PREPARATION OF ALKANES

Alkanes can be prepared by various ways. General methods of preparation of alkanes are--

(1) From Unsaturated Hydrocarbons

Alkanes are prepared by the catalytic reduction of unsaturated hydrocarbons such as alkene or alkyne in the presence of a suitable catalyst such as Ni, Pt, Pd, Raney Ni etc. Here hydrogen is added to an unsaturated hydrocarbon (alkene or alkyne) in the presence of a catalyst. This process is known as hydrogenation.

The catalytic hydrogenation reaction takes place at normal temperature and pressure if we use Raney Ni, Pt or Pd as a catalyst.

But if we use Ni as a catalyst, then the catalytic hydrogenation reaction takes place at 200 ⁰C – 300 ⁰C. The catalytic hydrogenation reaction in presence of a Ni catalyst at 200 ⁰C – 300 ⁰C is known as Sabatier-Senderens reduction.

(2) From Alkyl Halides

Alkanes can be prepared from alkyl halides in different ways---

(A) Through Grignard Reagent

When alkyl halide heated with magnesium powder in ether solution, alkyl magnesium halide (R―Mg―X) is produced. This alkyl magnesium halide (R―Mg―X) is known as Grignard reagent.

R―X + Mg → R―Mg―X

C₂H₅―I + Mg → C₂H₅―Mg―I

When Grignard reagent treated with water or dilute acid, alkane is produced.

R―Mg―X + H₂O → RH + Mg(OH)X

C₂H₅―Mg―I + H₂O → C₂H₆ + Mg(OH)I

(B) By Wurtz Reaction

In dry ether solution at normal temperature the reaction of two molecules of alkyl halide (preferably the bromide or iodide) with two molecules of pure and dry metallic sodium produce alkane.

R₁―X + 2Na + R₂―X → R₁―R₂ + 2NaX

CH₃―Br + 2Na + CH₃―Br → CH₃―CH₃ 

+ 2NaBr

If we use two types of alkyl halide, then mixture of alkanes is produced. It is difficult to separate this mixture of alkanes, because their close boiling point.

CH₃―Br + 2Na + C₂H₅―Br → CH₃―CH₃ + C₂H₅―C₂H₅ + CH₃―C₂H₅ + 2NaBr

This Wurth reaction is very useful for the synthesis of symmetrical alkane.

(C) By Corey-House Synthesis

In dry ether solution at normal temperature the reaction of alkyl halide with pure and dry metallic lithium produced alkyl lithium.

RX + 2Li → RLi + LIX

C₂H₅I + 2Li → C₂H₅Li + LiI

Reaction of alkyl lithium (RLi) with cuprous iodide (CuI) produced lithium dialkyl cuprate (R₂CuLi).

2RLi + CuI → R₂CuLi + LiI

2C₂H₅Li + CuI → (C₂H₅)₂CuLi + LiI

Reaction of lithium dialkyl cuprate (R₂CuLi) with another molecule of alkyl halide produced alkane.

R₂CuLi + R'X → R―R' + RCu +LiX

(C₂H₅)₂CuLi + CH₃I → C₂H₅―CH₃ + C₂H₅Cu + LiI

R and R' may be same or different. R group may be primary, secondary or tertiary but R' group always primary.

This Corey-House reaction is very useful for the formation of both symmetrical and unsymmetrical alkane.

(D) Reduction of Alkyl Halides

(i) Alkane is prepared by the reduction of alkyl halide by Zn and HCl.

C₂H₅Cl + Zn + HCl → C₂H₆ + ZnCl₂

(ii) Alkane is prepared by the reduction of alkyl halide by Zn and NaOH.

Zn + 2NaOH → Na₂ZnO₂ + 2[H]

R―X + 2[H] → R―H + HX

(iii) When alkyl halide is reduced by Zn-Cu couple in presence of alcohol alkane is produced.

Zn → Zn²⁺ + 2e

R―X + e → R∙ + X^-

R∙ + e → R:^-

R:^- + C₂H₅OH → R―H + C₂H₅O^-

(iv) When alkyl halide is reduced by LiAlH₄, NaBH₄ or Ph₃SnH alkane is produced.

R―X + H^- → R―H + X^-

(v) When alkyl halide is reduced by H₂ in presence of Pd-C or Raney Ni, alkane is produced.

R―X + H₂ → R―H + HX

(vi) When alkyl iodide is reduced by hydroiodic acid (HI) and red phosphorus at 150 ⁰C, alkane is produced.

C₂H₅―I + HI → C₂H₆ + I₂

(3) From Carbonyl Compounds

Alkane can be prepared from carbonyl compounds such as aldehyde and ketone by Clemensen reduction process and Wolf-Kishner reduction process.

(A) By Clemmensen Reduction

When carbonyl compounds such as aldehydes and ketones are heated with zinc amalgam (Zn/Hg) and concentrated HCl, carbonyl compounds reduced to form alkane.

(B) By Wolff-Kishner Reduction

Carbonyl compounds such as aldehydes and ketones react with hydrazine (H₂N―NH₂) to formed hydrazone. When the resulting hydrazone is heated with sodium ethoxide (NaOC₂H₅) and ethylene glycol (HOH₂C―CH₂OH) at 180 ⁰C, alkane is produced.

(4) From Carboxylic Acid

Alkane can be prepared from carboxylic acid by two methods. One is decarboxylation process and another is Kolbe’s electrolytic method.

(A) By Decarboxylation Process

Alkane is produced when carboxylic acid is heated with soda lime (NaOH + CaO). In this reaction CO₂ is removed, so it is called decarboxylation reaction.

This is a degradation reaction, because in this process the number of carbon atom in the produced alkane is one less than that of the carboxylic acid.

(B) Kolbe’s Electrolytic Method

Electrolysis of a concentrated and cool aqueous solution of sodium or potassium salts of monocarboxylic acid using platinum electrodes produced alkane at the anode.

2R―COOK + 2H₂O → [R―R + 2CO₂(anode)] + [H₂ + 2KOH (cathode)]

2CH₃COOK + 2H₂O → [C₂H₆ + 2CO₂(anode)] + [H₂ + 2KOH (cathode)]

Kolbe’s method of electrolysis is suitable for the preparation of even number of carbon atom alkane, but not suitable for the preparation of odd number of carbon atom alkane.

(5) From Alkyl Borane

Reaction of alkene with borane produced trialkyl borane.

3R―CH=CH₂ + B₂H₆ → (R―CH₂―CH₂)₃B

Alkane is produced by hydrolysis of the resulting trialkyl borane with acid.

(RCH₂CH₂)₃B + 3CH₃COOH → 3RCH₂CH₃ + (CH₃COO)₃B

(6) From Alcohol

When alcohol is reduced by red phosphorus (P) and hydroiodic acid (HI) in a closed vessel at 150 ⁰C, alkane is produced.