Hydrocarbon Class 11th Chemistry Notes

HYDROCARBONS

The compounds which contain only carbon and hydrogen are called hydrocarbons. Hydrocarbons are classified as aliphatic, alicyclic and aromatic hydrocarbons.

Aliphatic hydrocarbons contain open chains of carbon atoms, which are further classified as saturated and unsaturated hydrocarbons. 

Saturated hydrocarbon contains all C – C single bonds (alkanes) while unsaturated hydrocarbons contain C = C double bonds (alkenes) or C ≡ C  triple bonds (alkynes).

Alicyclic hydrocarbons contain rings of carbon atoms and have properties similar to that of aliphatic hydrocarbons.

Aromatic hydrocarbons contain one or more benzene rings in their structure.

ALKANES

Alkanes are aliphatic saturated hydrocarbons containing carbon – carbon and carbon – hydrogen single covalent bonds. Alkanes are also called paraffins because alkanes do not undergo chemical reactions easily. 

The general chemical formula of alkanes is C_nH_2n+2, where n is an integer and indicates the number of carbon atoms in the molecule. Some common alkanes are methane (CH₄), ethane (C₂H₆), propane (C₃H₈), Butane (C₄H₁₀), Pentane (C₅H₁₂) etc. Every carbon atom in alkanes is sp³ hybridized and has tetrahedral geometry. 

The tetrahedral are joined together and C – C and C – H bond lengths are 154pm and 112 pm respectively. The H – C – H, C – C – H and C – C – C bond angles are 109ᵒ28’.

CONFORMATION IN ALKANES

The different spatial arrangements of atoms in a molecule that can be converted into one another by free rotation about single bonds are called conformations or conformers or rotamers. These isomers cannot be usually isolated because they interconvert rapidly. Conformations can be represented by different formula, such as sawhorse and Newmann projection formula.

METHODS OF PREPARATION OF ALKANES

From unsaturated hydrocarbons : By hydrogenation

Alkanes are obtained by passing a mixture of alkenes or alkynes and hydrogen over Raney nickel and catalyst at about 473K to 573K. This reaction is called Sabatier and Sanderson reaction. Platinum or Palladium also catalyses the reaction at room temperature.

e.g. CH₂ = CH₂  +   H₂     ^{Raney Ni  &  Δ}   →    CH₃ – CH₃

          Ethene                                                        Ethane

       CH≡ CH  +   2H₂     ^{Raney Ni  &  Δ}   →    CH₃ – CH₃

        Ethene                                                        Ethane

From sodium of fatty acids : By decarboxylation

The reaction in which carbon dioxide is removed from carboxylic acid is called decarboxylation. When anhydrous sodium salt of fatty acid is fused with soda lime it gives alkane containing one carbon atom less than carboxylic acid.

By reduction of alkyl halides

Alkyl halides on reduction with zinc and dilute hydrochloric acid from alkane. The reduction of alkyl halides is due to nascent hydrogen obtained from reducing agent. Zn – Cu couple and alcohol can also be used as the reducing agents.

     R – X    +    2[H]      ^{zn, HCl}  →     R – H   +    HX

  Alkyl halide                                    Alkane

e.g.      CH₃ – I    +    2[H]       ^{Zn, HCl}  →    CH₄     +     HI

      Methyl iodide                                      Methane

By using active metal (Wurtz synthesis reaction)

Alkyl halides on heating with sodium metal in presence of dry ether as solvent give higher alkanes containing even number of carbon atoms.

R – X  +  2Na + X – R   ^{dry ether}  →  R – R  +  2NaX

e.g.      CH₃ – Cl    +    2Na    +    Cl – CH₃      ^{dry ether}  →    CH₃ – CH₃  +  2NaCl

       Methyl Chloride                                                            Ethane

PHYSICAL PROPERTIES OF ALKANES

• Alkanes are colourless and odourless they are insoluble in water, but readily dissolve in non – polar solvents like benzene, ether, chloroform etc.
• At room temperature, the first four alkanes are gases, next thirteen (C₅ to C₁₇) are liquids where as higher alkanes are waxy solids.
• Alkanes are non polar as C – H and C-C bonds in alkanes are nonpolar covalent bonds. All alkanes are less dense than water and their relative density increases with increase in molecular mass.
• Melting and boiling points increases with increase in molecular mass and decreases as branching increases.

CHEMICAL PROPERTIES OF ALKANES

Halogenations of alkanes

Alkanes reacts with halogens in presence of ultraviolet light as diffused sunlight or at high temperature (573K to 773K) to give mixture of alkyl halides.

Chlorination of Methane :

CH₄  ^{Cl2 – HCl} →  CH₃ – Cl   ^{Cl2 – HCl} →  CH₂Cl₂   ^{Cl2 – HCl} →  CHCl₃   ^{Cl2 – HCl} →  CCl₄

Nitration of alkanes

The reaction in which one hydrogen atom of alkanes in replaced by nitro (- NO₂) group is called nitration of alkanes. It is carried out in vapour phase, by heating a mixture of an alkane and conc. HNO₃ at about 423K to 698K.

e.g.  CH₃ – CH₃   +   HO – NO₂      ^{423 to 698K}  →    CH₃