Alkanes

The alkanes (i.e. the paraffins) are the simplest homologous series of organic compounds of hydrogen and carbon, where all atoms are linked by single bonds.

The general formula for the alkane series of hydrocarbons is CnH2n+2. The alkanes are said to be saturated because the maximum number of bonds are formed between each carbon atoms and its neighboring carbon and hydrogen atoms. The lack of any multiple bonds (i.e. double-bonds or triple-bonds) in the alkanes explain the relative chemical inertness of this series of hydrocarbons.


Methane		CH4 		CH4 
Ethane		C2H6 		CH3CH3 
Propane		C3H8 		CH3CH2CH3 
Butane		C4H10 		CH3CH2CH2CH3 
Pentane		C5H12 		CH3CH2CH2CH2CH3 
Heptane		C6H14		CH3CH2CH2CH2CH2CH3 
Hexane 		C7H16 		CH3CH2CH2CH2CH2CH2CH3 
Octane		C8H18 		CH3CH2CH2CH2CH2CH2CH2CH3 
Nonane 		C9H20 		CH3CH2CH2CH2CH2CH2CH2CH2CH3
Decane 		C10H22 		CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
The shapes of the alkanes shows that all bonds on the carbon atoms are identical and that the bond angles are close to 109 degrees. Thus, each carbon atom is at the center of a tetrahedral structure, with either carbon atoms or hydrogen atoms at the apices of the tetrahedron. All bonds between carbon atoms (i.e. C-C bonds), or between a carbon atom and hydrogen (i.e. C-H bonds), are s bonds (sigma bonds). These bonds are formed by the end-on overlap of sp3 hydride orbitals of the carbon atoms.


Alkanes Linear Structure

The alkane with the simplest structure is methane, CH4. Each succeeding member of the alkane series has a further methylene group, -CH2-, in the chain. The methane molecule consists of a central carbon atom, surrounded by four hydrogen atoms, with a tetrahedral shape. This tetrahedral structure of methane indicates that the geometry of the orbitals of the carbon in methane molecule, on which the geometry of the bonds depend, is significantly different from the structure of the orbitals in the carbon atom. This is due to hybridisation of four atomic orbitals in carbon atom to four identical molecular orbitals in methane molecule. These molecular orbitals are called SP3 Hybrid Orbitals. The single covalent bonds joining carbon to carbon and carbon to hydrogen in alkanes are sigma bonds formed by the end-on overlap of electron orbitals, so that each carbon atom is the center of a tetrahedron. There is free rotation about all bonds in alkanes.



	Methane CH4             CH4
	Ethane  C2H6            CH3CH3
	Propane C3H8            CH3CH2CH3
	Butane  C4H10           CH3CH2CH2CH3
	Pentane C5H12           CH3CH2CH2CH2CH3
	Heptane C6H14           CH3CH2CH2CH2CH2CH3
	Hexane  C7H16           CH3CH2CH2CH2CH2CH2CH3
	Octane  C8H18           CH3CH2CH2CH2CH2CH2CH2CH3        
	Nonane  C9H20   	CH3CH2CH2CH2CH2CH2CH2CH2CH3     
	Decane  C10H22		CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3


Alkanes Physical Properties


Name		State		Formula	    	MP degC	BP degC	Density(g/ml) 
  
====		=====		=======	    	=======	=======	=======
Methane		Gas		CH4		-183	-162	0.42
Ethane		Gas		C2H6		-172	-89	0.55
Propane		Gas		C3H8		-190	-45	0.58
Butane		Gas		C4H10		-135	-0.5	0.58
Pentane		Liquid		C5H12		-130   	36	0.63
Hexane		Liquid		C6H14		- 95   	69	0.66
Heptane		Liquid		C7H16		- 91   	98	0.68


Alkanes Structural Isomerism

Structural isomerism results from the fact that for a given formula, the carbon and hydrogen atoms can be arranged in a number of different ways while retaining the normal combining power of each element. For example, in the case of butane, C4H20, the atoms can be arranged in two different ways, yielding n-butane and iso-butane.

Because carbon has a valency of four, and because the carbon to carbon and the carbon to hydrogen covalent bonds are almost identical chemically, it is possible to arrange the linkages between the atoms in an alkane in many different ways. This leads to the concept of structural isomerism, where the isomers have identical formulae but different structural arrangements. Only one arrangement of the atoms within the molecule is possible for methane, CH4, ethane, CH3CH3, and propane, CH3CH2CH3. However, starting with butane, CH3CH2CH2CH3, alternative arrangements of the atoms is possible. The prefix n- (for normal) and iso- (for isomeric) is affixed to the name to distinguish between the isomers.


	n-Butane        C4H20           CH3CH2CH2CH3    

	Iso-Butane      C4H20           CH3CHCH3        
				           CH3       

The number of possible isomers increases rapidly as the number of carbons increases. Pentane has 3 isomers, and were known as


n-Pentane       C5H22           CH3CH2CH2CH2CH3 
				(Structure I)   

iso-Pentane     C5H22           CH3CHCH2CH3                                                           CH3       
				(Structure II)  

			           CH3       
neo-Pentane     C5H22           CH3CCH3                                                               CH3       
				(Structure III) 

Structure I is an unbranched chain alkane. Structures II and III are branched chain alkanes. Because the number of structural isomers increases rapidly with increasing number of carbon atoms in a molecule, a systematic method of naming the organic compounds has been promulgated by the International Union of Pure and Applied Chemistry, IUPAC.

In structure I, the longest continuous chain contains four carbon atoms and thus this compound is a pentane.

In structure II, the longest continuous chain contains four carbon atoms and thus this compound is a butane. As this also has a methyl group attached to the second carbon atom; it is therefore called 2-Methylbutane.

In structure III, the longest continuous chain contains three carbon atoms and thus this compound is a propane. As this also has two methyl group attached to the second carbon atom; it is therefore called 2,2-Dimethylpropane. Note that in this molecule, the central carbon atom is bonded to four carbon atoms, and the twelve hydrogen atoms are attached to the other four carbon atoms in the molecule.


Pentanes IUPAC Names


Old Name                Structure		IUPAC Name      
========		=========		==========

n-Pentane               CH3CH2CH2CH2CH3 	Pentane 

iso-Pentane     	CH3CHCH2CH3            	2- methylbutane
			   CH3       

			   CH3       
neo-Pentane     	CH3CCH3   		2,2-dimethylpropane
			   CH3


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