Chapter 11 Reaction of Alkanes

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					                          Chapter 11 Reaction of Alkanes

Alkanes:
General formula of alkanes is CnH 2n+2.
n=1CH4 Meth + ane, Methane
n=2 C2H6 Eth + ane , Ethane
n=3 C3H8 Prop + ane, n-Propane
n=4 C4H10 but + ane, n-Butane
n=5 pent + ane, n-Pentane

Bonding in alkanes:
Carbon has sp3 hybridization in alkanes. The geometry of carbon is tetraheral with 1090angle.

Structural isomers in alkanes:
Structural isomers have same formula but different connections between atoms.They have
different physical and chemical properties
Example: Pentane (C5H12) has three isomers:
                         n-Pentane          Isopentane       Neopentane




Cycloalkanes:
Monocyclic compounds:
They have general formula CnH2n :




Cyclo propane     Cyclo butane       Cyclo pentane    Cyclo Hexane




                                          3-ethyl-1,1-dimethylcyclohexane




Bicyclic compounds:Compounds containing two fused or bridged rings as bicycloalkanes. We
use the name of the alkane corresponding to the total number of carbon atoms in the rings as the
parent name. We name the compound based on following rule:
Bicyclo [# of carbon atoms in one side of bridge (bigger number) + # of carbon atoms in other
side of bridge + number of carbons on the bridge] + name of parent alkane Example:
                           Chapter 11 Reaction of Alkanes




Bicyclo[2.2.1]heptane                           Bicyclo[2.2.2]octane




 Bicyclo[3.2.1]octane               Bicyclo[3.2.0]heptane


Primary, Secondary and Tertiary carbon:
If a carbon is attached to one other carbon is called primary carbon.If the carbon is attached to
two other carbons is called secondary carbon.
If a carbon atom is attached to three other carbon is called tertiary carbon.
Primary, secondary and tertiary carbons have different reactivity in a chemical reaction,so it’s
important to identify each carbon as primary, secondary or tertiary carbon.

Physical properties of Alkanes and cycloalkanes:
1.Boiling point:
Boiling point of hydrocarbon depend on attractive forces between molecules.
Attractive forces between molecules are dipole-dipole and london dispersion.
Most hydrocarbons are nonpolar, so we just consider dispersion forces most of the time.

1.With increasing molecular weight of hydrocarbon, london dispersion force increases. So
boiling point of hydrocarbon goes up.
2.Branched hydrocarbons have lower boiling point than unbranched hydrocarbons. branch
prevents molecules come close to each other, so attractive forces become less and boiling point
goes down.

Solubility in water:
Alkanes are insoluble in water. Water is a polar substance and hydrocarbon is nonpolar, this
called hydrophobic effect.

Reaction of alkanes: Alkanes are quite unreactive compounds, because all covalent bonds in
alkanes are single bond ( ) and nonpolar. Under certain conditios, however, alkanes do react
with oxygen:
                               Chapter 11 Reaction of Alkanes

1. Combustion (oxidation) of alkanes:
CH4 + 3/2O2                         CO2                 + H2O + Heat

The heat released is called heat of combustion, H0.
 Heat of combustion increases with increasing number of carbons.
 Unbranched alkanes have higher heat of combustion than branched alkanes.

      2.Halogenation of Alkanes: If we mix methane with chlorine or bromine in the dark at room
      temperature, nothing happens. If we heat the mixture to 1000C or higher or expose it to light,
      a reaction begins at once:
                         heat or light
         CH4 + Cl2                                      CH3Cl + HCl



Mechanism of Halogenation of Alkanes: R-H + X2                                       R-X + H-X

Free Radical Mechanism:

1. Cl - Cl                           2Cl.

2. Cl.    +   H – CH3                       H-Cl + .CH3

3. Cl-Cl + .CH3                          CH3Cl + HCl




Stability of Free Radicals:

Free radicals, like carbocations need electron, so they are stabilized by substituents, such as alkyl
groups, that release electrons. Tert-radicals are most stable free radicals.


  •                                                 •                        H
                           •                    H         H                  •
                                H                                        H       H


Bond Dissociation Energy (BDE) :

 Homolytic bond cleavage                 R-H                     R +H

Heterolytic bond cleavage:          X-Y                       X+ + :Y-

The energy required for homolytic bond cleavage is called bond dissociation energy. More stable
radicals have less BDE.
                              Chapter 11 Reaction of Alkanes



        H

                                          •

                                                          H = +380 KJ


                H

                                               •
                                                            H = + 410 KJ


Relative Rate (chlorination):           R3CH        R2CH2        RCH3
                                         Tertiary    Secondry      Primary
                                           5.2        3.9             1
Relative Rate (bromination)              1640         82               1

                                   .
Allylic free radicals: C = C – C

           Allylic bonds are often weaker and are easily broken, for example compare the bond
            dissociation energies:




Allylic radicals are stabilized by resonance




Stability of free radicals:

Allylic > Tert > Sec > Pri > Methyl
                          Chapter 11 Reaction of Alkanes

Example of Allylic free radicals:




Benzylic and Allylic Free Radicals:

Formulas for the allyl and benzyl radicals are shown below..




Example of Benzylic free radical Hologenation of Alkylbenzenes:




      When treated with Br2 or Cl2 radical substitution of benzylic-H generates the benzyl
       halide and HX.
      The benzyl radical is quite stable so bromination will often by selective for the benzyl
       position.
      NBS is an alternate source for bromine.
      Reaction proceeds via a radical chain mechanism similar to that of alkanes.
                        Chapter 11 Reaction of Alkanes

Radical addition of HBr to Alkenes:




this is often known as the peroxide effect or anti-Markovnikov addition.

				
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