General Methods of Preparation of Alkanes
(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.
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
(ii) 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
RX + [RNa] → R-R + NaX
(b) Involving intermediate formation of free radicals:
RX + Na → [R] + Nax
R + R → R-R
(iii) By the reduction of alkyl halides:
(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) 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.
(v) 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).
(vi) 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.
(vii) 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.
(viii) 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
(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
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