Chemistry GENERAL METHODS OF PREPARATION, PROPERTIES AND REACTIONS
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GENERAL METHODS OF PREPARATION, PROPERTIES AND REACTIONS

Alkanes :

`text(Alkanes)` `text(Paraffin) :` The compounds of carbon and hydrogen with the general formula `C_n H_(2n+2)` are called as alkanes. These are also known as saturated hydrocarbons or `text(Paraffins)`.

`text(Structure and Reactivity :)` The simplest member of this family is methane (`CH_4`) where the `C`-atom is `sp^3` hybridized, which are overlapping with the `s`-orbitals of `H`-atoms at the corner of regular tetrahedron. This structure is verified by electron diffraction method. See fig.1.

The next higher member in the family is ethane `(C_2 H_6)`. See fig.2.

Here again each `C`-atom is `sp^3`-hybridized with a bond angle of approximately `109.5°`.

If one consider a molecule of methane or ethane or any other alkane, we find that all these molecules are non polar and hence the operative interactive forces are Van der Waal's forces.

Hence, within the family one would have stronger intermolecular forces if the surface area of molecule is large. See fig.3.

However, if branching of alkyl chain takes place, its surface area decreases and the molecular forces of attraction decrease.

e.g. n-Butane B.Pt. = `0°C` `CH_3-CH_2-CH_2-CH_3` Large Surface area

lsobutane B.Pt. = `-12°C` `(CH_3)_2CHCH_3` Smaller surface area

This shows as branching takes place the molecular force of attraction decreases so boiling point decreases. Being saturated hydrocarbons they are least reactive and most of the reactions correspond to the `C-H` bond cleavage which are very strong bonds.

Physical Properties :

(i) The first four members are colourless gases. The next `13` members (`C_5` to `C_17`) are colourless liquids and the higher ones are colourless solids.

(ii) The boiling points of the straight chain alkanes increases regularly with increase in molecular mass. On the average boiling point increases by `20-30``K`, for the addition of each carbon atom (`-CH_2-` group). Among the isomeric alkanes the boiling point of straight chain isomer is greater than branched chain isomers.

e.g. See fig.

Boiling point order is n-pentane > isopentane > neo-pentane.

However melting point order is Neopentane > Isopentane > n-pentane.

This is because as branching takes place the molecule assumes a spherical form that leads to a much more closed packing.

(iii) Alkanes are non-polar and hence are soluble in non-polar solvents and other organic solvents but insoluble in solvents such as water.

Preparations : Reduction Methods :

(i) `text(Hydrogenation of Alkenes and Alkynes :)`

When unsaturated hydrocarbons are catalytically hydrogenated we get alkane.

`C_nH_(2n) underset(200^oC -300^oC)overset(H_2 + Pt, Pd text(or) Ni) -> C_nH_(2n+2)`

`C_nH_(2n-2) underset(Ni, Pt text(or) Pd 200^oC - 300^oC)overset(2H_2)-> C_nH_(2n+2)`

e.g., `CH_2 = CH_2 underset(300^oC)overset(H_2//Ni)-> CH_3 - CH_3`

`CHequivCH +2H_2 underset(200-300^oC)overset(Ni)-> CH_3 -CH_3`


In the above reactions, Raney nickel is often used as an effective catalyst. It is obtained by boiling `Ni`-`Al` along with `NaOH`, when Aluminium dissolves leaving nickel in finely divided state, it is then filtered and dried. When palladium or platinum is used, the reactions can be carried even at room temperature.

(ii) `text(Red.uction of Alkyl Halides :)`

(a) `R-X underset(Delta C_2H_5OH)overset(Zn -Cu)-> R-H.`

`text(Mechanism :)`

`Zn -> Zn^(2+) + 2e^-`

`R-X -> R^* +X^* , R^* +e^(-) -> R^⊖ , X^* +e^(-) -> X^⊖`

`R^⊖ + H -OC_2H_5 -> R-H +C_2H_5O^⊖`

(b) `R-X underset(Delta)overset(Zn//HCl)-> R-H`

(c) `R-X underset(Delta)overset(Zn//CH_3COOH)-> R-H`

[Mechanisms for the reaction (b), (c) and (d) are the same as that of (a)]


(e) `R-X underset(Delta)overset(LiAlH_4)-> R-H`

[primary and secondary-alkyl halides give alkanes but tertiary halides gives alkenes]

(f) `R - X overset(NaBH_4)-> R-H`

[Here secondary- and tertiary-halides are converted to alkanes but not `pi`-halides]

(g) `R-X underset(Delta)overset(Pb_3SnH)-> R- H`.

[All types of alkyl halides undergo this reaction.]

`R - X` `underset(Delta)overset(HI//(text(Red) P)(->)` `R - H`

(iii) `text(Reduction of Alcohols :)`

(a) `R-OH +HI underset(Delta)overset(text(Red P))-> R- H`

`CH_3CH_2-OH +HI underset(Delta 150^oC)overset(text(Red P))-> CH_3 - CH_3 + I_2 + H_2O`

(iv) Reduction of CarbonyI Cornpounds (Aldehydes and Ketones) :

(a) `text(Using HI/Red P)`

See fig.1.

(b) `text(Clemmensen's Reduction :)` The reduction of carbonyl compounds to alkanes by using `Zn-Hg// HCl` is called Clemmensen's reduction.

See fig.2.

`text(NOTE :)` Clemmensen's reduction should not to be used if the molecule contains acid sensitive groups and `- NO_2` group because it also get reduced to `- NH_2` group.

(c) `text(Wolff Kishner Reduction :)` It is the reduction of carbonyl compounds to alkanes using hydrazine in the presence of alcoholic `KOH`. This method is not used if the molecule contains base sensitive groups apart from the carbonyl functional group.

See fig.3.

(iv) `text(Reduction of Carboxylic Acids :)`

`RCOOH underset(Delta)overset(HI text(Red P))-> R - CH_3`

`CH_3COOH underset(Delta 150^oC)overset(HI text(Red P))-> CH_3-CH_3`

Preparations : Decarboxylation Methods :

(i) `text(Soda Lime Decarboxylation)`

`RCOOH + NaOH -> RCOONa + H_2O`

`RCOONa + NaOH underset(630 K)overset(CaO)-> R- H + Na_2CO_3`

`CH_3COONa +NaOH underset(630 K)overset(CaO)-> CH_4 + Na_2CO_3`

(ii) `text(Kolbe's Electrolytic Decarboxylation)`

The electrolysis of aqueous salt solutions of carboxylic acids give alkane at anode.

`2RCOONa + 2H_2O oversettext(Electrolysis)-> R - R + 2NaOH + 2CO_2 + H_2`.

`2CH_3COONa (aq) oversettext(Electrolysis)-> CH_3 - CH_3 + 2NaOH + 2CO_2 + H_2.`

`text(Mechanism :)` See fig.1.

`H_2O -> H^(+)+ OH^(-)`

Reaction at cathode

`Na^(+) + e^(-) -> Na` (Not possible) `H^(+) + e^(-) -> 1/2 H_2` (Possible)

(Because the discharge potential of `H^(+)` is less than the discharge potential of `Na^(+)`)

Reaction at anode

`4OH^(-) -> 2H_2O + O_2 + 4e^(-)` (Not possible) `2RCOO^(-) -> R - R + 2CO_2 + 2e^(-)` (Possible)

(Because discharge potential of `RCOO^(-) < OH^(-)`)

`text(Detailed reaction at anode Side Reaction)` : See fig.2.

Also disproportionation leads to the formation of alkene and smaller alkane as side products.

(a) `CH_3CH_2^* + CH_3CH_2COO^* -> CH_3CH_2COOCH + 2CH_3`

`CH_3CH_2COO CH_2CH_3 overset(NaOH)-> CH_3CH_2COONa + C_2H_5OH` (side product)

(b) `CH_3CH_2^* -> CH_2=CH_2 +H^*`

`CH_3CH_2^* +H^* -> CH_3-CH_3`

Hence side products are `CH_2 = CH_2 + CH_3CH_3` also

`text(Limitation)`

(i) Methane cannot be perpared by this method.

(ii) Only symmetrical alkanes are prepared by this method.

Preparations : Through Formation of Organometallic Compounds :

(i) `text(Wurtz reaction :)`

When alkyl halides are reacted with `Na` in the presence of ether we get alkane.

`2R-X+ 2Na undersettext(ether)oversettext(Dry)-> R -R + 2NaX`

Other metals in a finely divided state may also be used in place of `Na` e.g. `Cu` or `Ag`.

`2CH_3Cl + Na undersettext(ether)oversettext(Dry)-> CH_3CH_3 + NaCl`

`text(Limitations :)`

(i) Methane cannot be prepared.

(ii) This reaction is not suitable for preparing unsymmetrical alkanes e.g. propane or pentane etc.

`text(Mechanism)`

`text(Ionic)`

`R-X -> R ^(.)·+X^(.) `

`Na -> Na^(+) + e^(-)`

`R^(.) +e^(-) -> R^-`

`X^(.) +e^(-) -> X^-`

`R^(⊖) + Na^(+) -> RNa`

`X^(⊖) + Na^(+) -> NaX`

`RNa +R - X -> R-R +NaX +` Side product

`text(Note :)` Side Products are possible due to elimination reaction on `R-X` by `RNa` giving a mixture of alkene and alkane.

`text(Free radical Mechanism)`

`R-X -> R^(.) +X^(.)`

`Na -> Na^(+) +e^(-)`

`X^(.) +e^(-) -> X^-`

`R^(.) +R^(.) -> R -R +` other side product

The side products are formed due to disproportionation of free radicals.


(ii) `text(From Grignard's Reagent :)`

When Grignard's reagent is made to react with any compound containing `H`-atom attached to highly electronegative atom like `O`, `N` or `F` will give alkane corresponding the alkyl part of Grignard's Reagent.

`RMg X+ H - OH -> R - H + MgX (OH)`

`R Mg X+ H - OR' -> R - H + MgX (OR')`

`R Mg X+ H - NH_2 -> R - H + MgX (NH_2)`

`R Mg X + H - NHR' -> R - H + MgX (NHR')`

`R Mg X -> Mg (R"COO) (X)`

(iii) `text(Frankland Reaction :)`

`RX + Zn -> R ZnX`

The alkyl zinc halide or dialkyl zinc may be isolated and made to react with different alkyl halide or same alkyl halide.

`RZnX + R'X -> undersettext[(unsymmetrical alkane)](R - R') + ZnX_2`

e.g., `CH_3ZnCl + CH_3CH_2Cl -> CH_3CH_2CH_3 + ZnCl_2`

`RZnX +R-X -> undersettext[(symmetrical alkane)][R-R] +ZnX_2`

e.g. `CH_3ZnCl + CH_3Cl -> CH_3CH_3 + ZnCl_2`

Hence Frankland reaction is useful for preparing both hydrocarbons containing odd number of `C`-atoms as well as even number of carbon atom. i.e. for preparing symmetrical as well as unsymmetrical alkanes.

(iv) `text(Corey House synthesis :)`

This reaction is very much useful in the preparations of unsymmetrical alkane.

`R_2CuLi + R'-X oversettext(ether)-> undersettext[(unsymmetrical)][R - R'] + R Cu + LiX.`

`R_2CuLi + R -X oversettext(ether)-> undersettext[(symmetrical)][R - R] + RCu + LiX`

`CH_3CH_2 -Br undersettext(Dry ether)overset(2 Li)-> CH_3CH_2Li +LiBr`

`2CH_3CH_2Li +CuI -> (CH_3CH_2)_2 CuLi + LiI`

`(CH_3CH_2)_2CuLi overset(CH_3-Cl)-> CH_3CH_2 - CH_3 + CH_3CH_2Cu + LiCl`.

`(CH_3CH_2)_2CuLi oversettext(CH_3CH_2-Cl)-> CH_3CH_2- CH_2CH_3 + CH_3CH_2Cu + LiCl`

(v) `text(From Carbides :)`

`Al_4C_3 + 12H_2O -> 4Al(OH)_3 + 3CH_4`

`Be_2C + 4H_2O -> 2Be(OH)_2 +CH_4`

Chemical Reactions :

Nitration :

The reaction in which `H` atom of an alkane is substituted by nitro group `(-NO_2)` is called nitration. See fig.1.

Normally hexane and higher alkanes can be Nitrated very easily but will give many side products. See fig.2.

`undersettext(Haxane)(H_3C -(CH_2)_4 - CH_3) + HNO_3 overset(Delta)-> H_2O + undersettext(Nitrohexene)(H_3C -(CH_2)_4 -CH_2-NO_2)`

Recently developed technique of vapour phase nitration, has made nitration of lower alkanes possible. But the vapour phase nitration also causes cleavage of `C-C` bond and results in the formation of more than one nitro compounds.

`undersettext(Ethane)(H_3C -CH_3) +HNO_3 underset(500^oC)ovesrset(V.P. )-> undersettext(Nitroethane)(H_3C-CH_2-NO_2) +undersettext(Nitromethane)(H_3C-NO_2)`

`undersettext(Propane)(H_3C-CH_2-CH_3) +HNO_3 underset(500^oC)ovesrset(V.P. )-> H_3C -CH_2-CH_2-NO_2+ undersettext[2-nitro propane][(CH_3)_2 CH-NO_2] + undersettext(Nitroethane)(H_3C -CH_2-NO_2) + undersettext(Nitromethane)(H_3-NO_2)`

Sulphonation : The reaction in which `H` atoms of alkanes are replaced by sulphonic acid group `(-SO_3H)` is called sulphonation. See fig.3.

The mixture of `H_2SO_4 + SO_3 -> H_2S_2O_7` is called as oleum or Fuming sulphuric acid.

Oxidation of alkanes : (a) Combustion (complete oxidation).

Like other organic compounds alkanes on complete combustion form `CO_2` and `H_2O`.

`undersettext(Methane)(CH_4) +2O_2 -> CO_2 + 2H_2O`

`undersettext(Ethane)(C_2H_6) +7/2 O_2 -> 2CO_2 +3H_2O`

(b) Chemical oxidation : Alkanes are usually not affected by oxidising agents like `KMnO_4` or `K_2Cr_2O_7`, but, alkanes having `3°` `H` atoms are oxidised on heating by these oxidising agents to alcohols. See fig.4.

 
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