HYDROCARBONS

Question 1

From unsaturated hydrocarbon

Answer 1

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. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen - hydrogen bond. Platinum and palladium catalyse the reaction at room temperature but relatively higher temperature and pressure are required with nickel catalysts. CH2=CH2 + H2 Pt/Pd/Ni → CH3-CH3

Ethene

Propane (9.1)

CH2-CH=CH2 + H2 Propane Pt/Pd/Ni → CH3-CH2CH3 Propane (9.2)

CH3-C=C-H + 2H Pt/Pd/Ni → CH3-CH2CH3

Propyne

Question 2

From alkyl halides

Answer 2

Alkyl halides (except fluorides highly reactive Sihey are on reduction with zinc and dilute hydrochloric acid give alkanes

CH-C1+H₂ CH Zn.H →CH+HC1 (9.4) Chloromethane Methane

Question 3

From alkyl halides

Answer 3

Alkyl halides (except fluorides highly reactive Sihey are on reduction with zinc and dilute hydrochloric acid give alkanes

CH-C1+H₂ CH Zn.H →CH+HC1 (9.4) Chloromethane Methane

Question 4

Wurtz reaction

Answer 4

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

CH,Br+2Na+BrCH, dry ether, CH,+2Na Bromomenthane Ethane (9.7)

C₂H₂br+2Na+BrC₂H₃ dether, C₂H₃-C₂H Bromoethane n-Butane

Question 5

What will happen if two different alkyl halides are taken in wurtz reaction

Answer 5

If two different alkyl halides are used in Wurtz reaction to prepare an alkane with odd number of carbon atoms, a mixture of three alkanes is actually produced.