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Alkanes: Properties

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Alkanes: Properties - Lesson Summary

Physical properties:
These properties depend on the attractive forces working on the molecules. The bonds present between Carbon-Carbon and Carbon-Hydrogen are covalent, and the electro-negativity between two carbon atoms or between carbon and hydrogen is almost negligible. Hence, the molecules formed are non-polar.

Therefore the attractive force acting on these molecules is the weak van der Waals forces. Therefore, the first four members of the series, C1 to C4, are gases; C5 to C17 are liquids while the molecules containing 18 or more than 18 carbon atoms are solid at 25 degree Celsius.   ALKANES   Formula               Name   State   CH 4             -     Methane  

  Gases   C 2H 6           -     Ethane   C 3H 8           -     Propane   C 4H 10         -      Butane   C 5H 12         -      Pentane







  Liquids   C 6H 14         -      Hexane   C 7H 16         -      Heptane   C 8H 18         -      Octane   C 9H 20         -       Nonane   C 10H 22       -        Decane   C 11H 24       -        Undecane   C 12H 26       -        Dodecane   C 13H 28       -        Tridecane   C 14H 30       -        Tetradecane   C 15H 32       -        Pentadecane   C 16H 34       -        Hexadecane   C 17H 36       -        Heptadecane   C 18H 38       -        Octadecane  

  Solids   C 19H 40       -         Nonadecane   C 20H 42       -         Icosane

They are all colourless and odourless. Due to their non-polar nature, these compounds are insoluble in polar solvents like water and soluble in non-polar solvents like benzene. 

Their boiling point is depends on the strength of the van der Waals force. Strength of the van der Waals force is determined by the number of electrons surrounding the molecule and its surface area. This force is very sensitive to distance. Hence, for a strong force of attraction, the molecules need to be held close together.

As alkanes are non-polar molecules, the van der Waals forces are weak, and hence their boiling points are much lower than expected. As the size of a molecule increases, its surface area increases. Thus, the strength of the intermolecular van der Waals force also increases.

A linear or straight chain alkanes like pentane has a higher boiling point than its branched isomers like 2-methyl butane and 2,2-dimethyl propane. This is because branching reduces the surface area of a molecule leading to a weakening of the van der Waals forces, which are then overcome at relatively lower temperatures. 
 
Chemical properties:
As alkanes are non-polar, their carbon bonds are quite stable, and so they do not easily react with acids and bases.

Redox reactions:
Even though they do undergo redox reactions under certain conditions as their carbon atoms are in strongly reduced states.

During this reaction, the hydrogen atom of an alkane is substituted and hence the reactions are called substitution reactions. When the element substituting the hydrogen is a halogen, the reaction is called halogenation.

It involves three steps - initiation, propagation and termination. During initiation, the chlorine molecule is homolysed in the presence of heat or light, forming chlorine free radicals.



During propagation, two reactions take place. First, the chlorine free radical attacks the methane molecule, forming a methyl free radical and hydrogen chloride. Then the methyl free radical attacks the second chlorine molecule, forming chloromethane and one more chlorine free radical setting up the chain reaction.



During termination, the reaction stops either because all the reactants have been consumed or because of the following possible chain terminating reactions.

One possible reaction is that, two chlorine free radicals combine to form a chlorine molecule. Another possible reaction is that, two methyl free radicals combine to form ethane. Or, a chlorine free radical and a methyl free radical combine to form chloromethane.
 

Thus, in the chlorination of methane, ethane is obtained as a by-product.
 
Combustion reaction:
On heating alkanes get completely oxidized to carbon dioxide and water along with heat, in the presence of air or di-oxygen. When alkanes undergo incomplete combustion, they form carbon black. It is used in the manufacturing of ink, printer ink, and black pigments and as filters.

Alkanes, like methane on heating at high pressure, with a regulated supply of oxygen and in the presence of a suitable catalyst, provides a variety of products like methanol and methanal. Ethane gives ethanoic acid.

 2CH 4 + O 2 Cu/523K/100atm →     2CH 3OH
                                                      Methanol

CH 4 + O 2        Mo 2O 3 →       HCHO + H 2O
                           Δ         Methanal

And an alkane with tertiary hydrogen atom like 2-Methylpropane gives corresponding alcohol. When a normal alkane like n-Hexane, is heated in the presence of anhydrous aluminium chloride and hydrogen chloride gas, it isomerizes to a branched chain alkane like 2-Methyl pentane and 3-Methyl pentane.

Alkanes undergo aromatization or reforming, that is, a normal alkane with six or more than six carbon atoms, like n-Hexane, when heated to 773 Kelvin, at10-20 atmospheric pressure in the presence of a catalyst like vanadium, molybdenum or chromium supported over alumina, gets dehydrogenated to form cyclic compounds like benzene or its homologues.

 
Methane when heated at 1000° C in the presence of Nickel along with steam to form carbon monoxide and dihydrogen gas.

Pyrolysis:
Higher alkanes at high temperatures decompose to form lower alkanes and alkenes, amongst others. This reaction is also known as Pyrolysis or cracking.

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