Alkanes Flashcards
What is a hydrocarbon
Compounds that contain only carbon and hydrogen atoms
Different types formulae (Empricical, molecular, general, structural, displayed and skeletal)
Empirical Formula
the simplest whole number ratio of atoms of each element in a compound
Molecular Formula
The true number of atoms of each element in a compound
General Formula
All members of a homologous organic series follow the general formula. For example, alkanes have the general formula CnH2n+2
Structural Formula
Shows the structural arrangement of atoms within a molecule e.g. CH3CH3 (formula for ethane)
Displayed Formula
Shows every atom and every bond in an organic compound
Skeletal Formula
Shows only the bonds in a compound and any non-carbon atoms. The verticies are carbon atoms and hydrogen is assumed to be bonded to them unless stated otherwise
What is a ‘homologous series’?
A series of organic compounds that follow a general formula and react in a very similar way.
Each consecutive member differs by CH2, and boiling point increases with chain length
What is a ‘functional group’?
Each homologous series has a functional group that allows the molecule to be recognised to be apart of that group because it makes it react in a certain way
How to name compounds (IUPAC)
1. Stem
Find the longest unbroken carbon chain in the compound (e.g. methane, hexane, etc.)
2. Functional groups
The ending of the compound name tells you the functional group present.
(e.g. alkane = -ane, alcohol = -ol)
If there is more than one present, it is added onto the end (e.g. eth-an-ol, meth-an-oic acid)
If a halogen is present it is added as a prefix (e.g. Bromo-methane, Chloro-ethene)
3. Side chains
* Functional groups and side chains are given with the name of the carbon they are attached to
* Numbers are seperated by commas
* Numbers and words are seperates by hyphens
* If more than one of that group/side chain is present then prefixes are added: di- (2) tri- (3) tetra- (4) etc
* The carbon chain is numbered in ascending order from the end of the chain nearest a functional group
* If multiple prefixes are present, they are put in alphabetical order
Prefixes for compounds up to C10
C1 = meth-
C2 = eth-
C3 = prop-
C4 = but-
C5 = pent-
C6 = hex-
C7 = hept-
C8 = oct-
C9 = non-
C10 = dec-
Reaction mechanisms
(addition, substitution, oxidation, reduction, polymerisation)
Types of reactions
Addition:
The reactants combine to form a single product
Substitution:
One functional group is replaced by a different functional group
Oxidation:
A species loses at least one electron, and is oxidised
Reduction:
A species gains at least one electron, and is reduced
Polymerisation:
Many small molecules (monomers) join together to form a long, repeating molecule called a polymer
What is strucural isomerism?
compounds that have the same molecular formula but different structural formulae
What is stereoisomerism?
Molecules with the same molecular formula but different spatial arrangement
OO
OO
OO
OO
The general formula for alkanes
Alkanes
CnH2n+2
Why are alkanes and cycloalkanes saturated hydrocarbons?
Alkanes
they contain only single bonds between carbon atoms
Fractional distillation of crude oil
Alkanes
Crude oil is a mixture of different hydrocarbons. Fractional distillation seperates it into seperate molecules, as the different chain lengths of molecules result in them having different boiling points
Steps:
* Mixture is vapourised and fed into the fractionating column
* Vapours rise, cool and condense
* Products are siphoned off for different uses
Products with short carbon chains have lower boiling points, meaning they rise higher up the fractionating column before reaching their boiling point and condensing, and are collected at the top
Compounds collected from the fractionating column can then be broken down further via cracking
Cracking of crude oil
Alkanes
Longer chains are not very useful, so they are broken down to form smaller, more useful molecules. The carbon-carbon bonds are broken, requiring harsh reaction conditions.
Thermal cracking:
-produces a high proportion of alkanes and alkenes
-needs high temperatures around 1200k and pressures around 7000kPa used to crack the carbon chains
-always produces an alkane, and the remaining atoms form at least one alkene
Catalytic cracking:
-produces aromatic compounds with carbon rings
-lower temperatures around 720K are needed along with normal pressure
-requires a zeolite catalyst to compensate for these less harsh conditions
Reforming of crude oil
Alkanes
Reforming is the conversion of straight chained hydrocarbons into branched and cyclic hydrocarbons by heating with a platinum catalyst for efficient combustion
Combustion of alkanes
Alkanes make good fuels because they release a lot of energy when burned.
With sufficient oxygen present, they undergo complete combustion, producing carbon dioxide and water:
CH4 + 2O2 —> CO2 + 2H2O
In insufficient oxygen, incomplete combustion takes place, where carbon monoxide or carbon particulates are formed alongside water:
2CH4 + 3O2 —> 2CO + 4H2O
Which pollutants can be formed during the combustion of alkane fuels?
Alkanes
-
Carbon monoxide (CO):
A toxic, odourless, colourless, gaseous product - Nitrogen dioxide
- Sulphur dioxide
also produced:
unburned hydrocarbons
carbon particulates
The problems arising from pollutants from the combustion of fuels
Alkanes
Oxides of nitrogen and sulphur are also produced as a byproduct of alkane combustion along with carbon particulates from unburnt fuel. In clouds, these oxides can react with water and form dilute acids, which result in acid rain.
How does a catalytic converter solve some problems caused by pollutants
Alkanes
Gaseous pollutants produced from the combustion of fuels can be removed from systems using a catalytic converter.
They use rhodium catalysts to convert harmful products into more stable products such as CO2 or H2O
The use of alternative fuels (comparing biofuel to non-renewable fossil fuels)
Alkanes
Biofuels release fewer, less harmful products when burned
Carbon dioxide is released during fuel combustion. It is a greenhouse gas so it causes global warming and contributes to climate change
Ethanol is a common biofuel. It is ‘carbon neutral’ as the carbon given out when burned is equal to the carbon taken in by the crop during the growing process.
Ethanol is produced by fermentation, where enzymes break down starch from crops into sugars which can then be fermented to form ethanol.
Biofuels are also sustainable. This means their supply can be maintained at the rate they are being used, so they will not run out - unlike fossil fuels
Free radical substitution mechanism steps
Alkanes
Initiation:
1. A halide molecule’s bond is broken by U.V. light, producing two halide free radicals
Propagration:
2. A halide free radical reacts with an alkane, producing an alkane free radical and a halide-hydrogen (as the hydrogen from the alkane is bonded to the halide instead)
3. The alkane free radical reacts with another halide molecule, producing a halide free radical and an alkane-halide. (the free radicals from these steps of propagation form a chain reaction)
Termination:
4. Halide free radical + halide free radical —> halide molecule
5. Halide free radical + alkane free radical —> Alkane-halide molecule
6. Alkane free radical + alkane free radical —> different alkane (free radical formula x2)
what is a free radical?
a species with an unpaired electron, represented in mechanisms by a single dot
What is homolytic fission?
Describes the breaking of a covalent bond so that the bonding electrons are shared equally between the atoms involved
What condition is required in the first step of the free radical substitution mechanism?
U.v light to break the halogen molecule bond
The reactions of alkanes with:
* Oxygen in the air (combustion)
* Halogens (in terms of radical substitution)
Alkanes with oxygen:
* In good oxygen supply, the combustion of alkanes produces carbon dioxide and water
* In insufficient oxygen supply, carbon monoxide or carbon particulates can be produced alongside water
Alkanes with halogens:
produces halognenoalkanes via free radical substitution
ex. C2H6 + Br2 –> CH5Br + BrH
The limitations of the use of free radical substitution reactions in the synthesis of organic molecules
- It forms a mixture of products
- can take place at any point along the carbon chain
- low atom economy
- difficult to control
What the different catalysts are used for
Rhodium:
Catalytic converters
Zeolite:
Catalytic cracking
Platinum:
Reforming of crude oil
