
What are the IUPAC rules for alkane nomenclature?
Find and name the longest continuous carbon chain
Identify and name groups attached to chain
Number chain consecutively starting at end nearest a substitute to group
Designate location of each substitute to group by appropriate number and name
Assemble name listing groups in alphabetical order
Prefixes: di,tri,tetra are used to designate several groups of the same kind so are not considered when alphabetising
What is methane?
Simplest named alkane
Alkanes are organic compounds that consist entirely of single boned carbon and hydrogen atoms
No double bonds
State what you know about alkanes
Saturated hydrocarbons
General formula:CnH2n+2
Unbranched chains,branched chains or rings
Unbranched are straight chains with bond angle 109.5 degrees
Ring alkanes have same general formula as alkenes- CnH2n
What are the physical properties of alkanes?
Polarity- almost non polar as carbon and hydrogen have close electronegativities
Only intermolecular forces are van der waals
The larger the molecule the stronger the van deer waals forces and larger electron density cloud surrounding the molecules
What is special about the boiling points of alkanes?
As chain length increases boiling point increases
Shorter chains are gases at RT
Medium chains are liquid at RT
Long chains are solids at RT
This is due to van deer waals forces
Branched chains with the same number of carbons as a chain alkane have a lower boiling point as they cannot get as closely together as a chain alkane
What about solubility related to alkanes?
Alkanes are insoluble in water so do mix with other relatively non polar liquids
Water molecules are held together by hydrogen bonding which are stronger than the van der waals that act between alkane molecules
A non polar substance will not dissolve in a polar liquid
How reactive are alkanes?
Strong carbon carbon bonds and strong carbon hydrogen bonds
They burn in oxygen as complete and incomplete combustion
React with halogens under suitable conditions
Relatively unreactive as they do not react with : acids,bases and oxidising or reducing agents
Where does fractional distillation take place?
In a fractionating column
What do you know about crude oil?
It’s a fossil fuel formed millions of years ago
Non renewable so takes many years to form
Mixture of mostly alkanes - branched and unbranched
Formed at high temperatures and pressures from dead animals an plants
What is the definition of fractional distillation?
Used to separate the mixture by heating it and separating the fractions based on their boiling points
What are the steps in fractional distillation?
Crude oil is heated and the mixture of liquid and gas is passed into the fractionating tower
Vapour passes up the tower via a series of trays and bubble caps until it reaches a point at which they condense to a liquid and run off
Shorter chains condense near the top of the tower where as longer chains condense nest the bottom of the tower
What is needed to break alkanes down?
Extremely harsh conditions
What are the two main types of cracking?
Thermal cracking- high temperature and high pressure
Catalytic cracking- lower temperature(720K) and lower pressure(101KPa) plus a catalyst- usually turning longer chain alkanes into motor fuel
What are the typical products of cracking?
Gases
Branched alkanes
Cycloalkanes
Aromatic compounds based on benzene rings
State what you know about thermal cracking
7000Pa
700-1200K
Expensive and dangerous as c-c bond breaks someone electron goes to each carbon forming a free radical to get 2 free radicals
2 shorter chain free radicals are produced which are both extremely reactive intermediates and react to form a variety of shorter chain molecules
Not enough hydrogens so one chain must have a carbon carbon double bond which is an alkene
More than one carbon carbon double bond may break during this process
What else can be produced during thermal cracking?
Hydrogen
Almagest are kept at a high temperature and pressure for an extremely short time to combat chains being broken down too much
What is combustion?
Burning a substance in oxygen where all carbon reacts with a plentiful amount of oxygen
These reactions give out heat and have large negative enthalpies
More carbons the greater the heat output
What is incomplete combustion?
Burning of a substance in a limited supply of oxygen
Carbon monoxide is formed instead of carbon dioxide which is very dangerous
When even less oxygen is present soot is produced as longer chain alkanes tend to combust in this way as there are more carbons that need to react with oxygen
What does carbon monoxide do?
Attach to haemoglobin in place of oxygen so that oxygen can’t reach cells in the body so this could result in death
Not detectable A’s has no colour or smell so is a poisonous gas
Carbon particulates can be detected
What are examples of polluting products?
Unburnt hydrocarbons
Carbon monoxide
Nitrogen dioxide
Sulfur dioxide
Water vapour ,carbon dioxide which are greenhouse gases
Carbon particles
State what you know about nitrogen oxides
Needs to be enough energy for nitrogen and oxygen to combine
Happens in petrol engines where there is enough heat and a spark to ignite fuel
Contributes to acid rain and photochemical smog as it can react with water to produce nitric acid
State what you know about sulfur dioxide
Produced from impurities in crude oil
Contributes to acid rain due to forming sulfuric acid when reacted with water vapour and oxygen
State the effect of carbon particles
Make asthma worse and in some cases cause cancer
State the effect of unburnt hydrocarbons
Enter atmosphere and increase greenhouse effect
Contribute to photochemical smog ad cause health issues
State the effect of carbon dioxide
Always produces when hydrocarbons burn in enough oxygen
Levels of carbon dioxide are rising which increases earths temperature and has an effect on our climate
Water vapour also contributes to this greenhouse effect
What are the ways we can reduce the amount of pollution?
Flue gas desulfurisation- process of removing sulfur dioxide gases in a power station
Method 1: mixture of calcium oxide and water is sprayed into the gases and the gas reacts with them to form calcium sulfite which can be oxidised to calcium sulphate
Method 2:use calcium carbonate
Catalytic converters: reduce output of CO and NOx gases and unburnt hydrocarbons
A honeycomb: large SA made of ceramic materials then coated in platinum or rhodium catalysts
What is glacial warming and the greenhouse effect?
Gases behave like glass in a greenhouse trapping infrared radiation causing the earths atmosphere to heat up
Needed for life to be sustained on earth as CO2 levels are increasing earths temperature increases
Water and methane contribute to this but their levels have stayed relatively constant
What is a photochemical reaction?
Reaction between a halogen and an alkane can occur in bright sunshine or UV light
Reaction is called free radical substitution and has 3 stages: initiation ,propagation and termination
One or more of the atoms in a reaction has been replaced by a different atom
What happens in initiation ?
cl-cl bond is broken into 2cl free radicals
Cl2 absorbs UV light
Both atoms are identical so one electron goes to each atom from the bond
This forms a free radical( particle with an unpaired electron)
C-H bond is stronger then Cl-Cl so does not break with UV light as free radicals are highly reactive
What are the two stages of propagation?
Cal free radical takes a hydrogen from methane to form HCl leaving a methyl free radical or alkyl group
Methyl free radical is very reactive and reacts with another Cl2 molecule producing another Cl free radical and chloromethane
Chain reaction starts here and can happen many times before termination
What is termination?
The step where free radicals are removed and so collide with one another
Two free radicals will always react together to form a stable product
What is special about longer chain alkanes?
Lots of isomers can be formed due to Cl replacing any of hydrogens in the chain
State what you know about CFCs
Chlorofluorocarbons
Destroy ozone layer as they are in the stratosphere
Break it down and it decomposes oxygen
Too much ozone can cause lung irritation at plastic degradation at ground level
Important in the atmosphere as it protects the earth from harmful exposure to UV light
Free radicals are catalysts and react with ozone to form an intermediate and an oxygen molecule
Chlorine free radicals are formed when CFCs are released into atmosphere
Chlorine free radical is regenerated and goes straight to another ozone molecule
