Preparation of Alkanes
1. From Unsaturated Hydrocarbons
The unsaturated hydrocarbons (alkenes and alkynes) are converted into alkanes by catalytic hydrogenation. In this process hydrogen is passed through alkenes or alkynes in the presence of catalyst such as Ni, Pt or Pd at 150 – 200° C.
2. From Alkyl halides
Alkyl halides (halogen derivatives of alkanes) can be converted into alkanes by the following methods:
a. Wurtz reaction: When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing twice the number of carbon atoms of alkyl halide is obtained.
This method gives alkane with even number of carbon atoms. If two different alkyl halides are taken for preparing an alkane with odd number of carbon atoms, a mixture of products is obtained. When ethyl bromide and methyl bromide react in the presence of sodium, a mixture of ethane, propane and butane is obtained. This is because the two alkyl halides not only react with each other but they react among themselves also. For example:
b. Reduction of alkyl halides. Alkyl halides are reduced to alkanes by suitable reducing agents. For example,
i. With zinc and HCl:
ii. With hydrogen in the presence of platinum or palladium (catalytic hydrogenation):
iii. With hydrogen iodide in the presence of red phosphorus.
The purpose of red phosphorus is to removeiodine. It combines with iodine forming phosphorus tri-iodide (PI3). Therefore, it makes the reaction to proceed in the forward direction. If I2 is not removed, it will convert back alkane into alkyl halide.
iv. By zinc copper couple and alcohol:
By the use of Grignard reagent: Alkyl halides react with magnesium metal in diethyl ether to form alkyl magnesium halides. The alkyl magnesium halides are called Grignard reagents. These are highly reactive and can be easily decomposed by water or alcohol to form alkanes. For example:
3. By the decarboxylation reaction of carboxylic acid
Principle. When sodium salt of carboxylic acid is heated with soda lime ( NaOH + CaO), then alkane is produced. In this reaction, the carboxyl group is eliminated as carbon dioxide. That is why this reaction is called decarboxylation reaction. For example:
R-COONa + NaO H (CaO) = R – H + Na2CO3
Example: By this reaction methane and ethane could be obtained by heating sodium ethanoate (CH3COONa) and sodium propanoate (CH3CH2COONa) respectively.
CH3COONa + NaOH (CaO) = CH4 + Na2CO3 (CaO) CH3CH2COONa + NaOH (CaO) = C2H6 + Na2CO3 (CaO)
d. Kolbe’s reaction: When an aqueous solution of sodium or potassium salt of carboxylic acid is electrolysed, alkane is evolved at the anode.
At anode: 2CH3COO– = 2CH3COO– + 2e–
2CH3COO = CH3CH3 + 2CO2
At cathode: K+ as well as H+ ions formed by the electrolysis of water will move towards the cathode. But H+ ions will be preferentially discharged at the cathode because the reduction potential of H+ ions is greater than that of K+ ion. The H+ ion will be discharged to produce H2 gas, which is liberated at cathode.
H+ + e– = H
H + H = H2
The K+ ions remain in the solution. Thus, the reaction may be written as: