Alkanes

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Alkanes

Alkanes are saturates hydrocarbons. There is a whole series of them, with different numbers of carbon atoms. To show they are alkanes, they all have names ending in -ane. The first part of the name shows how many carbon atoms there are in the chain

  • Hydorcarbons are compounds which contain only carbon and hydrogen
  • A saturated organic compound is one which contains only single bonds between carbon atoms

Important definitions;

  • Functional Group: An atom or group of atoms that gives a compound it's characteristic chemical reactivity
  • Homologous Series: A series of compounds that have the same functional group and the same general formula, each member of the series differing from the last by an extra -CH2 unit. The alkanes form a homologous series
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Representations of organic structures

In alkanes, the shape around each carbon atom is tetrahedral and the bond angles are all 109.5 degrees

There are several ways of representing molecules;

  • The molecular formula - simply states the number of each type of atom contained in a molecule of the substance
  • The empirical formula - gives the simplest whole number ratio of atoms of each element in a compound, in it's lowest terms
  • The displayed formula - shows every atom and every bond in the molecule explicitly but does not show correct bond angles
  • The skeletal formula - represents C-C bonds as a line and leaves out the hydrogens attached to each carbon. Any other groups present are shown in the normal way
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Isomerism

  • Isomers are different compounds that have the same molecular formula

This can happen if molecules of the two substances contain the same number of atoms of each element, but are connected up in a different sequence. This is called structural isomerism;

  • Structural isomers are compounds that have the same molecular formula but different structural formulae

In alkanes with more than three carbons, there is the possibility of structural isomerism becuase the carbon chain mey be straight or branched

In the case of alkanes, the isomers have very similar chemical properties, though this is not always the case with other types or organic compound. Alkane isomers have significant differences in physical properties

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Alkanes

Substituted and branched-chain alkanes

  • A branch in an alkane is called an alkyl group. The general formula of alkyl groups is CnH2n+1
  • Alkanes can also be substituted. This means one or mroe hydrogens is replaced by a different atom or group of atoms

Cycloalkanes

  • It is possible for carbon atoms to form rings as well as chains
  • A saturated hydrocarbon based on a ring of carbon atoms is called a cycloalkane. They are named with the number of carbon atoms indicated in the usual way and the prefix cyclo- to indicate the ring
  • Cycloalkanes have very similar properties to alkanes. They undergo the same types of reactions
  • The general formula of cycloalkanes is CnH2n
  • Substituted cycloalkanes are named in the same way as other compounds 
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Alkanes

Occurrence of Alkanes

  • Natural gas is mainly methane, with some other hydrocarbons and gases
  • Crude oil contains a large number of liquid hydrocarbons, including alkanes as well as other compounds
  • Fractional distillation is used to separate the components of crude oil on the basis of difference in their boiling points
  • The products of fractional distillation can be used as fuels, or processed by the petrochemical industry to make other organic compounds

Physical properties of Alkanes

Melting and boiling points

  • Boiling or melting involved breaking intermolecular forces
  • Carbon and hydrogen have virtually equal electronegativity, so C-H bonds are non-polar. Therefore, alkanes cannot form permanent dipole - permanent dipole interactions or hydrogen bonds. They can only form van der Waal forces, which are the weakest type of intermolecular force. As a result, alkanes have relatively low melitng and boiling points
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Physical properties of Alkanes

The more carbons in the molecule, the higher the boiling point

  • Boiling points generally increase as the size of the molecule increases because;
  • The van der Waal forces become stronger as the number of electrons pre molecule increases
  • There are more van der Waal forces to break as the surface contact with other molecules increases
  • The first 4 straight chain alkanes are gases at room temperature, the next thirteen are liquids and the remainder are solids

Straight chain alkanes have higher boiling points than their branched isomers

  • Even though they have the same number of electrons, isomeric alkanes generally have different boiling points
  • This is because the staight chain isomer has a larger surface area to form van der Waals forces with other molecules, so more energy is needed to break these interactions

Density

  • All solid and liquid alkanes are less dense than water and so will float on water
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Chemical properties of Alkanes

Solubility

  • All alkanes are virtually insoluble, because they are non-polar, so cannot form hydrogen bonds to the water molecules. However, the smaller alkanes are soluble in organic solvents like ethanol

Chemical properties of Alkanes

  • Alkanes are in general, unreactive. This is mainly because they do not contain features such as polar bonds or double bonds which play an important role in the mechanism of many organic reactions

Combustion

  • Burning alkanes is highly exothermic. This is exploited in their extensive use as fuels
  • In complete combustion, where oxygen is plentiful, the products are carbon dioxide and water
  • In more limeted supplies of oxygen, carbon monoxide or solid carbon may be formed. This is called incomplete combustion
  • Carbon monoxide is a toxic gas. It binds very strongly to haemoglobin in blood, limiting the supply of oxgen to the vital organs 
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Alkanes

  • For this reason, prolonged exposure to carbon monoxide can be fatal
  • It is produced by car exhausts as the fuel in the engine is burned in a limeted supply of oxygen. It can also form from faulty boiler systems in the home where the supply of oxygen has been limeted by a blockage
  • Boilers should be serviced regularly to prevent this happening and carbon monoxide detectors can also be fitted

Using alkanes as fuels

  • Alkanes obtained from fossil fuels - crude oil and natural gas - are the most important fuels used in the UK at present. They are used;
  • In industry and the home for heating - natural gas is mainly methane
  • In transportation - petrol contains mainly alkanes with 6 to 10 carbons. Diesel contains alkanes with 10 to 15 carbons
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Alkane Fuels

Advantages of alkane fuels

  • They burn in air to release large amounts of energy
  • They can be easily ignited and can be made to burn steadily
  • They can be stored and transported conveniently
  • Ther are readily available at a reasonable price
  • The products of combustion can easily be disposed of

Disadvantages of alkane fuels

  • Combustion of alkanes can pollute the atmosphere
  • In the short term, e.g. by releasing oxides of nitrogen (which can cause acid rain), carbon monoxide, unburned hydrocarbons ect.
  • In the long term by increasing the amount of carbon dioxide in the atmosphere, which may be leading to an increase in atmospheric temperatures and hence climate change
  • Fossil fuels are in effect, non renewable as they take millions of years to form
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Biofuels

  • These are fuels produced from recently living biological material
  • A simple example is wood. Other examples which are becoming increasingly important are biodiesel - made from plant oils - and ethanol, which can be made from sugars in plants by fermentation

Advantages of biofuels

  • As new plants can be readily grown, biofuels are renewable fuels
  • CO2 is removed from the air as plants grow, then returned to the air if they are made into fuel and burned. So they are carbon neutral

Disadvantages of biofuels

  • There is a limit to the amount of land which can be used to produce crops, and the production of biofuels can reduce the production of food
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Petrochemical Processes

Cracking, Isomerisation and Reforming

  • Oil companies use a variety of processes to convert some of the less useful and valuble fractions into more useful compounds

Cracking

  • Involves breaking down longer alkane molecules into smaller alkane and alkene molecules
  • The smaller compounds are more useful because;
  • Smaller alkanes are used in fuels, especially petrol
  • Alkenes are used to make other products, such as polymers and ethanol
  • Cracking involves a high temperaure, up to 850 degrees centigrade and often the use of a catalyst

Isomerisation

  • Involves the conversion of straight chain alkanes into branched chain alkanes. It is carried out at a high temperature, often in the presence of a catalyst
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Petrochemical Processes

  • The branched chain alkanes are better fuels than straight chain compounds because they burn more smoothly in an engine (prevent knocking)

Reforming

  • This involves the conversion of alkanes into cyclical compounds such as cycloalkanes or arenes. Hydorgen is also formed
  • The conditions used are similar to those used for isomerisation
  • Cyclic compounds, like branched chain alkanes are better fuels than straight chain alkanes as they burn more smoothly
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Reaction of alkanes with halogens

  • The reaction of alkanes with chlorine or bromine to produce halogenoalkanes is important
  • This is called a substitution reaction, because a hydrogen atom in the alkane is substituted by a bromine atom

Key Conditions

  • The reaction does not occur in the dark
  • The reaction occurs at room temperature upon exposure to UV light

Observations

  • The bromine is decolourised
  • Steamy, acidic fumes of HBr are produced
  • Chlorine reacts similarly to bromine, but the reaction is more vigorous
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