JEE Main & Advanced Chemistry Hydrocarbons / हाइड्रोकार्बन Alkanes [Paraffines]

Alkanes [Paraffines]

Category : JEE Main & Advanced

“Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecules.”

Alkanes are less reactive so called paraffins; because under normal conditions alkanes do not react with acids, bases, oxidising agents and reducing agent.

General formula : \[{{C}_{n}}{{H}_{2n+2}}\]

Examples are \[C{{H}_{4}},\,{{C}_{2}}{{H}_{6}},\,{{C}_{3}}{{H}_{8}}\],

(1) General Methods of preparation

(i) By catalytic hydrogenation of alkenes and alkynes (Sabatie and sanderen’s reaction)

\[\underset{\text{Alkene}}{\mathop{{{C}_{n}}{{H}_{2n}}}}\,+{{H}_{2}}\underset{\text{heat}}{\mathop{\xrightarrow{Ni}}}\,\underset{\text{Alkane}}{\mathop{{{C}_{n}}{{H}_{2n+2}}}}\,\]; \[\underset{Alkyne}{\mathop{{{C}_{n}}{{H}_{2n-2}}}}\,+2{{H}_{2}}\underset{\text{heat}}{\mathop{\xrightarrow{Ni}}}\,\underset{\text{Alkane}}{\mathop{{{C}_{n}}{{H}_{2n+2}}}}\,\]

Methane is not prepared by this method

(ii) Birch reduction :

\[R-CH=C{{H}_{2}}\underset{2.\,C{{H}_{3}}OH}{\mathop{\xrightarrow{1.\,Na/N{{H}_{3}}}}}\,R-C{{H}_{2}}-C{{H}_{3}}\]

(iii) From alkyl halide

(a) By reduction : \[RX+{{H}_{2}}\xrightarrow{Zn/HCl}RH+HX\]

(b) With hydrogen in presence of pt/pd : \[RX+{{H}_{2}}\xrightarrow{Pd\,orPt.}RH+HX\]

(c) With HI in presence of Red phosphorus : \[\underset{\text{Purpose of Red }P\text{ is to remove }{{I}_{\text{2}}}\text{ in the form of }P{{I}_{\text{3}}}}{\mathop{RBr+2HI\xrightarrow{{}}RH+HBr+{{I}_{2}}}}\,\]

(iv) By Zn-Cu couple :

\[2C{{H}_{3}}C{{H}_{2}}OH+\underset{\text{Zn-Cu}\,\text{couple}}{\mathop{Zn}}\,\xrightarrow{Cu}\underset{\text{Zinc}\,\text{ethoxide}}{\mathop{{{(C{{H}_{3}}C{{H}_{2}}O)}_{2}}Zn}}\,+2H\]

\[RX+2H\xrightarrow{{}}RH+HX\]

(v) Wurtz reaction :

\[R-Br\] or \[RI\] preferred in this reaction. The net result in this reaction is the formation of even no. of carbon atoms in molecules.

(vi) Frankland’s reaction :

\[2RX+Zn\xrightarrow{{}}R-R+Zn{{X}_{2}}\]

(vii) Corey-house synthesis

\[C{{H}_{3}}-C{{H}_{2}}-Cl\underset{2.\,CuI}{\mathop{\xrightarrow{1.\,Li}}}\,{{(C{{H}_{3}}-C{{H}_{2}})}_{2}}LiCu\xrightarrow{C{{H}_{3}}-C{{H}_{2}}-Cl}\]\[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}\]

Reaction is suitable for odd number of Alkanes.

(viii) From Grignard reagent

(a) By action of acidic ‘H’ :

\[\underset{\text{halide}}{\mathop{\underset{\text{Alkyl magnesium}}{\mathop{RMgX}}\,}}\,+\underset{\text{Water}}{\mathop{HOH}}\,\xrightarrow{{}}\underset{\text{Alkane}}{\mathop{RH}}\,+Mg(OH)X\]

(b) By reaction with alkyl halide :

(ix) From carboxylic acids

(a) Laboratory method [Decarboxylation reaction or Duma reaction] \[R\,\,COONa+NaOH\underset{CaO}{\mathop{\xrightarrow{heat}}}\,\underset{\text{Alkane}}{\mathop{R-H}}\,+N{{a}_{2}}C{{O}_{3}}\]

NaOH and CaO is in the ratio of 3 : 1. (Sodalime)

(b) Kolbe’s synthesis :

At anode [Oxidation] :

\[\underset{O\,\,\,\,\,\,\,\,\,\,\,}{\mathop{\underset{|\,|\,\,\,\,\,\,\,\,\,\,\,}{\mathop{2R-C-{{O}^{-}}-2{{e}^{-}}}}\,}}\,\xrightarrow{{}}2R-\underset{O}{\mathop{\underset{|\,|}{\mathop{C}}\,}}\,-\overset{\bullet }{\mathop{O}}\,\xrightarrow{{}}2\overset{\bullet }{\mathop{R}}\,+2C{{O}_{2}}\]

\[2\overset{\bullet }{\mathop{R}}\,\xrightarrow{{}}R-R\] (alkane)

At cathode [Reduction] :

\[2N{{a}^{+}}+2{{e}^{-}}\xrightarrow{{}}2Na\xrightarrow{2{{H}_{2}}O}2NaOH+{{H}_{2}}\] \[(\uparrow )\]

Both ionic and free radical mechanism are involved in this reaction.

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+6HI\underset{p}{\mathop{\xrightarrow{\operatorname{Re}duction}}}\,\underset{\text{Ethane}}{\mathop{C{{H}_{3}}C{{H}_{3}}}}\,+2{{H}_{2}}O+3{{I}_{2}}\]

(x) By reduction of alcohols, aldehyde, ketones or acid derivatives

\[\underset{\text{(Methyl alcohol)}}{\mathop{\underset{\text{Methanol}}{\mathop{C{{H}_{3}}OH}}\,}}\,+2HI\underset{{{150}^{o}}C}{\mathop{\xrightarrow{\text{Red}\,P}}}\,\underset{\text{Methane}}{\mathop{C{{H}_{4}}}}\,+{{H}_{2}}O+{{I}_{2}}\]

\[\underset{\text{(Ethanal)}}{\mathop{\underset{\text{Acetaldehyde}}{\mathop{C{{H}_{3}}CHO}}\,}}\,+4HI\underset{{{150}^{o}}C}{\mathop{\xrightarrow{\text{Red}\,P}}}\,\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,+{{H}_{2}}O+2{{I}_{2}}\]

\[\underset{\text{(Propanone)}}{\mathop{\underset{\text{Acetone}}{\mathop{C{{H}_{3}}COC{{H}_{3}}}}\,}}\,+4HI\underset{{{150}^{o}}C}{\mathop{\xrightarrow{\text{Red}\,P}}}\,\underset{\text{Propane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{3}}}}\,+{{H}_{2}}O+2{{I}_{2}}\]

\[\underset{\text{(Ethanoyl chloride)}}{\mathop{\underset{\text{Acetyl chloride}}{\mathop{C{{H}_{3}}-\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,-Cl}}\,}}\,+6HI\underset{{{200}^{o}}C}{\mathop{\xrightarrow{\text{Red}\,P}}}\,\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,+{{H}_{2}}O+HCl+3{{I}_{2}}\]

\[\underset{\text{(Ethanamide)}}{\mathop{\underset{\text{Acetamide}}{\mathop{C{{H}_{3}}-\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,-N{{H}_{2}}}}\,}}\,+6HI\underset{{{200}^{o}}C}{\mathop{\xrightarrow{\text{Red}\,P}}}\,\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,+{{H}_{2}}O+N{{H}_{3}}+3{{I}_{2}}\]

Aldehyde and ketones when reduced with amalgamated zinc and conc. \[HCl\] also yield alkanes.

Clemmenson reduction :

\[\underset{\text{(Ethanal)}}{\mathop{\underset{\text{Acetaldehyde}}{\mathop{C{{H}_{3}}CHO}}\,}}\,+4H\underset{HCl}{\mathop{\xrightarrow{Zn-Hg}}}\,\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,+{{H}_{2}}O\]

\[\underset{\text{(Propanone)}}{\mathop{\underset{\text{Acetone}}{\mathop{C{{H}_{3}}COC{{H}_{3}}}}\,}}\,+4H\underset{HCl}{\mathop{\xrightarrow{Zn-Hg}}}\,\underset{\text{Propane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{3}}}}\,+{{H}_{2}}O\]

Aldehydes and ketones \[(>C=O)\] can be reduced to hydrocarbon in presence of excess of hydrazine and sodium alkoxide on heating.

Wolff-kishner reduction :

(xi) Hydroboration of alkenes

(a) On treatment with acetic acid

\[\underset{\text{Alkene}}{\mathop{R-CH=C{{H}_{2}}}}\,\xrightarrow{{{B}_{2}}{{H}_{6}}}\underset{\text{Trialkyl borane}}{\mathop{{{(R-C{{H}_{2}}-C{{H}_{2}})}_{3}}B}}\,\xrightarrow{C{{H}_{3}}COOH}\]\[\underset{\text{Alkane}}{\mathop{R-C{{H}_{2}}-C{{H}_{3}}}}\,\]

(b) Coupling of alkyl boranes by means of silver nitrate

\[6[R-CH=C{{H}_{2}}]\xrightarrow{2{{B}_{2}}{{H}_{6}}}{{[2R-C{{H}_{2}}-C{{H}_{2}}-]}_{3}}B\underset{NaOH}{\mathop{\xrightarrow{AgN{{O}_{3}}\,{{25}^{o}}C}}}\,\]\[3[RC{{H}_{2}}C{{H}_{2}}-C{{H}_{2}}C{{H}_{2}}R]\]

(2) Physical Properties

(i) Physical state : Alkanes are colourless, odourless and tasteless.

Alkanes State

\[{{C}_{1}}-{{C}_{4}}\] Gaseous state

\[{{C}_{5}}-{{C}_{17}}\] Liquid state [Except neo pentane which is gas]

\[{{C}_{18}}\] and above Solid like waxes

(ii) Density : Alkanes are lighter than water.

(iii) Solubility : Insoluble in water, soluble in organic solvents, solubility \[\propto \frac{1}{\text{Molecular mass}}\]

(iv) Boiling points and Melting points : Melting points and boiling points. \[\propto \] Molecular mass \[\propto \frac{1}{\text{No}\text{.}\,\text{of}\,\text{branches}}\]

Alkane :	\[{{C}_{3}}{{H}_{8}}\]	\[{{C}_{4}}{{H}_{10}}\]	\[{{C}_{5}}{{H}_{12}}\]	\[{{C}_{6}}{{H}_{14}}\]	\[{{C}_{7}}{{H}_{16}}\]	\[{{C}_{8}}{{H}_{18}}\]
M.P.(K) :	85.9	138	143.3	179	182.5	216.2

Melting points of even > Odd no. of carbon atoms, this is because, the alkanes with even number of carbon atoms have more symmetrical structure and result in closer packing in the crystal structure as compared to alkanes with odd number of carbon atoms.

(3) Chemical properties

(i) Substitution reactions of Alkanes

(a) Halogenation : \[R-H+X-X\xrightarrow{{}}R-X+HX\]

The reactivity of halogen is : \[{{F}_{2}}>C{{l}_{2}}>B{{r}_{2}}>{{I}_{2}}\]

Fluorine can react in dark \[C{{l}_{2}},\,B{{r}_{2}}\] require light energy. \[{{I}_{2}}\] doesnot show any reaction at room temperature, but on heating it shows iodination.
Iodination of methane is done in presence of oxidising agent such as \[HN{{O}_{3}}/HI{{O}_{3}}/HgO\] which neutralises \[HI\].

Chlorination of methane :

\[C{{H}_{4}}+2Cl-Cl\underset{-2HCl}{\mathop{\xrightarrow{u.\,v.\,light}}}\,C{{H}_{2}}-C{{l}_{2}}\underset{-HCl}{\mathop{\xrightarrow{u.\,v.\,light,\,C{{l}_{2}}}}}\,\]\[CHC{{l}_{3}}\underset{C{{l}_{2}}}{\mathop{\xrightarrow{-HCl}}}\,CC{{l}_{4}}\]

(ii) Reaction based on free radical mechanism

(a) Nitration : \[\underset{\text{Alkane}}{\mathop{R-H}}\,+HON{{O}_{2}}\underset{temp.}{\mathop{\xrightarrow{High}}}\,\underset{\text{Nitroalkane}}{\mathop{R-N{{O}_{2}}}}\,+{{H}_{2}}O\]

Nitrating mixture : (i) \[(Con.\,HN{{O}_{3}}+Con.\,{{H}_{2}}S{{O}_{4}})\] at \[{{250}^{o}}C\]

(ii) \[(HN{{O}_{3}}\,\text{vapour}\,\text{at }\,{{400}^{o}}-{{500}^{o}}C)\].

(b) Sulphonation : Free radical mechanism \[R-H+HOS{{O}_{3}}H\underset{\text{Prolonged}\,\text{heating}}{\mathop{\xrightarrow{\,\,\,\,\,\,\text{S}{{\text{O}}_{\text{3}}}\,\,\,\,}}}\,R-S{{O}_{3}}H+{{H}_{2}}O\]

Lower alkanes particularly methane, ethane, do not give this reaction.

(iii) Oxidation

(a) Complete Oxidation or combustion :

\[{{C}_{n}}{{H}_{2n+2}}+\left( \frac{3n+1}{2} \right){{O}_{2}}\xrightarrow{{}}nC{{O}_{2}}+(n+1){{H}_{2}}O+Q\]

This is exothermic reaction.

(b) Incomplete combustion or oxidation

\[2C{{H}_{4}}+3{{O}_{2}}\xrightarrow{Burn}2CO+4{{H}_{2}}O\]

\[C{{H}_{4}}+\,\,{{O}_{2}}\xrightarrow{{}}C\,+2{{H}_{2}}O\]

(c) Catalytic Oxidation : \[C{{H}_{4}}+[O]\underset{100\,atm/{{200}^{o}}C}{\mathop{\xrightarrow{Cu-tube}}}\,\,C{{H}_{3}}OH\]

This is the industrial method for the manufacture of methyl alcohol.

Higher alkanes are oxidised to fatty acids in presence of manganese stearate.

\[C{{H}_{3}}{{(C{{H}_{2}})}_{n}}C{{H}_{3}}\underset{100-{{160}^{o}}C}{\mathop{\xrightarrow{{{O}_{2}}}}}\,C{{H}_{3}}{{(C{{H}_{2}})}_{n}}COOH\]

(d) Chemical oxidation :

\[\underset{\text{Isobutane}}{\mathop{{{(C{{H}_{3}})}_{3}}CH}}\,\xrightarrow{KMn{{O}_{4}}}\underset{\text{Tertiary butyl alcohol}}{\mathop{{{(C{{H}_{3}})}_{3}}.C.OH}}\,\]

(iv) Thermal decomposition or cracking or pyrolysis or fragmentation

\[\underset{\text{Methane}}{\mathop{C{{H}_{4}}}}\,\xrightarrow{{{1000}^{o}}C}C+2{{H}_{2}}\]

\[\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,\underset{C{{r}_{2}}{{O}_{3}}+A{{l}_{2}}{{O}_{3}}}{\mathop{\xrightarrow{{{500}^{o}}C}}}\,\underset{\text{Ethylene}}{\mathop{C{{H}_{2}}=C{{H}_{2}}}}\,+{{H}_{2}}\]

\[{{C}_{3}}{{H}_{8}}\xrightarrow{{}}{{C}_{2}}{{H}_{4}}+C{{H}_{4}}\] or \[{{C}_{3}}{{H}_{6}}+{{H}_{2}}\]

This reaction is of great importance to petroleum industry.

(v) Isomerisation :

\[\underset{n\text{-Butane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{3}}}}\,\underset{{{200}^{o}}C,\,35atm}{\mathop{\xrightarrow{AlC{{l}_{3}}+HCl}}}\,\underset{\text{Isobutane}}{\mathop{C{{H}_{3}}\overset{C{{H}_{3}}\,\,\,\,\,\,\,}{\mathop{\overset{|\,\,\,\,\,\,\,\,\,\,\,\,\,}{\mathop{CHC{{H}_{3}}}}\,}}\,}}\,\]