General methods of preparation of alkanes

(i) Decarboxylation :

(a) Decarboxylation means removal of CO₂ from molecules having - COOH gp.

(b) Saturated monocarboxylic acid salt of sodium potassium on dry distillation with soda lime gives alkane.

RCOONa R-H

or RCOONa + NaOH R-H + Na₂CO₃

(c) The alkane formed always contains one carbon atom less than the original acid.

(d) The yield is good in case of lower members but poor for higher members.

(e) Soda lime is prepared by soaking quick lime CaO in caustic soda solution and then drying the products. It is generally written as NaOH + CaO. Its reaction is milder than caustic soda. Otherwise the reaction will occur violently. Also CaO used alongwith NaOH keeps it dry (NaOH is hygroscopic) to aid fusion.

(f) The decarboxylation of sodium formate yields H₂.

HCOONa + NaOH H₂ + Na₂CO₃

CH₃COONa + NaOH CH₄ + Na₂CO₃

^methane

(ii) By hydrogenation of alkenes : Sabatier and senderen's method :

(a) Alkenes and alkynes on catatlytic hydrogenation give alkanes

CH₂ = CH₂ + H₂ CH₃-CH₃

CH≡CH + 2H₂ CH₃-CH₃

(b) Catalyst Ni is used in finely divided form. If Pt or Pd are used as catalyst, reaction occurs at normal temperature. Also some times Raney nickel is used as catalyst. It is obtained by boiling Ni-AI alloy with NaOH, when AI dissolves leaving Ni in finely divided state. The filtered, washed and died Ni is known as Raney Nickel. Raney Ni is effective at room temperature and atmospheric pressure.

(iii) Reduction of alkyl halides(haloalkanes):

(a) Alkyl halides on reduction with nascent hydrogen form alkanes.

R-X R-H + HX

(b) The nascent hydrogen may be obtained by any one of the following

(i) Zn + HCI

(ii) Zn + CH₃COOH

(iii) Zn-Cu couple in ethanol

(iv) Red P + HI

(v) Al-Hg + ethanol

(c) Alkyl halides can also be reduced catalytically to alkane by H₂/Pd or LiAIH₄ or by H₂/Ni.

(d) The yields are generally high and the hydrocarbons formed are pure.

Note : Zn-Cu couple is prepared by adding Zn granules in aqueous CuSO₄ solution where copper is deposited on the Zn pieces.

(iv) Kolbe's electrolysis method:

(i) Alkanes are formed, on electrolysis of concentrated aqueous solution of sodium or potassium salt of saturated mono carboxylic acids

(ii) Electrolysis of an acid salt gives symmetrical alkane. However, in case of mixture of carboxylic acid salts, all probable alkanes are formed.

R¹COOK + R²COOK R¹-R² + 2CO₂ + H₂ + 2NaOH

(R¹-R¹ and R²-R² are also formed).

(v) By Grignard reagents:

(i) Organic compounds in which a metal atom is directly linked to carbon atom are known as organometallic compound.

e.g. HC≡CNa, (C₂H₅)₄ Pb, (C₂H₅)₂ Zn

(ii) Alkyl or aryl magnesium halide (R-MgX) are also called Grignard reagents or organometallic compounds.

(iii) Grignard reagent on double decomposition with water or with other compounds having active H(the hydrogen attached on O, N, F or triple bonded carbon atom are known as active hydrogen) give alkane.

(vi) Wurtz reaction:

(a) A solution of alkyl halide in ether on heating with sodium gives alkane.

R-X + 2Na + X-R R-R + 2NaX

(b) An alkyl halide on Wurtz reaction leads to the formation of symmetrical alkane having an even number of carbon atoms.

(c) Two different alkyl halides, on Wurtz reaction give all possible alkanes.

CH₃X + Na + C₂H₅X → CH₃CH₂CH₃ + CH₃CH₃ + CH₃CH₂CH₂CH₃

The different steps are:

CH₃X + 2Na + C₂H₅X → CH₃CH₂CH₃ + 2NaX

CH₃X + 2Na + C₂H₅X →CH₃CH₃ + 2NaX

C₂H₅X + 2Na + C₂H₅X → C₂H₅C₂H₅ + 2NaX

(d) The separation of mixture into individual members is not easy because their boiling points are near to each other and thus Wurtz reaction is not suitable for the synthesis of alkanes containing odd number of carbon atoms.

(e) If Zn is used in place of Na, the reaction is named as Frankland method.

(f) Limitations of wurtz reaction :

(a) Methane can not be obtained by this method

(b) The reaction fails in case of tertiary halides

(g) Mechanism : The mechanism of Wurtz reaction is although not clear however two mechanisms are proposed for this reaction.

(a) Involving intermediate formation of an organometallic compound:

RX + 2Na → [RNa] + NaX

^Intermediate

RX + [RNa] → R-R + NaX

(b) Involving intermediate formation of free radicals:

RX + Na → [R] + Nax

^{Free radicals}

R + R → R-R

^Alkane

(vii) By Reduction of Carbonyl compounds :

(i) The reduction of carbonyl compounds by amalgamated zinc and conc. HCI also yields alkanes. This is Clemmensen reduction.

CH₃CHO + 2H₂ CH₃CH₃ + H₂O

CH₃COOH + 2H₂ CH₃CH₂CH₃ + H₂O

(ii) Carbonyl compounds may also be reduced to alkanes by Wolf Kishner reaction

(viii) By reduction of alcohols, aldehydes, ketones or fatty acids and their derivatives:

(i) The reduction of either of the above in presence of red P & HI gives corresponding alkane.

(ix) By the hydrolysis of AI or Be carbides:

(i) Only CH₄ can be obtained by the hydrolysis of Be or Al carbides.

AI₄C₃ + 12H₂O 4AI(OH)₃ + 3CH₄

Be₂C + 4H₂O 2Be(OH)₂ + CH₄

Note :

1. Calcium carbide reacts with water to give acetylene.

2. Magnesium carbide, Mg2C₂ reacts with water to give propyne.

3. CH₄ can be obtained by passing a mixture of H₂S and CS₂ through red and Cu tube

CS₂ + 2H₂S CH₄ + 4Cu₂S

(x) By hydroboration of alkenes :

Alkenes on hydroboration give trialkyl borane as a result of addition of diborane on olefinic bond. This trialkyl borane on treatment with acetic acid or propanoic acid yields alkane.

2R - CH=CH₂ 2(RCH₂CH₂)3B 2RCH₂CH₃

(xi) ByCorey-House synthesis :

Alkyl chloride say chloroethane reacts with lithium in presence of ether to give lithium alkyl then reacts with CuI to give lithium dialkyl cuprate. This lithium dialkyl cuprate now again reacts with alkyl chloride to given alkane.

CH₃CH₂CI + 2Li CH₃CH₂Li + LiCl

2CH₃CH₂Li + CuI → Li(CH₃CH₂)₂ Cu + LiL

Li(CH₃CH₂)₂Cu + CH₃CH₂CI → CH₃CH₂CH₂CH₃ + CH₃CH₂Cu + LiCl