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alkanes

                Alkanes 

Alkanes are hydrocarbons in which the carbon atoms are held together by single bonds.  Alkanes also called parafins. Their general formula is CnH2n+2 for molecules which do not contain ring structures.Alkanes are also called saturates or saturated hydrocarbons. The carbon atoms that make up the carbon backbone are linked together to form a chain (linear or branched alkanes), a circle (cyclic alkenes), or a mixture of both. The alkanes are distinguished from the other hydrocarbons by the fact that they are completely saturated with hydrogen. This means that no additional hydrogen atoms can be added to these hydrocarbons without destroying the carbon backbone. 

Alkanes are in some respect the most boring of the organic compounds, since they are unreactive (mostly) towards acids, bases, oxidizing agents, reducing agents, and most of the other reagents that organic chemistry have in their arsenals.  On the other hand, alkanes are important for their ability to undergo combustion with molecular oxygen (O2):  alkanes of various lengths are the major components of the hydrocarbon fuels that we burn for energy, whether for cooking (methane, propane, butane) or for transportation (gasoline, diesel fuels):

CnH2n+2  +  (3n+1)/2O2  ®  nCO2  +  (n+1)H2O  +  energy 


Straight  chain  and  branched  alkanes

The straight-chain (normal) and branched alkanes have the general formula CnH2n+2.  The molecules consist of either straight chains of carbon atoms, connected one after the other, with the appropriate number of hydrogen atoms on each carbon, or as   branched chains of carbon atoms having carbon substituents (alkyl groups) attached at various points along the chain.

Straight-chain alkanes are named by using a stem that indicates the number of carbon atoms (meth = 1 C, eth = 2 C's, prop = 3 C's, etc.) to which is added the suffix -ane, indicating that the molecule is an alkane (that is,  the carbon atoms are all connected by single bonds).  Thus, the word "propane" indicates that there are three carbon atoms in the chain, all connected by single bonds.      


 Cycloalkanes :-

have one or more rings of carbon atoms. The simplest examples of this class consist of a single, unsubstituted carbon ring, and these form a homologous series similar to the unbranched alkanes. The IUPAC names of the first five members of this series.  gives the general formula for a cycloalkane of any size. If a simple unbranched alkane is converted to a cycloalkane two hydrogen atoms, one from each end of the chain, must be lost. Hence the general formula for a cycloalkane composed of n carbons is CnH2n.  
              source, chemistry.msu.edu

Physical properties of alkanes :-

  • Alkanes are non-polar compounds. The difference in the electronegativities of Carbon and Hydrogen is almost non-existent, hence they have an almost complete absence of polarity. 
  • Alkanes generally have relatively lower boiling points and melting points. This is because their atoms have weak Van Der Waals force and so the atomic bonds break easily.
  • However, as the molecules get bigger the force gets stronger. So more complex alkane has higher boiling and melting points. 
  • They can exist as solids liquids and gases in their natural states. Unbranched alkanes usually are gases in their natural state. The examples are methane, ethane etc. The alkanes bigger than hexadecane are all solids. 
  • they are completely insoluble in water, again due to the weak van der Waal forces.
  • However, they are soluble in organic solids. Here the van der Waal forces of alkane break and are replaced by newer van der Waal forces.
Melting point and boiling point of  

     Alkanes  :-

Both the melting points and boiling points of alkanes are characteristic of the intermolecular forces found between the molecules. The electronegativity difference between carbon and hydrogen (2.1 – 1.9 = 0.2) is small; therefore, the C-H bond is nonpolar, meaning that the only attractions between one molecule     and its neighbors will be      london dispersion forces. 
* These forces will be very small for a molecule like methane but will increase as the size of the molecules increase.
* Therefore, the melting and boiling points of the alkanes increases with the molecular size, due to the increase in London dispersion forces. (i.e. the intermolecular forces are stronger in larger hydrocarbons, therefore, more energy is required to cause phase changes).  
* the melting and boiling point trends for the first 16 hydrocarbons. Notice that the first four alkanes are gases at room temperature, and solids do not start to appear until about C17H36.
* the more branched the chain, the lower the boiling point tends to be. London dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, bulky molecules (with substantial amounts of branching) to lie close together (compact) compared with long, thin molecules. Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points, melting points, and densities than alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact. 

Solubility 

Alkanes (both normal and cycloalkanes) are virtually insoluble in water but dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds. When a molecular substance dissolves in water, the following must occur:
• Due to very little difference in electronegitivity between carbon and hydrogen and the covalent nature of C-C bond or C-H bond, alkanes are generally non-polar molecules.
• breaking of the intermolecular   forces within the substance. In the   case of the alkanes, these are the     Van der Waals dispersion forces.
• breaking of the intermolecular forces in the water so that the substance can fit between the water molecules. In water, the primary intermolecular attractions are hydrogen bonds. 
• Breaking either of these attractions requires energy, although the amount of energy required to break the van der walls , dispersion forces in a compound, such as methane, is relatively negligible; this is not true of the hydrogen bonds in water.
•  To simplify, a substance will     dissolve if sufficient energy is   released when the new bonds are formed between the substance and the water to make up for the energy required to break the original attractions.
 The only new attractions between the alkane and the water molecules are the Van der Waals forces. These forces to do not release a sufficient amount of energy to compensate for the energy required to break the hydrogen bonds in water. Therefore, the alkane does not dissolve. 
Solubility in organic solvents :-
In most organic solvents, the primary forces of attraction between the solvent molecules are the Van der Waals forces composed of either dispersion forces or dipole -dipole attractions. 
Therefore, when an alkane dissolves in an organic solvent, the Van der Waals forces are broken and are replaced by new Van der Waals forces. 
The two processes more or less cancel each other out energetically; thus, there is no barrier to solubility.

Chemical properties of alkanes 


Combustion of alkanes :-

In excess of oxygen O2, alkanes readily undergo combustion producing carbon dioxide gas, water and energy in the form of heat and light.Alkane + Oxygen → Carbon Dioxide gas + Water +  EnergyC4H10(g) + 6½O2(g) → 4CO2(g)  + 5H2O(l) + 2874 KJ mol-1 

The above reaction is the combustion reaction of butane. With the increasing molar mass of straight-chain alkane the energy released increases. Also with the increasing carbon chain length the combustion energy increases. In the absence of sufficient oxygen, alkanes try to undergo incomplete combustion. Incomplete combustion produces water and carbon monoxide or carbon. 

Halogenation :-

Alkanes are less reactive. Without ultraviolet light, they do not react with halogens. With UV light halogenated alkane is produced. It is a substitution reaction in which one or more hydrogen atoms are substituted by halogen atoms.CH3-CH2-CH3 + Br3 →  CH3-CH2-CHBr + HBr .
 propane is reacting with bromine. The general substitution reaction equation can be given as 
R-H +X2 → R-X + H-X
where R is a carbon chain and X is a halogen.

Usages of alkanes :-

• petroleum
• refining crude oil
• natural gas ( heating ,cooling ac houses, oven ,fuel cells ,antifreeze, motor fuel).
• biogas
• renewable diesel 

                              

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