Ethene

CHEMISTRY OF ETHENE
INTRODUCTION
   Molecular formula = C2H4
   Empirical formula = CH2
   Molecular mass = 28
   Empirical formula mass = 14
   Homologous series = alkene
ORBITAL STRUCTURE
OF ETHENE
   COMPOSITION OF ETHENE:
   Ethene molecule consists of two carbon atoms and four H-atoms i.e.    CH2=CH2
   NATURE OF HYBRIDIZATION:
   In ethene molecule each C-atom is Sp2-hybridized. Due to Sp2-hybridization each C-atom generates    three Sp2-hybrid orbitals. In this way there exist six Sp2-hybrid orbital. These Sp2-orbital are arranged in trigonal order and 120o apart. Remaining ‘2pz‘ unhybrid orbital of carbon atom are placed    perpendicular to the plane of Sp2-orbitals.
   SIGMA BOND FORMATION:
   One Sp2-hybrid orbital of C-atom overlaps with One Sp2-hybrid orbital of second C-atom to produce    one sigma bond between two carbon atoms. Remaining two Sp2-orbitals overlap with two H-atom to    produce sigma bond. ‘2pz‘ orbitals of two C-atoms are un-hybridized and make parallel overlapping to    produce pi-bond as shown below:

ethene_orbital

   BOND LENGTH:
   C=C bond length is 1.34A.
   C-H bond length is 1.09A.
ethene

METHODS OF PREPARATION
   BY VICINAL DIHALIDE:
   When vicinal dihalide is heated with zinc dust, two halide atoms are removed from the compound and ethene is formed.
            CH2-CH2 + Zn irreversible_arrow CH2=CH2 + ZnCl2
                    |
              Cl     Cl
 
   BY THE REDUCTION OF ETHYNE:
   Under control conditions, ethyne adds two H-atoms to form ethene in the presence of Nickel(Ni) at    200oC.
             + H2 irreversible_arrow H-C=C-H
                  
   BY THE DEHYDROHALOGENATION OF ALKYL HALIDE:
   When alkyl halide (ethyl chloride) is treated with alcoholic KOH, b-elimination takes place and ethene
    is obtained.
            CH3-CH2Cl + KOH irreversible_arrow CH2=CH2 + KCl + H2O
   BY THE DEHYDRATION OF ALCOHOL:
   When ethyl alcohol is heated with conc.H2SO4 at 170oC, dehydration of alcohol takes place and 
    ethene is formed.
            C2H5OH irreversible_arrow C2H4 + H2O
OR
   Ethyl alcohol may also be converted into ethene by passing vapors of ethyl alcohol over a catalyst    (Al2O3, H3PO4/Al2O3). The reaction is carried out at 350C in the presence of Al2O3 and at 250C
    when H3PO4/Al2O3 is used.
            C2H5OH irreversible_arrow C2H4 + H2O
CHEMICAL PROPERTIES
   ADDITION REACTIONS OF ETHENE:
   Addition of chlorine:
            CH2=CH2 + Cl2 irreversible_arrow CH2Cl-CH2Cl (1,2-dichloroethane)
   Addition of bromine:
            CH2=CH2 +Br2 irreversible_arrow CH2Br-CH2Br (1,2-dibromoethane)
   Addition of iodine:
            CH2=CH2 + I2 irreversible_arrow CH2I-CH2I (1,2-diiodoethane)
   Order of reactivity:
            Cl2 > Br2 > I2
   ADDITION OF HYDROGEN:
            CH2=CH2 + H2 irreversible_arrow CH3-CH3
   ADDITION OF H2SO4:
            CH2=CH2 + H2SO4 irreversible_arrow CH3-CH2-HSO4
   ADDITION OF H2O:
            CH2=CH2 + HOH irreversible_arrow CH3-CH2-OH
   ADDITION OF HCl:
            CH2=CH2 + HCl irreversible_arrow CH3-CH2-Cl
   ADDITION OF HBr:
            CH2=CH2 + HBr irreversible_arrow CH3-CH2-Br
   ADDITION OF HI:
            CH2=CH2 + HI irreversible_arrow CH3+CH2-I
   ADDITION OF HYPOHALOUS ACID:
            CH2=CH2 + HOCl irreversible_arrow OH-CH2-CH2-Cl (Ethane chlorohydrin)
   REACTION WITH CHLORINE WATER:
   When Cl2 is mixed with water, two molecules of acids are formed.
            Cl2 + H2Oirreversible_arrow  HCl + HOCl
            CH2=CH2 + HCl irreversible_arrow CH3-CH2-Cl
            CH2=CH2 + HOCl irreversible_arrow OH- CH2-CH2-Cl
   COMBUSTION REACTION:
            CH2=CH2 + 3O2 irreversible_arrow 2CO2 + 2H2O
   OXIDATION REACTION:
   When ethene is treated with KMnO4 solution, purple colour of KMnO4 disappears due to deformation of a colorless compound “Ethylene Glycol”.
            CH2=CH2 + H2O +[O] irreversible_arrow OH-CH2-CH2-OH
   FORMATION OF MUSTARD GAS:
   b,b-Dichloroethyl Sulphide is commonly known as “Mustard gas”. It is a very poisonous gas. It can    produce blisters on skin.
            2CH2=CH2 + S2Cl2 irreversible_arrow Cl-CH2-CH2-S-CH2-CH2-Cl + S
   POLYMERIZATION:
            n(CH2=CH2) irreversible_arrow n(-CH2-CH2-)