Alkanes – The Saturated Hydrocarbons

Class 11 Chemistry Chapter 13 Hydrocarbons

Alkanes are the saturated hydrocarbon in which carbon and hydrogen also, carbon and carbon are linked with single bond. Among alkanes, methane is the first member of the family. Second member of family is obtained by replacing one hydrogen atom with carbon atom and by joining sufficient amount of carbon atom to fullfill its valence. It is called as Ethane. Thus by using same method other member of alkane family can also be obtained as propane, butane, pentane, hexane, heptane, octane, nonane, decane, etc. the general formula of alkanes can be given as CnH2n+2. Here n is the number of carbon atoms. Series of molecules having same general formula is called homologous series. In normal conditions, hydrocarbons are non reactive or inert in acids, bases or any reagent. Thus alkanes are called as paraffins.

Isomerism of Alkanes

Two or more compounds having same molecular formula but different arrangements of atoms are called isomers. First three alkanes have only one structure. However, higher alkane can have more than one structure. For example butane has two isomers n butane and isobutane. In same way, pentane have three isomers. Difference in property of alkanes is because of their structures. Therefore, such isomers are structural isomers. Isomers can have chain or branched structures. Chain isomers are the structural isomers that differ in chain of carbon atoms.

Primary, Secondary, Tertiary carbon atom

The carbon atom is called primary or 1°, if it is not attached to any other carbon atom in methane or to one carbon atom in ethane. Thus all the terminal atoms are primary if they are attached to single carbon atom. Similarly, the carbon atom attached to two carbon atoms are secondary or 2°. If carbon atom is attached to three other carbon atoms then it is called tertiary 3°. Similarly, if it is attached to four carbon atoms then it is qua-ternary 4°.




Preparation of Alkanes

  • From unsaturated hydrocarbons: Unsaturated hydrocarbons undergo hydrogenation in presence of finely divided catalyst like platinum, palladium or nickel, produces corresponding alkanes. At room temperature platinum or palladium is used as a catalyst. To use nickel as catalyst temperature should be higher about 443 k to 453 k.
                                                 Pt/Pd/Ni
                CH3-CH=CH2 + H2     →      CH3-CH2-CH3
                     Propene                                Propane

  • From alkyl halides: All alkyl halides reacts with zinc and dilute hydrochloric acid to form alkanes. However only alkyl fluorides do not form alkanes.
                                         Zn,H+
                CH3-Cl + H2     →      CH4 + HCl
                 Chloromethane           Methane
    Alkyl halides undergo treatment with sodium metal in dry ether solution gives higher alkanes. This reaction is called Wurtz reaction.

                                                       dry ether
                CH3-Br+ 2Na + BrCH3     →      CH3-CH3 + 2NaBr
                 Bromomethane                             Ethane

  • From Carboxylic Acids: Sodium salt of carboxylic acid is heated with sodium hydroxide in presence of calcium oxide to form alkane. Formed alkane have one carbon atom less than the carboxylic acid. This reaction is also known as decarboxylation.
                                                       Δ,CaO
               CH3COO–Na+ + NaOH     →      CH4 + Na2CO3
                Sodium ethanoate

    The electrolysis of aqueous solution of sodium or potassium salt of carboxylic acid gives alkanes having even number of carbon atoms at anode. This method is known as Kolbe’s electrolytic method.

                                                       Electrolysis
               2CH3COO–Na+ + 2H2O     →     CH3-CH3 + 2CO2 + H2 + 2NaOH
                Sodium ethanoate

Physical properties of alkane

Alkanes are made of C-H and C-C single covalent bond. Due to weak van der waals forces the first four members (C1 to C4) of alkane family are gases. C5 to C17 are liquids whereas from C18 are solids. Alkanes are colourless and odourless compounds. Alkanes are nonpolar. Hence, alkanes are insoluble in water. However, alkanes are soluble in oil, grease which are non polar in nature. With increase in molecular size, van der Waals forces between molecules increase, and therefore the boiling point also increase. It can also be observed that with increase in branching boiling point decreases. With more and more branching the molecule attains the shape of sphere, which have smaller area of contact and therefore smaller van der Waals forces. These forces can be easily overcome by even small rise in temperature and so such molecules have lower boiling point.

Chemical properties of alkane

  • Substitution Reaction: The reaction in which hydrogen atom of alkane is substituted by the other groups are substitution reaction. When hydrogen atom is replaced by halogen then the reaction is known as halogenation. Halogenation, generally takes place either at higher temperature (573k – 773k) or in presence of diffused sunlight or ultraviolet light. However, nitration and sulphonation of alkanes is possible only in case of higher alkanes. For example: chlorination of methane.

                                      hv
                 CH4+Cl2     →      CH3Cl + HCl
                                              Chloromethane

                                           hv
                 CH3-Cl +Cl2     →      CH2Cl2 + HCl
                                                  Dichloromethane

                                             hv
                 CH2-Cl2 +Cl2     →      CHCl3 + HCl
                                                  Trichloromethane

                                           hv
                 CH-Cl3 +Cl2     →      CCl4 + HCl
                                                  Tetrachloromethane

  • Combustion: Heating alkanes in presence of oxygen, undergo oxidation so as to produce carbon dioxide and water releasing large amount of heat .

                                  
                 CH4 + 2O2     →      CO2 + 2H2O           ΔcH⊝= -890 kJ/mol
                                 
    If there is incomplete combustion of alkanes produces carbon black. Carbon black is used in manufacture of ink, printer inks, black pigments and filters.

                                  Incomplete Combustion
                 CH4 + O2     →      C + 2H2O
                                                 

  • Controlled oxidation: Controlled oxidation is helpful in obtaining different product.

                                  Cu/523 K/100atm
                 2CH4 + O2     →      2CH3OH
                                                 Methanol
    Alkanes having tertiary atom can be oxidised using potassium permanganate to form corresponding alcohols
                                  KMnO4, Oxidation
                 (CH3)3CH     →      (CH3)3COH
                2-Methylpropane        2-Methylpropan-2-ol

  • Reaction with steam: Methane reacts with steam at 1273 k in presence of nickel catalyst to form carbon monoxide and dihydrogen. This method is used in industrial preparation of dihydrogen.

                                       Ni,Δ
                 CH4 +H2O     →      CO + 3H2
               

  • Pyrolysis: On heating, higher alkanes undergo decomposition to smaller fragments, forming lower alkanes and alkenes is called pyrolysis or cracking. For example, hexane undergo pyrolysis to form cyclohexane, isobutylene, propene, ethene and methane.

Keywords: Combustion, Halogenation, Pyrolysis, Oxidation, Unsaturated Hydrocarbons, Isomerism

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Comments

  1. Divyesh Choudhary

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