`color{green}("𝟏. 𝐅𝐫𝐨𝐦 𝐮𝐧𝐬𝐚𝐭𝐮𝐫𝐚𝐭𝐞𝐝 𝐡𝐲𝐝𝐫𝐨𝐜𝐚𝐫𝐛𝐨𝐧𝐬:")`

Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium(catalyse the reaction at room temperature) or nickel(catalyse at higher temperature and pressure) to form alkanes. This process is called `color{green}("𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧𝐚𝐭𝐢𝐨𝐧")` .Dihydrogen gas gets adsorbed on metal surfaces and the hydrogen – hydrogen bond gets activated.

`color{red}(underset("Ethene")(CH_2 = CH_2) + H_2 overset(" Pt/Pd / Ni")→ underset(" Ethane")(CH_3 - CH_3))` ................(13.1)

`color{red}(underset("Propene")(CH_3 - CH = CH_2) + H_2 overset("Pt/Pd/Ni")→ underset("Propane")(CH_3 - CH_2 - CH_3))` ............(13.2)

`color{red}(underset("Propyne")(CH_3 - C equiv C - H) +2H_2 overset(" Pt/Pd/Ni")→ underset("Propane")(CH_3 - CH_2 - CH_3))` ..........(13.3)


`color{green}("𝟐. 𝐅𝐫𝐨𝐦 𝐚𝐥𝐤𝐲𝐥 𝐡𝐚𝐥𝐢𝐝𝐞𝐬 ")`

i) Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.

`color{red}(underset("Chloromethane")(CH_3 - Cl) + H_2 overset(Zn , H^(+))→ underset("Methane")(CH_4) + HCl)` ...........(13.4)

`color{red}(underset("Chloroethane")(C_2H_5 - Cl) + H_2 overset(Zn , H^(+))→ underset("Ethane")(C_2H_6)+HCl)` ................(13.5)

`color{red}(underset("1-Chloropropane")(CH_3CH_2CH_2Cl)+H_2 overset(Zn , H^(+))→ underset("Propane")(CH_3CH_2CH_3) +HCl)` ..........(13.6)

ii) Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as `color{green}("𝐖𝐮𝐫𝐭𝐳 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧")` and is used for the preparation of higher alkanes containing even number of carbon atoms.

`color{red}(underset("Bromomethane")(CH_3Br) +2Na+ BrCH_3 overset("dry ether")→ underset("Ethane")(CH_3-CH_3) + 2NaBr)` .........(13.7)

`color{red}(underset("Bromoethane")(C_2H_5Br) + 2Na + BrC_2H_5 overset("dry ether")→ underset("n-Butane")(C_2H_5 - C_2H_5))` ........(13.8)



`color{green}("𝟑. 𝐅𝐫𝐨𝐦 𝐜𝐚𝐫𝐛𝐨𝐱𝐲𝐥𝐢𝐜 𝐚𝐜𝐢𝐝𝐬")`

i) Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as `color{green}("𝐝𝐞𝐜𝐚𝐫𝐛𝐨𝐱𝐲𝐥𝐚𝐭𝐢𝐨𝐧.")`

`color{red}(underset("Sodium ethanoate")(CH_3COO^(-) Na^(+)) + NaOH underset(Delta) overset(CaO)→ CH_4+Na_2CO_3)`

ii) `color{green}("𝐊𝐨𝐥𝐛𝐞’𝐬 𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐥𝐲𝐭𝐢𝐜 𝐦𝐞𝐭𝐡𝐨𝐝")` An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode.


The reaction is supposed to follow the following path :

i) `color{red}(2CH_3COO^(-) Na^(+) ⇌ 2 CH_3 - overset(overset(O)(||))C - O^(-) +2Na^(+))`

ii) `color{green}("𝐀𝐭 𝐚𝐧𝐨𝐝𝐞:")` `color{red}(underset("Acetate ion")(2CH_3 - overset(overset(O)(||))C- O^(-)) + overset(-2e^(-))→ underset("Acetate free radical")( 2CH_3 - overset(overset(O)(||))C - underset(. .) overset(.)O ": ") → underset("Methyl free radical")( 2 overset(.)CH_3+ 2CO_2 ↑))`

iii) `color{red}(H_3 overset(.)C + overset(.)CH_3 → H_3C - CH_3 ↑)`

iv) `color{green}("𝐀𝐭 𝐜𝐚𝐭𝐡𝐨𝐝𝐞 ")`: `color{red}(H_2O + e^(-) → text()^(-)OH + overset(.)H)`

`color{red}(2 overset(.)H → H_2 ↑)`

• Alkanes are almost non-polar molecules because of the covalent nature of `color{red}(C-C)` and `color{red}(C-H)` bonds and due to very little difference of electronegativity between carbon and hydrogen atoms.

• They possess weak van der Waals forces. Due to the weak forces, the first four members, `color{red}(C_1)` to `color{red}(C_4)` are gases, `color{red}(C_5)` to `color{red}(C_(17))` are liquids and those containing 18 carbon atoms or more are solids at `298 K`.

• They are colourless and odourless.

• Petrol is a mixture of hydrocarbons and is used as a fuel for automobiles.

• Petrol and lower fractions of petroleum are also used for dry cleaning of clothes to remove grease
stains.

• Grease (mixture of higher alkanes) is non-polar and, hence, hydrophobic in nature.

• It is generally observed that polar substances are soluble in polar solvents, whereas the non-polar ones in non-polar solvents i.e., like dissolves like.

• Boiling point (b.p.) of different alkanes are given in Table 13.2 from which it is clear that there is a steady increase in boiling point with increase in molecular mass. This is due to the fact that the intermolecular van der Waals forces increase with increase of the molecular size or the surface area of the molecule.

• With increase in number of branched chains, the molecule attains the shape of a sphere. This results in smaller area of contact and therefore weak intermolecular forces between spherical molecules, which are overcome at relatively lower temperatures. It is observed (Table 13.2) that pentane having a continuous chain of five carbon atoms has the highest boiling point (309.1K) whereas 2,2 - dimethylpropane boils at 282.5K.

Alkanes undergo the following reactions under certain conditions.

1. `color{green}("𝐒𝐮𝐛𝐬𝐭𝐢𝐭𝐮𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧𝐬:")`

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}("𝐇𝐚𝐥𝐨𝐠𝐞𝐧𝐚𝐭𝐢𝐨𝐧")`

`color{red}(CH_4 + Cl_2 overset(hv)→ underset("Chloromethane")(CH_3Cl) + HCl)` ...................(13.10)

`color{red}(CH_3Cl + Cl_2 overset(hv)→ underset("Dichloromethane")(CH_2Cl_2) + HCl)` ..........(13.11)

`color{red}(CH_2Cl_2 + Cl_2 overset(hv)→ underset("Trichloromethane")(CHCl_3) + HCl)` ................(13.12)

`color{red}(CHCl_3 + Cl_2 overset(hv)→ underset("Tetrachloromethane")(C Cl_4) + HCl)` ...............(13.13)

`color{red}(CH_3- CH_3+Cl_2 overset(hv)→ underset("Chloroethane")(CH_3 - CH_2 Cl) + HCl)` ...............(13.14)


• It is found that the rate of reaction of alkanes with halogens is `color{red}(F_2 > Cl_2 > Br_2 > I_2).`
• Rate of replacement of hydrogens of alkanes is : `color{red}("3° > 2° > 1°")`.
• Fluorination is too violent to be controlled.
• 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 ⇌ CH_3I + HI)` .....................(13.15)

`color{red}(HIO_3 + 5HI → 3I_2 + 3H_2O)` .................(13.16)

`color{green}("𝐌𝐄𝐂𝐇𝐀𝐍𝐈𝐒𝐌 𝐎𝐅 𝐇𝐀𝐋𝐎𝐆𝐄𝐍𝐀𝐓𝐈𝐎𝐍:")`

(i) `color{green}("𝐈𝐧𝐢𝐭𝐢𝐚𝐭𝐢𝐨𝐧 :")` The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The `color{red}(Cl–Cl)` bond is weaker than the `color{red}(C–C)` and `color{red}(C–H)` bond and hence, can be easily broken.

`color{red}(Cl - Cl underset("homolysis") overset(hv)→ underset("Chlorine free radicals")(overset(•)Cl + overset(•)Cl))`

(ii) `color{green}("𝐏𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐢𝐨𝐧 :")` 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(•)Cl overset(hv)→ overset(•)CH_3 + H - Cl)`

(b) `color{red}(overset(•)CH_3 +Cl - Cl overset(hv)→ underset("Chlorine free radical" )(CH_3 - Cl + overset(•)Cl))`

The methyl radical thus obtained attacks the second molecule of chlorine to form `color{red}(CH_3 – 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}("𝐓𝐞𝐫𝐦𝐢𝐧𝐚𝐭𝐢𝐨𝐧:")` 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(•)Cl + overset(•)Cl → Cl - Cl)`

(b) `color{red}(H_3 overset(•)C + overset(•)CH_3 → H_3C - CH_3)`

(c) `color{red}(H_3 overset(•)C + overset(•)Cl → H_3C - Cl)`

Though in (c), `color{red}(CH_3 – 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.

𝟐. `color{green}("𝐂𝐨𝐦𝐛𝐮𝐬𝐭𝐢𝐨𝐧:")`

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) → CO_2(g) +2H_2O (l) ; Delta_cH^(⊖) = -890 kJ mol^(-1))` .........(13.17)

`color{red}(C_4H_(10) (g) + 13/2 O_2 (g) → 4CO_2 (g) + 5H_2O(l) ; Delta_cH^(⊖) = -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 → 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")→ C(s) + 2H_2O (l))` ....................(13.20)



𝟑. `color{green}("𝐂𝐨𝐧𝐭𝐫𝐨𝐥𝐥𝐞𝐝 𝐨𝐱𝐢𝐝𝐚𝐭𝐢𝐨𝐧:")`

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")→ underset("Methanol")(2CH_3OH))` ................(13.21)

(ii) `color{red}(CH_4 + O_2 underset(Delta) overset(Mo_2O_3)→ underset("Methanal")(HCHO) + H_2O)` .............(13.22)

(iii) `color{red}(2CH_3CH_3 + 3O_2 overset((CH_3COO)_2 Mn)→ 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)→ underset("2-Methylpropan-2-ol")((CH_3)_3COH))` ..............(13.24)

𝟒 . `color{green}("𝐈𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧:")`

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)→underset("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)

𝟓. `color{green}("𝐀𝐫𝐨𝐦𝐚𝐭𝐢𝐳𝐚𝐭𝐢𝐨𝐧:")`

`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 𝑎𝑟𝑜𝑚𝑎𝑡𝑖𝑧𝑎𝑡𝑖𝑜𝑛 or 𝑟𝑒𝑓𝑜𝑟𝑚𝑖𝑛𝑔.



𝟔. `color{green}("𝐑𝐞𝐚𝐜𝐭𝐢𝐨𝐧 𝐰𝐢𝐭𝐡 𝐬𝐭𝐞𝐚𝐦:")`

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")→ CO + 3H_2)` .............(13.27)


𝟕. `color{green}("𝐏𝐲𝐫𝐨𝐥𝐲𝐬𝐢𝐬:")`

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")→ underset("Heptane")(C_7 H_(16)) + underset("Pentene")(C_5H_(10)) + " other products")` .................(13.29)