Name: Physical Properties of Alkanes
Uploaded: 2019-07-19
Duration: 4 min 41 s
Description: Alkanes are organic compounds that consist of single-bonded carbon and hydrogen atoms. Let’s watch the video and learn some of the important physical propert...

Question

Describe any three methods for the preparation of aliphatic saturated hydrocarbons and their general chemical reactions.

Accepted Answer

The three methods for the preparation of aliphatic saturated hydrocarbons are:
(a) From alkyl halides by Wurtz reaction
When an alkyl halide (mostly iodide or bromide) is heated with metallic sodium in a dry ethereal solution, a higher symmetrical alkane is formed. This reaction is known as the Wurtz reaction. The hydrocarbon formed contains double the number of carbon atoms present in the alkyl halide.

For example,

$\underset{Ethyl bromide}{2 {CH}_{3} {CH}_{2} Br} + 2 Na \overset{dry ether}{\to} \underset{Butane}{{CH}_{3} {CH}_{2} {CH}_{2} {CH}_{3}} + 2 NaBr$

(b) By Kolbe's electrolytic method: In this method, a concentrated solution of sodium or potassium salt of fatty acid is electrolyzed, the alkane is obtained at the anode and hydrogen gas is liberated at the cathode. The various reactions involved are:

(i) $2 {CH}_{3} {COO}^{+} {Na}^{-} \to 2 {CH}_{3} {COO}^{-} + 2 {Na}^{+}$
(ii) At anode:
$\underset{Acetate ion}{2 {CH}_{3} {COO}^{-}} \overset{- 2 e^{-}}{\to} \underset{Acetate free radical}{2 {CH}_{3} {COO}^{∙}} \to \underset{Methyl free radical}{2 \overset{∙}{C} H_{3}} + {CO}_{2}$

iii) $H_{3} \overset{∙}{C} + \overset{∙}{C} H_{3} \to \underset{Ethane}{H_{3} C - {CH}_{3}}$
iv) At cathode:
$H_{2} O + e^{-} \to {OH}^{-} + \overset{∙}{H} 2 \overset{∙}{H} \to H_{2}$

(c) By Clemmensen reduction: Aldehydes and ketones are reduced to alkanes by zinc amalgam and concentrated hydrochloric acid.

$\underset{Propanal}{{CH}_{3} {CH}_{2} CHO} \to_{HCl}^{Zn - Hg} \underset{Propane}{{CH}_{3} {CH}_{2} {CH}_{3}}$

Some of the general chemical reactions of aliphatic saturated hydrocarbons are:
(a) Substitution reaction: In the substitution reactions of alkanes, the direct replacement of one or more hydrogen atoms by halogen atoms or other monovalent groups takes place. It includes halogenation, nitration, and sulphonation.

$\underset{Alkane}{R - H} + X_{2} \overset{hv}{\to} R - X + HX \underset{Alkane}{R - H} + \underset{Nitric acid}{{HONO}_{2}} \overset{400 ° C}{\to} \underset{Nitroalkane}{R - {NO}_{2}} + \underset{Water}{H_{2} O} \underset{Alkane}{R - H} + \underset{Sulphuric acid}{{HOSO}_{3} H} \overset{}{\to} \underset{Alkyl sulphonic acid}{R - {SO}_{3} H} + \underset{Water}{H_{2} O}$

(b) Combustion: Alkanes readily burn on heating in the presence of air or oxygen, producing carbon dioxide, water vapour, and a lot of heat energy is called combustion.

$\underset{Alkane}{C_{n} H_{2 n + 2}} + \underset{Oxygen}{(\frac{3 n + 1}{2}) O_{2}} \overset{∆}{\to} \underset{Carbon dioxide}{{nCO}_{2}} + \underset{Water}{(n + 1) H_{2} O} + Heat$

(c) Pyrolysis or cracking: The thermal decomposition of alkanes to produce lower hydrocarbons (saturated or unsaturated) is known as pyrolysis or cracking. When alkanes are heated at high temperatures in the absence of air, cracking occurs.

Alkane (large molecule) $\overset{Crack}{\to}$ Alkane (small molecule) + Alkene + Hydrogen

$\underset{Propane}{3 {CH}_{2} - {CH}_{2} {CH}_{3}} \overset{600 ° C}{\to} \underset{Propylene}{{CH}_{3} - CH = {CH}_{2}} + \underset{Hydrogen}{H_{2}} + \underset{Ethylene}{2 {CH}_{2} = {CH}_{2}} + \underset{Methane}{2 {CH}_{4}}$

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