Text Box: Alkanes
Text Box: Contents on Alkane Ø General Ø Methods of preparation Ø Physical properties Ø Chemical properties
 

 

 

 

 


Alkanes are the saturated hydrocarbons containing carbon and hydrogen only. The molecules of the hydrocarbons are characterized by the presence of single covalent linkages between carbon and carbon, and carbon and hydrogen. All the four valencies of carbon atoms are thus completely satisfied and they can take up no more hydrogen or any other atoms. Thus being fully saturated in respect of chemical combination, these are called 'paraffins' (Latin: parum àNO, affinsà affinity).

Alkanes are very stable compounds and do not react with the reagents like potassium permanganate, alkalis and even with the stronger acids under the ordinary conditions because bond fission between C and H is low as:

1. C - C and C-H are strong 'PI' bond.

2.  Low electronegativity difference between C and H

 

 

 

General methods of preparation

 

1.                  Decarboxylation Of Fatty Acids

Cao
 
When sodium salts of fatty acids (RCOONa) are heated with soda-lime  (NaOH+CaO), a molecule of carbon dioxide is lost and an alkane is obtained. This is called DECARBOXYLATION. The alkane obtained contains one carbon atom less than the fatty acids.

               

                R COONa  +  NaO H                   RH  + Na2CO3

                Sod. Salt  of acid                            Alkane   

 

Cao
 
 

Eg:  CH3 COONa + NaO H                   CH4 + Na2CO3

         Sod.acetate                                                Methane

 

 

2.                  Reduction Of Alkyl Halides With Nascent Hydrogen

 Alkyl Halides on treatment with Zn/HCl or Zn/NaOH or Zn-Cu couple on alcohol or LiAlH4 in ether , are reduced to alkanes.

                RX      +    2H                     RH     +     HX

           Alkyl Halide                                Alkane

 

  Eg:  CH3I            +   2H                     CH4     +     HI

      Methyl  Iodide                                                     Methane

         

3.                  Hydrogenation Of Unsaturated HydroCarbons

When the mixture of alkene or alkyne is passed over heated Nickel around temperature of                 250- 300oC, alkanes are formed.

 

I)            CnH2n    +    H2                     CnHn+2

              Alkene                                                     Alkane                 

 

             CH2=CH2  +  H2                      CH3.CH3

               Ethene                                                     Ethane

 

II)          CnH2n-2 +  2H2                       CnH2n+2

               Alkyne                                                     Alkane

       

              CHºCH +   2H2                      CH3.CH3

              Ethyne                                                         Ethane

 

However methane cannot be prepared by this method.

               

4.                  By the Reduction of Alcohols

Reduction of alcohols may be carried out by heating them with hydroiodic acid and phosphorous at 150oC.

       

                ROH    +   2HI                    RH  + H2O +I2

                Alcohol                                                Alkane

 

                CH3CH2OH + 2HI                  CH3CH3 + H2O + I2

                 Ethyl Alcohol                                           Ethane

 

5.                  By the reduction of Aldehydes and Ketones

Like alcohols , their reduction may also be carried heating with conc. Hydroiodic acid at 150oC , under pressure in the presence of Red Phosphorus.

 

                RCHO   +   4HI                   RCH3 + H2O + 2I2

                Aldehyde                                               Alkane

               

                CH3CHO + 4HI                     CH3CH3 + H2O +2I2

                Acetaldehyde                                             Ethane

 

                RCOR + 4HI                       RCH2R + H2O + 2I2

                 Ketone                                                  Alkane

               

                CH3CHOCH3 + 4HI               CH3CH2CH3 + H20 +2I2

                   Acetone                                                 Propane

 

 6.           By the reduction of the carboxylic acids

Fatty acids when reacted with HI in the presence  of red phosphorus at 150oC in a sealed tube yield alkanes.

 

                RCOOH  +  6HI                    RCH3  + 2H2O + 3I2

                 Acid                                                      Alkane

               

                CH3COOH + 6HI                  CH3CH3 + 2H2O + 3I2  

                 Acetic acid                                              Ethane

 

7.          From Grignard Reagents

Grignard's reagents is Alkyl Magnesium halide which is obtained by treating alkyl halide with Magnesium metal in the presence of carbon tetra chloride as solvent.

When Grignard's reagent is boiled with water it undergoes hydrolysis to give alkane.                  

boil                             

                 R-MgX + H-OH                    RH+ Mg(OH)X  

                                                                       

                                                boil

                CH3MgBr   + HOH                   CH4 + Mg(OH)Br

 

 

Physical properties

 

1.     The first four hydrocarbons methane ,ethane , propane and butane are colourless and odourless gases. The next eleven members of the series (C5H12 TO C15H32) are volatile liquids and the rest of them are solids.

2.     Their boiling points and specific gravities rise with increasing molecular masses. Branched chain hydrocarbons have lower boiling points than corresponding straight chain ones.

3.     Their melting points go on incresing with increase in their molecular masses

4.     They are insoluble in water but soluble in organic solvents like alcohol , ether , acetone etc.

Chemical properties

 

          Undoubtedly alkanes are relatively inert chemically . They do not react with strong acids , alkalis and oxidising agents under the ordinary conditions. But modern researches have shown that they gives many reactions under the drastic conditions.The general reactions of the alkanes are given below:

       

a.     Halogenation

The replacement of one or more hydrogen atoms by halogens is called the halogenation. These reactions occur slowly in darkness but more rapidly in sunlight.

Chlorine and bromine react with alkanes with considerable ease in the presence of sunlight or by the use of a catalyst like iron fillings.

When the hydrogen of latter are replaced in turn to form four different halogen derivatives.

       

        CH4  +  Cl2                    CH3Cl  +  HCl

                                       Methyl chloride

 

        CH3Cl  +  Cl2                   CH2Cl2  +  HCl

                                        Methylene Chloride

 

        CH2Cl2 +  Cl2                   CHCl3  +  HCl

                                           Chloroform

 

        CHCl3  + Cl2                    CCl4    +  HCl

                                      Carbon Tetra chloride

 

Iodine does not react with alkanes as the HI produced during iodination reduces the alkyl iodide back to the hydrocarbon.

 

        RH  +  I2                     RI  +  HI  

 

To  make the reaction move forward , it is carried out in the presence of an oxidising agents like conc.  Nitric acid or iodic acid which oxidises the hydroiodic acid to iodine.

       

                5HI  +  HIO3                  3I2  + 3H2O

 

b.    Nitration

The replacement of hydrogen of alkane by nitro group (NO2 Group) is known as Nitration . Nitration occurs in vapour Phase. Alkanes may be converted into nitrodervatives by heating the hydrocarbons with the vapours of the nitric acid at the temperature of about 450oC.

                       

                RH  + HO-NO2                   RNO2  +  H2O

                                                      Nitro alkane

 

                CH3CH3 + HO-NO2                  CH3CH2NO2  +  H2O

                   Ethane                                  Nitro ethane

       

c.     Sulphonation

It is a process in which a hydrogen atom of an alkane is replaced by a sulphonic acid group, S03H.  Alkanes from hexane onwards when treated with fuming sulphuric acid, undergo sulphonation yielding sulphonic acid. Thus:

 

C6H11-H  + HO-SO3H                        C6H11SO3H  +  H2O

 

The lower gaseous hydrocarbons are more stable and dissolve in sulphuric acid and are not sulphonated.

 

d.    Oxidation

I)  Normal alkanes are not oxidised by potassium permanganate  but it readily oxidises a tertiary hydrogen atoms to a hydroxyl group. Thus isobutane is oxidised to a tertiary butanol.

(CH3)3CH +[O]                    (CH3)3C.OH

  Isobutane                              Tertiary Butanol

 

II) Lower hydrocarbons when heated with limited supply of air at 350 –500oC, yield the corresponding aldehydes .thus

 

CH4 + O2                    HCHO  +  H2O

            Formaldehyde

Higher alkane on slow oxidation yield long chain of  fatty  acids.

 

CH3(CH2)9CH3  +  O2                       CH3(CH2)9.COOH

     Undecane                                          Undecanoic acid

 

III) When burnt in air or oxygen, alkanes are completely oxidised to carbon dioxide and water.

CH4  +  2O2                    CO2  +  2H2O