Alkanes undergo the following reactions under certain conditions.
1. `color{green}("๐๐ฎ๐๐ฌ๐ญ๐ข๐ญ๐ฎ๐ญ๐ข๐จ๐ง ๐ซ๐๐๐๐ญ๐ข๐จ๐ง๐ฌ:")`
One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane is given below:
`color{green}("๐๐๐ฅ๐จ๐ ๐๐ง๐๐ญ๐ข๐จ๐ง")`
`color{red}(CH_4 + Cl_2 overset(hv)โ underset("Chloromethane")(CH_3Cl) + HCl)` ...................(13.10)
`color{red}(CH_3Cl + Cl_2 overset(hv)โ underset("Dichloromethane")(CH_2Cl_2) + HCl)` ..........(13.11)
`color{red}(CH_2Cl_2 + Cl_2 overset(hv)โ underset("Trichloromethane")(CHCl_3) + HCl)` ................(13.12)
`color{red}(CHCl_3 + Cl_2 overset(hv)โ underset("Tetrachloromethane")(C Cl_4) + HCl)` ...............(13.13)
`color{red}(CH_3- CH_3+Cl_2 overset(hv)โ underset("Chloroethane")(CH_3 - CH_2 Cl) + HCl)` ...............(13.14)
โข It is found that the rate of reaction of alkanes with halogens is `color{red}(F_2 > Cl_2 > Br_2 > I_2).`
โข Rate of replacement of hydrogens of alkanes is : `color{red}("3ยฐ > 2ยฐ > 1ยฐ")`.
โข Fluorination is too violent to be controlled.
โข Iodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like `color{red}(HIO_3)` or `color{red}(HNO_3)`.
`color{red}(CH_4 + I_2 โ CH_3I + HI)` .....................(13.15)
`color{red}(HIO_3 + 5HI โ 3I_2 + 3H_2O)` .................(13.16)
`color{green}("๐๐๐๐๐๐๐๐๐ ๐๐
๐๐๐๐๐๐๐๐๐๐๐๐:")`
(i) `color{green}("๐๐ง๐ข๐ญ๐ข๐๐ญ๐ข๐จ๐ง :")` The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The `color{red}(ClโCl)` bond is weaker than the `color{red}(CโC)` and `color{red}(CโH)` bond and hence, can be easily broken.
`color{red}(Cl - Cl underset("homolysis") overset(hv)โ underset("Chlorine free radicals")(overset(โข)Cl + overset(โข)Cl))`
(ii) `color{green}("๐๐ซ๐จ๐ฉ๐๐ ๐๐ญ๐ข๐จ๐ง :")` Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the `color{red}(C-H)` bond to generate methyl free radical with the formation of `color{red}(H-Cl)`.
(a) `color{red}(CH_4 + overset(โข)Cl overset(hv)โ overset(โข)CH_3 + H - Cl)`
(b) `color{red}(overset(โข)CH_3 +Cl - Cl overset(hv)โ underset("Chlorine free radical" )(CH_3 - Cl + overset(โข)Cl))`
The methyl radical thus obtained attacks the second molecule of chlorine to form `color{red}(CH_3 โ Cl)` with the liberation of another chlorine free radical by homolysis of chlorine molecule. The chlorine and methyl free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are those which directly give principal products, but many other propagation steps are possible.
(iii) `color{green}("๐๐๐ซ๐ฆ๐ข๐ง๐๐ญ๐ข๐จ๐ง:")` The reaction stops after some time due to consumption of reactants and / or due to the following side reactions :
The possible chain terminating steps are :
(a) `color{red}(overset(โข)Cl + overset(โข)Cl โ Cl - Cl)`
(b) `color{red}(H_3 overset(โข)C + overset(โข)CH_3 โ H_3C - CH_3)`
(c) `color{red}(H_3 overset(โข)C + overset(โข)Cl โ H_3C - Cl)`
Though in (c), `color{red}(CH_3 โ Cl)`, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.
๐. `color{green}("๐๐จ๐ฆ๐๐ฎ๐ฌ๐ญ๐ข๐จ๐ง:")`
Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
`color{red}(CH_4(g) + 2O_2(g) โ CO_2(g) +2H_2O (l) ; Delta_cH^(โ) = -890 kJ mol^(-1))` .........(13.17)
`color{red}(C_4H_(10) (g) + 13/2 O_2 (g) โ 4CO_2 (g) + 5H_2O(l) ; Delta_cH^(โ) = -2875.84 kJ mol^(-1))` .........(13.18)
The general combustion equation for any alkane is :
`color{red}(C_n H_(2n+2) + ((3n+1)/2) O_2 โ nCO_2 + (n+1) H_2O) ` .............(13.19)
Due to the evolution of large amount of heat during combustion, alkanes are used as fuels. During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters.
`color{red}(CH_4(g) + O_2(g) underset("combustion") overset("Incomplete")โ C(s) + 2H_2O (l))` ....................(13.20)
๐. `color{green}("๐๐จ๐ง๐ญ๐ซ๐จ๐ฅ๐ฅ๐๐ ๐จ๐ฑ๐ข๐๐๐ญ๐ข๐จ๐ง:")`
Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.
(i) `color{red}(2CH_4 + O_2 overset("Cu / 523K / 100 atm")โ underset("Methanol")(2CH_3OH))` ................(13.21)
(ii) `color{red}(CH_4 + O_2 underset(Delta) overset(Mo_2O_3)โ underset("Methanal")(HCHO) + H_2O)` .............(13.22)
(iii) `color{red}(2CH_3CH_3 + 3O_2 overset((CH_3COO)_2 Mn)โ underset("Ethanoic acid")(2CH_3COOH)+2H_2O)` .................(13.23)
(iv) Ordinarily alkanes resist oxidation but alkanes having tertiary `color{red}(H)` atom.
`color{red}(underset("2-Methylpropane")((CH_3)_3CH) underset("Oxidation") overset(KMnO_4)โ underset("2-Methylpropan-2-ol")((CH_3)_3COH))` ..............(13.24)
๐ . `color{green}("๐๐ฌ๐จ๐ฆ๐๐ซ๐ข๐ฌ๐๐ญ๐ข๐จ๐ง:")`
n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.
`color{red}(underset("n - Hexane")(CH_3(CH_2)_4 CH_3) overset(" Anhy ." AlCl_3 // HCl)โunderset("2-Methylpentane")(CH_3 underset(underset(CH_3)(|))CH - (CH_2)_2 - CH_3) + underset("3-Methylpentane")(CH_3CH_2 - underset(underset(CH_3)(|))CH- CH_2 - CH_3))` ................(13.25)
๐. `color{green}("๐๐ซ๐จ๐ฆ๐๐ญ๐ข๐ณ๐๐ญ๐ข๐จ๐ง:")`
`n`-Alkanes having six or more carbon atoms on heating to `773K` at `10-20` atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as ๐๐๐๐๐๐ก๐๐ง๐๐ก๐๐๐ or ๐๐๐๐๐๐๐๐๐.
๐. `color{green}("๐๐๐๐๐ญ๐ข๐จ๐ง ๐ฐ๐ข๐ญ๐ก ๐ฌ๐ญ๐๐๐ฆ:")`
Methane reacts with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen. This method is used for industrial preparation of dihydrogen gas.
`color{red}(CH_4 + H_2O underset(Delta) overset("Ni")โ CO + 3H_2)` .............(13.27)
๐. `color{green}("๐๐ฒ๐ซ๐จ๐ฅ๐ฒ๐ฌ๐ข๐ฌ:")`
Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.
Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, dodecane, a constituent of kerosene oil on heating to 973K in the presence of platinum, palladium or nickel gives a mixture of heptane and pentene.
`color{red}(underset("Dodecane")(C_(12)H_(26)) underset(973K)overset("Pt/Pd/Ni")โ underset("Heptane")(C_7 H_(16)) + underset("Pentene")(C_5H_(10)) + " other products")` .................(13.29)
Alkanes undergo the following reactions under certain conditions.
1. `color{green}("๐๐ฎ๐๐ฌ๐ญ๐ข๐ญ๐ฎ๐ญ๐ข๐จ๐ง ๐ซ๐๐๐๐ญ๐ข๐จ๐ง๐ฌ:")`
One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane is given below:
`color{green}("๐๐๐ฅ๐จ๐ ๐๐ง๐๐ญ๐ข๐จ๐ง")`
`color{red}(CH_4 + Cl_2 overset(hv)โ underset("Chloromethane")(CH_3Cl) + HCl)` ...................(13.10)
`color{red}(CH_3Cl + Cl_2 overset(hv)โ underset("Dichloromethane")(CH_2Cl_2) + HCl)` ..........(13.11)
`color{red}(CH_2Cl_2 + Cl_2 overset(hv)โ underset("Trichloromethane")(CHCl_3) + HCl)` ................(13.12)
`color{red}(CHCl_3 + Cl_2 overset(hv)โ underset("Tetrachloromethane")(C Cl_4) + HCl)` ...............(13.13)
`color{red}(CH_3- CH_3+Cl_2 overset(hv)โ underset("Chloroethane")(CH_3 - CH_2 Cl) + HCl)` ...............(13.14)
โข It is found that the rate of reaction of alkanes with halogens is `color{red}(F_2 > Cl_2 > Br_2 > I_2).`
โข Rate of replacement of hydrogens of alkanes is : `color{red}("3ยฐ > 2ยฐ > 1ยฐ")`.
โข Fluorination is too violent to be controlled.
โข Iodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like `color{red}(HIO_3)` or `color{red}(HNO_3)`.
`color{red}(CH_4 + I_2 โ CH_3I + HI)` .....................(13.15)
`color{red}(HIO_3 + 5HI โ 3I_2 + 3H_2O)` .................(13.16)
`color{green}("๐๐๐๐๐๐๐๐๐ ๐๐
๐๐๐๐๐๐๐๐๐๐๐๐:")`
(i) `color{green}("๐๐ง๐ข๐ญ๐ข๐๐ญ๐ข๐จ๐ง :")` The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The `color{red}(ClโCl)` bond is weaker than the `color{red}(CโC)` and `color{red}(CโH)` bond and hence, can be easily broken.
`color{red}(Cl - Cl underset("homolysis") overset(hv)โ underset("Chlorine free radicals")(overset(โข)Cl + overset(โข)Cl))`
(ii) `color{green}("๐๐ซ๐จ๐ฉ๐๐ ๐๐ญ๐ข๐จ๐ง :")` Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the `color{red}(C-H)` bond to generate methyl free radical with the formation of `color{red}(H-Cl)`.
(a) `color{red}(CH_4 + overset(โข)Cl overset(hv)โ overset(โข)CH_3 + H - Cl)`
(b) `color{red}(overset(โข)CH_3 +Cl - Cl overset(hv)โ underset("Chlorine free radical" )(CH_3 - Cl + overset(โข)Cl))`
The methyl radical thus obtained attacks the second molecule of chlorine to form `color{red}(CH_3 โ Cl)` with the liberation of another chlorine free radical by homolysis of chlorine molecule. The chlorine and methyl free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are those which directly give principal products, but many other propagation steps are possible.
(iii) `color{green}("๐๐๐ซ๐ฆ๐ข๐ง๐๐ญ๐ข๐จ๐ง:")` The reaction stops after some time due to consumption of reactants and / or due to the following side reactions :
The possible chain terminating steps are :
(a) `color{red}(overset(โข)Cl + overset(โข)Cl โ Cl - Cl)`
(b) `color{red}(H_3 overset(โข)C + overset(โข)CH_3 โ H_3C - CH_3)`
(c) `color{red}(H_3 overset(โข)C + overset(โข)Cl โ H_3C - Cl)`
Though in (c), `color{red}(CH_3 โ Cl)`, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.
๐. `color{green}("๐๐จ๐ฆ๐๐ฎ๐ฌ๐ญ๐ข๐จ๐ง:")`
Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
`color{red}(CH_4(g) + 2O_2(g) โ CO_2(g) +2H_2O (l) ; Delta_cH^(โ) = -890 kJ mol^(-1))` .........(13.17)
`color{red}(C_4H_(10) (g) + 13/2 O_2 (g) โ 4CO_2 (g) + 5H_2O(l) ; Delta_cH^(โ) = -2875.84 kJ mol^(-1))` .........(13.18)
The general combustion equation for any alkane is :
`color{red}(C_n H_(2n+2) + ((3n+1)/2) O_2 โ nCO_2 + (n+1) H_2O) ` .............(13.19)
Due to the evolution of large amount of heat during combustion, alkanes are used as fuels. During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters.
`color{red}(CH_4(g) + O_2(g) underset("combustion") overset("Incomplete")โ C(s) + 2H_2O (l))` ....................(13.20)
๐. `color{green}("๐๐จ๐ง๐ญ๐ซ๐จ๐ฅ๐ฅ๐๐ ๐จ๐ฑ๐ข๐๐๐ญ๐ข๐จ๐ง:")`
Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.
(i) `color{red}(2CH_4 + O_2 overset("Cu / 523K / 100 atm")โ underset("Methanol")(2CH_3OH))` ................(13.21)
(ii) `color{red}(CH_4 + O_2 underset(Delta) overset(Mo_2O_3)โ underset("Methanal")(HCHO) + H_2O)` .............(13.22)
(iii) `color{red}(2CH_3CH_3 + 3O_2 overset((CH_3COO)_2 Mn)โ underset("Ethanoic acid")(2CH_3COOH)+2H_2O)` .................(13.23)
(iv) Ordinarily alkanes resist oxidation but alkanes having tertiary `color{red}(H)` atom.
`color{red}(underset("2-Methylpropane")((CH_3)_3CH) underset("Oxidation") overset(KMnO_4)โ underset("2-Methylpropan-2-ol")((CH_3)_3COH))` ..............(13.24)
๐ . `color{green}("๐๐ฌ๐จ๐ฆ๐๐ซ๐ข๐ฌ๐๐ญ๐ข๐จ๐ง:")`
n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.
`color{red}(underset("n - Hexane")(CH_3(CH_2)_4 CH_3) overset(" Anhy ." AlCl_3 // HCl)โunderset("2-Methylpentane")(CH_3 underset(underset(CH_3)(|))CH - (CH_2)_2 - CH_3) + underset("3-Methylpentane")(CH_3CH_2 - underset(underset(CH_3)(|))CH- CH_2 - CH_3))` ................(13.25)
๐. `color{green}("๐๐ซ๐จ๐ฆ๐๐ญ๐ข๐ณ๐๐ญ๐ข๐จ๐ง:")`
`n`-Alkanes having six or more carbon atoms on heating to `773K` at `10-20` atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as ๐๐๐๐๐๐ก๐๐ง๐๐ก๐๐๐ or ๐๐๐๐๐๐๐๐๐.
๐. `color{green}("๐๐๐๐๐ญ๐ข๐จ๐ง ๐ฐ๐ข๐ญ๐ก ๐ฌ๐ญ๐๐๐ฆ:")`
Methane reacts with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen. This method is used for industrial preparation of dihydrogen gas.
`color{red}(CH_4 + H_2O underset(Delta) overset("Ni")โ CO + 3H_2)` .............(13.27)
๐. `color{green}("๐๐ฒ๐ซ๐จ๐ฅ๐ฒ๐ฌ๐ข๐ฌ:")`
Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.
Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, dodecane, a constituent of kerosene oil on heating to 973K in the presence of platinum, palladium or nickel gives a mixture of heptane and pentene.
`color{red}(underset("Dodecane")(C_(12)H_(26)) underset(973K)overset("Pt/Pd/Ni")โ underset("Heptane")(C_7 H_(16)) + underset("Pentene")(C_5H_(10)) + " other products")` .................(13.29)