Hydrocarbon
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ALKANES
Introduction of Alkanes
(a) Branched and unbranched aliphatic saturated open chain hydrocarbons are called member of alkanes.
(b) CH4 is also known as Marsh gas (fire damp).
(c) Calore gas : Mixture of n–butane and isobutane.
(d) LPG (Liquefied petroleum gas) : liquid propane, isobutane.
(e) Natural gas : 80% methane + 10% ethane + 10% propane + small amounts of H2, N2, CO2 etc.
(f) Water gas : CO + H2
(g) Synthesis gas : CO + 3H
General Methods of Preparation
(1) From alkenes and alkynes
(Sabatier and Senderens reaction) or (By hydrogenation of
alkenes and alkynes) : Alkenes and alkynes on catalytic hydrogenation give alkanes
Catalyst :
(a) Pd/Pt at ordinary temperature and pressure
(b) Ni, 200–300° C (sabatier)
(c) Raney Nickel at room temp.
(d) Methane can not be prepared by this method
(2) From alkyl Halides (By reduction) :
Catalyst :
(i) Zn + HCl (ii) Zn + CH3 COOH (iii) Zn—Cu couple in C2H5OH
(iv) Red P + HI (v) Al – Hg + ethanol
(3) From alkyl halide
(By Wurtz reaction): A solution of alkyl halide in ether on heating with sodium
gives alkane
(a) Two moles of alkyl halide are treated with Na in presence of dry ether. If ether is wet then we
obtain alcohol(b) Methane can not be prepared by this method. The alkane produced is higher and symmetrical
i.e. it contains double the number of carbon atoms present in the alkyl halide taken.
(c) Two different alkyl halides, on wurtz reaction give all possible alkanes.
(d) The separation of mixture into individual members is not easy because their B.P. are near to
each other and thus wurtz reaction is not suitable for the synthesis of alkanes containing odd
number of carbon atom
(4) From Frankland Reagent:
(5) From Carboxylic Acid
(By decarboxylation) : Sodium salt of saturated monocarboxylic acid on
dry distillation with sodalime give alkane
The process of elimination of Carbon-di-oxide from Carboxylic acid called decarboxylation.
The alkane formed by decarboxylation contains one carbon atom less than the original acid.
Decarboxylation of sodium formate gives H2O
if a compound two caroxylic group present and they are attached to same carbon atom then
decarboxylation of one of the carboxylic groups takes place simply on heating.
CH3-—CH2-—CH3 can be prepared by Butanoic acid and 2-–Methyl propanoic acid
b-Keto acids are decarboxylated readily simply on heating (soda lime is not required)
(6) From carboxylic acid (By Kolbe's electrolysis process) : Alkanes are formed on electrolysis
of concentrated aqueous solution of sodium or potassium salt of saturated monocarboxylic acids
Electrolysis of an acid salt gives symmetrical alkane, However in case of a mixture of Carboxylic acid
salts, all probable alkanes are formed
At anode alkane and CO2 gas is formed while at cathode NaOH and H2 gas is formed.
The concentration of NaOH in solution is increased with time so pH of solution is also increased
(7) From alkanol, alkanals,
Alkanone and alkanoic acid (By reduction) :
The reduction of either of the above compound in presence of red P and HI gives corresponding
alkane
In the above reaction I2 is formed which may react with alkane to form alkyl halide. So red P is
added in the reaction to remove I2 formed in the reaction
(8) From alkanones
(By Clemmensen's method) : Carbonyl compound may also be reduced with
Zinc amalgam and concentrated HCl (Zn–Hg/HCl), this reaction is called Clemmensen reduction
(9) From alkanals and alkanones (By Wolf Kishner reaction) :
(10) From G.R. :
(a) Formation of alkanes with same number of C atoms : Grignard reagent reacts with the
compounds having active hydrogen to form alkane
This reaction is used to determine the number of active H-atoms in the compound this is known
as Zerewitnoff's method
(b) G.R. react with alkyl halide to give higher alkanes :
11) Corey-House Synthesis : This method is suitable for the preparation of unsymmetrical alkanes
i.e. those of type R—R'
12) From metal carbide (By hydrolysis) :
Only CH4 can be obtained by the hydrolysis of Be or Al carbides
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