Alkanes, Nomenclature and Structure of alkanes

 

Alkanes:

  Alkanes are the simplest organic compounds made of carbon and hydrogen only, first member methane (OR,) have the structural formula.

and its molecule is built of one carbon atom joined to four hydrogen

atoms by covalent bonds. Other hydrocarbons of this series: ethane, propane, butane

                Ethane                      Propane                     Butane

Saturated hydrocarbons:

  Since all the four valencies of the carbon atoms in these compounds are fully satisfied, they can take up no more hydrogen or any other atoms. Thus being 'fully saturated' in respect of chemical combination, these hydrocarbons are classed as Saturated hydrocarbons.

  Due to less reactivity, alkanes are also called Paraffins (Latin, parum affinis=little affinity).

Structure of alkanes:

 Hybridization of alkanes:
     Alkanes are Sp3 hybridized, so they have tetra hedral structures

Angles:
  There is 109.5^◦ angle  in alkanes between carbon­­-hydrogen  bond
Bond length:
   1-09A^◦ (C H)

   1.54A^◦ (C C)

 

Nomenclature:

Alkanes form a homologous series having the general formula C_n H2n+2. There are two systems of naming these Hydrocarbons.

        (1) The common System          (2) IUPAC System
(1) The common System:

  The first four members of the series are Methane, Ethane, Propane and Butane. The remainings are:

       

The alkanes having a straight or normal chain are called normal (or 'n') hydrocarbons. This is indicated by prefixing n- to the name of the alkane.

                                                 

The alkanes in which the carbon chain has branches are called branched chain hydrocarbons. Such alkanes as posses a one carbon branch on the second carbon of the normal or basic chain, are referred to as Iso hydrocarbons

e.g.

                                                  

When the normal carbon chain present in the molecule has two  carbon branches on the second carbon from the end, the alkane is referred to as Neo hydrocarbon.

                                               

Types of Carbons atoms in Alkanes:

   The structural formulas of alkanes and their derivatives contain four types of carbon atoms

(I) Primary (1°) carbon atom which is bonded to one (or no other carbon atom.)

(3) Secondary (2°) carbon atom which is bonded to two other carbon atoms.

(3) Tertiary (3°) carbon atom which is bonded to three other carbon atoms.

(4) Quaternary (4°) carbon atom which is bonded to four other carbon atoms.

 

Naming of Alkyl radicals:

A group of atoms that has an unused valency (represented by dash) is called a radical. The radicals derived from alkanes (R-H) by removal of one H atom are called Alkyl radical (R - ). The names of the alkyl radicals corresponding to normal and isoalkanes are obtained by simply changing the suffix ·ane to -yl.

ISOMERISM IN ALKANES:

     The first three hydrocarbons of the series, methane, ethane and propane do not exhibit isomerism. The next hydrocarbons butane (C4H10) exists in two isomeric forms:     

All higher hydrocarbons show chain isomerism and the number of isomers goes on increasing rapidly with the increase in the number of carbon atoms. Thus:

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