Alkanes and Their Significance

Fractional distillation of crude oil Crude oil is a complex mixture of hydrocarbons formed by geological action on decayed aquatic plant and animal matter over long periods (i.e. millions of years). The first step in oil refining is fractional distillation. For separating components of crude oil it is carried out in large steel towers up to 40 metres high. In this process, the components of oil are separated according to their boiling points. Since boiling point increases as molecular weight increases, the separation is roughly in order of increasing atomic weights (or increasing number of carbon atoms per molecule). The crude oil is then vaporised by heating, they fed into the bottom of the fractionating column which contains a series of trays. The temperature falls as the vapour rise up through the column. The least volatile components, i.e. those with the highest boiling points and hence the largest molecular weights condense near the bottom of the columns while the most volatile components do not condense until they reach the top of the column. Liquids are drawn off from the column at various heights and these are the various fractions which are collected. Lubricating oils are obtained from the least volatile fraction by distilling under a vacuum because they will boil off at a lower temperature that would be needed at atmospheric pressure). Greases are separated from the remaining nonvolatile material by solvent extraction and the final residue is asphalt or tar. Fraction Name natural gas

Boiling Point Range (°C) 275 (refinery liquid) (refinery solid) (hard solid) (hard solid)

5-7 6-7 6-8 5-10 12-18 13-18 16-20 18-22 20-30 30-40

Uses household gas, making methanol and hydrogen in plants very inflammable solvent safer dry cleaning solvent than carbon tetrachloride solvent motor cars (the main refinery product) aviation and tractor fuel diesel engine fuel for trucks and trains lubricants pharmaceuticals candles, cartons, surf-board wax roof flashing, road asphalt

Classifying hydrocarbons The most important fossil fuels i.e. natural gas, petroleum and coal, are all mixtures of hydrocarbons. Hydrocarbons are compounds that contain the elements carbon and hydrogen (not to be confused with carbohydrates which contain the elements oxygen, carbon and hydrogen). There are three main groups of hydrocarbons: 

aliphatic

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alicyclic

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aromatic

IUPAC (International Union of Practical and Applied Chemistry) has developed a systematic system to name hydrocarbons and other organic compounds. The purpose of adopted this naming system (nomenclature) is to establish an international standard of naming organic compounds. Each organic compound is given a name which effectively describes its structure. When naming organic compounds, attention must be paid to: 

The number of carbon atoms in the hydrocarbon chain

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The presence of any functional groups in the compounds.

Alkanes Homologous series are groups of related chemical that obey a general formula and share the same functional groups, i.e. particular aspects of their structure, such as double bond or triple. In alkanes, the general formula is CnH(2n+2) where n is an integer. In alkenes, the general formula is CnH2n where n is an integer (except when there is more than one double bond).

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Alkanes are described as saturated hydrocarbons since each carbon is bonded to four other atoms, meaning that no other atoms can be incorporated into its structure. In all alkane molecules, each carbon atom forms four single bonds.

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When naming alkanes, the suffix –ane is used.

No of Carbons 1 2 3 4 5 6 7

Name methane ethane propane butane pentane hexane heptane

No of Carbons 8 9 10 11 12 13 14

Name octane nonane decane undecane dodecane tridecane tetradecane

No of Carbons 20 21 22 30 31 40 50

Name icosane henicosane docosane triacontane hentriacontane tetracontane pentacontane

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Methane (CH4), Ethane (C2H6), Propane (C3H8) and Butane (C4H10) are gases at room temperature. The boiling point increases with chain length, so the next twelve alkanes are liquids and the rest are solid at room temperature.

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Alkanes are found in natural gas and crude oil. Natural gas is mostly methane, with small amounts of ethane, propane and butane. Crude oil is a much more complicated mixture of hydrocarbons, and can contain alkanes with up to 100 carbon atoms in their molecules.

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Alkanes are unreactive, apart from combustion. For example acids and alkalis have no effect on them. However they do burn well in a good supply of oxygen, forming carbon dioxide and water vapour. The reactions give out plenty of heat, so alkanes are often used as fuels. When propane burns, the reaction is: C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g) + heat

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Although alkanes do undergo substitution reactions with some very reactive halogens such as fluorine and chlorine.

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Both propane and butane are used as camping gas, and in gas lighters. Calor gas is mainly butane, and natural gas is used for cooking and heating in homes.

Alkenes 

Alkenes are unsaturated hydrocarbons that contain at least one carbon double bond. The suffix –ene is used to indicate the double bond present. The double bond in alkenes is the functional group.

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For alkenes containing four of more carbon atoms, structural isomerisation occurs. Isomers have the same molecular formula but are arranged differently. When naming alkenes, the longest chain must contain the double bond, and is numbered so that double bond has the lowest number possible.

No of Carbons Name No of Carbons Name No of Carbons 1 n/a 3 propene 5 2 ethene 4 butene 6  Alkanes also burn in oxygen like alkanes, and have very similar chemical properties.

Name pentene hexene

e.g. C2 (g) + 3O2 (g) → 2CO2 (g) + 2H2O(g) + heat 

However, unlike alkanes which are relatively unreactive compounds, alkenes are not, reacting with hydrogen and other compounds, e.g. C2H4 (g) + H2 (g) → C2H6 (g)

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Alkenes are more reactive than alkanes. The double bond within carbon atoms can break to form single bonds, so can combine easily with other elements such as hydrogen or oxygen.

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Ethene is unsaturated since its molecules can add on more atoms, while ethane is saturated since its molecules cannot fit in more atoms because there are no double bonds to break, and each carbon atom already has four single bonds.

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Alkene molecules can combine with each other, due to their double bonds in a process called polymerisation. During polymerisation, many small molecules, called monomers, join together to from very large molecules, called polymers.

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Polythene is an example of such a molecule which is formed by this process. It is formed, when ethene is heated, under high pressure. More than 1,000 ethene molecules can combine through this process to make a single molecule of polythene. Polythene is a solid. It is unreactive, as there are no double bonds present. It can be rolled into thin sheets and moulded into different shapes, and because that is easy to mould, it is called a plastic.

There are two ways of testing a hydrocarbon to determine whether it is an alkane or alkene. 1.

The addition of bromine water. Bromine water is an orange solution of bromine in water. It turns colourless in the presence of an alkene, because the bromine adds on to the alkene, to form a colourless compound: C2H4 (g) + Br2 (aq) → C2H4Br2

2.

ethane + bromine water → 1,2-dibromoethane

The addition of potassium manganate. This is purple, but turns colourless when an alkene is present.

Alkynes 

Alkynes are unsaturated hydrocarbons that contain at least one carbon triple bond. The suffix –yne is used to indicate the triple present.

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When naming alkynes, the longest chain must contain the triple bond, and is numbered so that triple bond has the lowest number possible.

Branched groups 

When an alkane (or alkene etc.) has a branched group which replaces one of the hydrogen atoms, then the longest chain in the molecule still gives the basic name of the compound.

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The chain is numbered to give any double bonds or triple bonds as before, but where possible number so that the branch has the lowest number possible (note: the presence of a double or triple bond takes precedence).

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Name the group joined to the chain, and state the number of the carbon atom to which it is joined.

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Alkyl groups are similar to alkanes instead they have one less hydrogen, for example: methane (CH4) becomes methyl (CH3). The suffix for alkyl groups are –yl. For a chain with multiple branches, both branches are named together. Prefixes should also be added (such as di-, tri- and tetra-).

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If there are two or more different branches, the groups are written alphabetically.

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When numbering, in the case of a conflict, the one that comes first alphabetically takes precedence provided there are no other constituents.

Branch Name bromo chloro

Structure Br– Cl–

Branch Name fluoro iodo

Structure F– I–

Properties of alkanes and alkenes 

Alkanes and alkenes are both non-polar substances. The bonds between the molecules are weak intermolecular forces. As a result these substances have low melting and boiling point, and many of them exist as gases or liquids at room temperature. Generally the longer the carbon chain, however, the higher the melting and boiling points, which is which methane is gas, while octane is a liquid. This is because there are more bonds to break in a longer carbon chain, which means more energy is needed.

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Alkenes tend to have lower melting and boiling points than alkanes because alkenes are smaller, and therefore fewer bonds that need to be broken, and therefore less energy required to change its state from solid to liquid, or liquid to gas.

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The density naturally increases as the carbon chain increases (i.e. the molecular weight increases). Alkanes and alkenes, when present as a liquid are less than water. They also are insoluble in water because they are non-polar substance and water prefers being bonded to its own atoms (as it is a polar substance). This means when placed in water, alkanes and alkenes will float on-top of the water, forming an immiscible layer.

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Alkanes and alkenes have a high volatility, which means they go from a liquid to gaseous state much easier than low volatile liquids. Volatility also means a relatively high partial pressure of their gas above the liquid’s surface.

Flash points The flash point is the lowest temperature where if a small flame is applied to the vapour above the fuel will cause it to ignite in a closed container with air. Chemicals that have a low flash point, such as hydrocarbons have low boiling points. In fact, all hydrocarbons up to octane have a flash point less the room temperature meaning that potential to ignite at room temperature (25°C).

Uses of hydrocarbons The major uses of hydrocarbons are as fuels such as: 

natural gas for domestic uses, such as cooking and heating, and also in industry

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LPG for caravans and barbeques (and even some cars)

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petrol and diesel for cars, trucks, vans and trucks

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kerosene for jet aircraft

Higher molecular weight hydrocarbons are used as lubricating oils and greases. Vaseline is a highly purified form of these high molecular weight hydrocarbons, while paraffin waxes are a mixture of high molecular weight alkanes. Low molecular weight hydrocarbons are used as propellants in aerosol sprays. Many hydrocarbons are solvents as well.

Safety issues with hydrocarbons Hydrocarbons are extremely flammable (especially the low molecular weight), and are toxic in high concentrations. They also have a high volatility, meaning they can quickly form a flammable and explosive mixture with air. As a result the following safety precautions are undertaking when using hydrocarbons: 

A well-maintained cylinders and fittings for gaseous hydrocarbons are used

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These cylinders are stored away from sources of electrical sparks

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Odours are added for early detection of leaks

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Sturdy containers are used for liquids

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The quantities of these substances used are minimised, especially in everyday usage.

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These substances are kept away from exposed flames and sparks.

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Warning signs are placed around the storage of these liquids as well as on the containers which store these substances.

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These substances should never be handled in confined spaces, instead well-ventilated rooms or a fume-hood should be used.