General Methods of Preparation of Alkanes

Decarboxylation

Decarboxylation refers to the process of removal of CO₂ from the molecules having -COOH group. Saturated monocarboxylic acid salt of sodium potassium on dry distillation with soda lime gives alkane.

or

The alkane formed by decarboxylation process always contains one carbon atom less than the original acid.The yield is good in case of lower members but poor for higher members.

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.

The decarboxylation of sodium formate yields H₂.

HCOONa + NaOH  H₂ + Na₂CO₃

CH₃COONa + NaOH  CH₄ + Na₂CO₃                        

Wurtz Reaction

In wourtz reaction  a solution of alkyl halide in ether on heating with sodium gives alkane.

CH₃X + Na + C₂H₅X → CH₃CH₂CH₃ + CH₃CH₃ + CH₃CH₂CH₂CH₃

The different involved are steps are:

CH₃X + 2Na + C₂H₅X → CH₃CH₂CH₃ + 2NaX

CH₃X + 2Na + C₂H₅X →CH₃CH₃ + 2NaX

C₂H₅X + 2Na + C₂H₅X → C₂H₅C₂H₅ + 2NaX

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.

Limitations of Wurtz reaction :

a.Methane can not be obtained by this method

b.The reaction fails in case of tertiary halides

Mechanism of Wurtz reaction : 

The mechanism of Wurtz reaction is although not clear however two mechanisms are proposed for this reaction.

The first proposed mechanism of Wurtz reaction involved  formation of an  Intermediate organometallic compound:

 RX + 2Na →   [RNa]  + NaX

                     ^Intermediate

RX + [RNa] → R-R + NaX

Another proposed mechanizm of Wurtz reaction involved  formation of Intermediate  free radicals:
 RX + Na → R^. + Nax

                 ^{Free radicals}

 R^. + R^. → R-R

By the Reduction of Alkyl Halides

Alkyl halides on reduction with nascent hydrogen form alkanes.  R-X  + 2[H]→R-H + HX. The nascent hydrogen may be obtained by any one of the following

1. Zn + HCI                              

2. Zn + CH₃COOH

3. Zn-Cu couple in ethanol

4. Red P + HI

5. Al-Hg + ethanol

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

By Hydrogenation of Alkenes((>C=C<) : Sabatier and Senderen's Method 

Alkenes and alkynes on catatlytic hydrogenation give alkanes

CH₂ = CH₂ + H₂  CH₃-CH₃

CH≡CH + 2H₂  CH₃-CH₃

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.

Kolbe's Electrolysis Method

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

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).

By Grignard Reagents

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

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

By Reduction of Alcohols, Aldehydes, Ketones or Fatty Acids and their Derivatives

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

By Reduction of Carbonyl Compounds

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

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

By the Hydrolysis of AI or Be Carbides

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

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

By Corey- 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

Refer the below mentioned links to get an immediate solution to all queries on Organic chemistry:

JEE Organic Chemistry Syllabus

CBSE online coaching

CBSE online class 10