General Methods of Preparation of Alkanes
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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.
![RCOONa\xrightarrow[-CO^{_{2}}]{Soda Lime} R-H](https://files.askiitians.com/cdn1/cms-content/common/latex.codecogs.comgif.latexrcoona_xrightarrow-co_2sodalimer-h.jpg)
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.
| An alkyl halide on Wurtz reaction leads to the formation of symmetrical alkane having an even number of carbon atoms. Two different alkyl halides, on Wurtz reaction give all possible alkanes. |
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.
| When Zn is used in Wurtz reaction in place of Na, the reaction is named as Frankland method. |
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
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
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.
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
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
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