1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts
From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes.
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.
Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.
2. From alkyl halides
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.
Reducing agents can be used to add hydrogen to the halide and remove the halogen atom.
i) Zinc + HCl is one reducing agent.
ii) Catalytic hydrogenation using Pd or Pt as catalyst
iii) Hydrogen iodide (halogen acid) in the presence of red phosphorous also acts as reducing agent. In this reaction phosphorous combines with iodine to form phosphorous triiodide.
iv) 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 which are called as Grignard reagents.
Grignard reagents are highly reactive and are easily decomposed by water or alcohol to form alkanes
RMgX + HOH (H2O) ---> RH + Mg(OH)X
3. From carboxylic acids
i) Reduction of carboxylic acid: Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group.
ii) When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
RCOONa + NaOH -->RH + Na2CO3
In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.
iii) Kolbe's reaction
When an acqueous solution of sodium or potassium salt of carboxylic acid is eletrolysed alkane is evolved at the anode.
Kolbe's reaction can also be used like wurtz reaction for preparing alkanes with even number of carbon atoms.
The methods in this section can be summarised as
R-COONa ---> RH
R-COOK ---> R-R - Kolbe's reaction
Industrial method: Petroleum provides the natural source of alkanes.