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 soda-lime R-H

or RCOONa + NaOH cao 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 cao H2 + Na2CO3

CH3COONa + NaOH cao 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 catalyst-ni CH3-CH3

CH≡CH + 2H2 catalyst-ni 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 chemical-2h 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

kolbes-electrolysis-method

(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 electrolysis 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.

grignard-reagent-on-double-decomposition

(vi) Wurtz reaction:

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

R-X + 2Na + X-R dry-ether 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 chemical-3 CH3CH3 + H2O

CH3COOH + 2H2 chemical-3 CH3CH2CH3 + H2O

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

carbonyl-compounds-reduced

(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.

reduction



(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 delta 4AI(OH)3 + 3CH4

Be2C + 4H2O delta 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 chemical-cu 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 chemical-1 2(RCH2CH2)3B chemical-2 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 ether CH3CH2Li + LiCl

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

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