NCERT Solutions for Class 11 Chemistry Chapter 9 Hydrocarbons

Hydrocarbons occur naturally and form the basis of crude oil, natural gas, coal, and other important energy sources. The term ‘hydrocarbon’ is self-explanatory which means compounds of carbon and hydrogen only. Hydrocarbons play a key role in our daily life. You must be familiar with the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the abbreviated form of liquefied petroleum gas whereas CNG stands for compressed natural gas.

Hydrocarbons are of different types. Depending upon the types of carbon-carbon bonds present, they can be classified into three main categories:

• (A) Saturated
• (B) Unsaturated and
• (C) Aromatic hydrocarbons

alkanes are saturated open chain hydrocarbons containing carbon – carbon single bonds. Methane (CH₄) is the first member of this family. Methane is a gas found in coal mines and marshy places. If you replace one hydrogen atom of methane by carbon and join the required number of hydrogens to satisfy the tetravalence of the other carbon atom, you get C₂H₆. This hydrocarbon with molecular formula C₂H₆ is known as ethane. Thus you can consider C₂ H₆ as derived from CH₄ by replacing one hydrogen atom by -CH₃ group. General formula for alkanes is CₙH₂ₙ₊₂.

Petroleum and natural gas are the main sources of alkanes. However, alkanes can be prepared by following methods:
From Unsaturated Hydrocarbons
Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes. This process is called hydrogenation.
CH₂ = CH₂ + H₂ ⟶ CH₃¬¬¬¬ – CH₃
Ethene Ethane

From Alkyl Halides
Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.
CH₃ – Cl + H₂ ⟶ CH₄ + HCl
Chloromethane Methane

From carboxylic acids 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 decarboxylation.
CH₃COO⁻Na⁺ + NaOH ⟶ CH₄ + Na₂CO₃

Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms. C₁ to C₄ are gases, C₄ to C₁₇ are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless.

alkanes are generally inert towards acids, bases, oxidising and reducing agents.
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.
CH₄ (g) 2O₂ ⟶ (g) CO₂ (g) 2H₂O(l)

Alkenes are unsaturated hydrocarbons containing at least one double bond. If there is one double bond between two carbon atoms in alkenes, they must possess two hydrogen atoms less than alkanes. Hence, general formula for alkenes is CₙH₂ₙ. Alkenes are also known as olefins (oil forming) since the first member, ethylene or ethene (C₂H₄) was found to form an oily liquid on reaction with chlorine.

Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen are liquids and the higher ones are solids. Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless, insoluble in water but fairly soluble in nonpolar solvents like benzene, petroleum ether.

Alkenes are the rich source of loosely held pi (p) electrons, due to which they show addition reactions in which the electrophiles add on to the carbon-carbon double bond to form the addition products. Some reagents also add by free radical mechanism. There are cases when under special conditions, alkenes also undergo free radical substitution reactions. Oxidation and ozonolysis reactions are also quite prominent in alkenes.