Monday, November 11, 2013

Aliphatic Compounds

      Aliphatic compounds are open-chain or acyclic compounds. [ Greek - aliphos ( fat, oil ) ]                      

Acyclic meaning:        
          Acyclic compound with a linear structure, rather than a cyclic one. An open-chain compound having no side chains.
          Aliphatic compounds are otherwise known as non-aromatic compounds. Aliphatic compounds can be saturated joined by single bonds (alkanes) or unsaturated with double bonds (alkenes) or triple bonds (alkynes).
              In organic chemistry alkane or paraffin is a saturated hydrocarbon. Alkanes contain only two elements carbon and hydrogen. In alkane all of the carbon-carbon bonds are single bonds. Each carbon atom forms four bonds and each hydrogen atoms form a single bond. Alkane general chemical formula   CnH2n+2 .
Methane :
     Paraffins are the saturated hydrocarbons. Main source of paraffins is mineral oil or petroleum. The simplest paraffin is methane CH4. Methane contain one atom of carbon and four atoms of hydrogen. It is a alkane mainly occurs in natural gas. Methane is a product of organic decay in swamps and marshes, the gas obtained by the action of bacteria, this method of formation of natural gas given rise to the name "marsh-gas" for methane.
Structure of methane:

        Methane is a tetrahedral molecule with 4 equivalent C - H bonds. The molecular formula of methane is CH4.   SP3  hybridization.
Properties of methane:
  • Water insoluble.
  • Methane is a colourless, odourless and non-poisonous gas.
  • Methane is quite soluble in ethanol and ether.
  • Melting point - 90.7 K
  • Boiling point - 109 - 113 K.
  • Liquid methane does not burn unless subjected to high pressure.
  • Gas at room temperature.
  • It burns with a non-luminous flame in air or oxygen, forming carbon dioxide and water.

                            CH4     +     2O2     →     CO2     +     2H2O
Substitution reaction of methane:
                 In a substitution reaction, a functional group in a particular chemical compounds is replaced by another group.
                A good example of a substitution reaction is the photochemical chlorination of methane forming methyl chloride.
                            CH4     +     Cl2     →     CH3Cl     +     HCl
                                                              (methyl chloride)
                A mixture of  methane and chlorine is exposed to sunlight, that time substitution reaction occurs. But the reaction doesn't stop here, all four hydrogen atoms in methane are successively replaced by chlorine atoms.
                           CH4     +     Cl2     →     CH3Cl     +     HCl
                                                           (methyl chloride)
                           CH3Cl     +     Cl2     →     CH2Cl2     +     HCl
                                                            (Methylene chloride)
                           CH2Cl2     +     Cl2     →     CHCl3     +     HCl
                           CHCl3     +     Cl2     →     CCl4     +     HCl
                                                             (carbon tetrachloride)
             Methane react with chlorine, the hydrogen atoms are replaced, and chlorine atoms take their place. This type of reaction is known as substitution reaction. The atom or group that has replaced the hydrogen atom is called the substitution and when a substituent atom or group is replaced by some other atom or group, the reaction is referred to as a replacement reaction. It should be noted that in substitution or replacement reaction there is no change in structure.
             Methane reacts with bromine in the presence of UV light to form bromomethane.
                         CH4     +     Br2     →     CH3Br     +     HBr
                                                         (bromo methane)
             Methane reacts explosively with gaseous fluorine. The initial reaction is possible.
                         CH4     +     2F2     →     C     +     4HF
Step 1 :
Initiation Step :
UV light breaking a bromine molecules into free radicals.
                            Cl - Cl      →     Cl●     +     ●Cl
                            [Cl2       →     2Cl●]
Step II
Propagating Steps :
                          CH4     +     Cl●     →     CH3●     +     H-Cl 
                          CH3●     +     Cl2     →     CH3Cl     +     Cl● 
Step III
Terminating Steps:
                         Cl●     +     Cl●             →     Cl2
                         Cl●     +      CH3●        →     CH3Cl
                        CH3●     +     CH3●      →     CH3CH3

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