Chemistry Preparation of alkanes and their physical and chemical properties

Topics to be covered

`=>` Preparation of alkanes
`=>` Physical properties
`=>`Chemical properties

Preparation of alkanes

`color{green}("๐Ÿ. ๐…๐ซ๐จ๐ฆ ๐ฎ๐ง๐ฌ๐š๐ญ๐ฎ๐ซ๐š๐ญ๐ž๐ ๐ก๐ฒ๐๐ซ๐จ๐œ๐š๐ซ๐›๐จ๐ง๐ฌ:")`

Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium(catalyse the reaction at room temperature) or nickel(catalyse at higher temperature and pressure) to form alkanes. This process is called `color{green}("๐ก๐ฒ๐๐ซ๐จ๐ ๐ž๐ง๐š๐ญ๐ข๐จ๐ง")` .Dihydrogen gas gets adsorbed on metal surfaces and the hydrogen โ€“ hydrogen bond gets activated.

`color{red}(underset("Ethene")(CH_2 = CH_2) + H_2 overset(" Pt/Pd / Ni")โ†’ underset(" Ethane")(CH_3 - CH_3))` ................(13.1)

`color{red}(underset("Propene")(CH_3 - CH = CH_2) + H_2 overset("Pt/Pd/Ni")โ†’ underset("Propane")(CH_3 - CH_2 - CH_3))` ............(13.2)

`color{red}(underset("Propyne")(CH_3 - C equiv C - H) +2H_2 overset(" Pt/Pd/Ni")โ†’ underset("Propane")(CH_3 - CH_2 - CH_3))` ..........(13.3)

`color{green}("๐Ÿ. ๐…๐ซ๐จ๐ฆ ๐š๐ฅ๐ค๐ฒ๐ฅ ๐ก๐š๐ฅ๐ข๐๐ž๐ฌ ")`

i) Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.

`color{red}(underset("Chloromethane")(CH_3 - Cl) + H_2 overset(Zn , H^(+))โ†’ underset("Methane")(CH_4) + HCl)` ...........(13.4)

`color{red}(underset("Chloroethane")(C_2H_5 - Cl) + H_2 overset(Zn , H^(+))โ†’ underset("Ethane")(C_2H_6)+HCl)` ................(13.5)

`color{red}(underset("1-Chloropropane")(CH_3CH_2CH_2Cl)+H_2 overset(Zn , H^(+))โ†’ underset("Propane")(CH_3CH_2CH_3) +HCl)` ..........(13.6)

ii) Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as `color{green}("๐–๐ฎ๐ซ๐ญ๐ณ ๐ซ๐ž๐š๐œ๐ญ๐ข๐จ๐ง")` and is used for the preparation of higher alkanes containing even number of carbon atoms.

`color{red}(underset("Bromomethane")(CH_3Br) +2Na+ BrCH_3 overset("dry ether")โ†’ underset("Ethane")(CH_3-CH_3) + 2NaBr)` .........(13.7)

`color{red}(underset("Bromoethane")(C_2H_5Br) + 2Na + BrC_2H_5 overset("dry ether")โ†’ underset("n-Butane")(C_2H_5 - C_2H_5))` ........(13.8)

`color{green}("๐Ÿ‘. ๐…๐ซ๐จ๐ฆ ๐œ๐š๐ซ๐›๐จ๐ฑ๐ฒ๐ฅ๐ข๐œ ๐š๐œ๐ข๐๐ฌ")`

i) Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as `color{green}("๐๐ž๐œ๐š๐ซ๐›๐จ๐ฑ๐ฒ๐ฅ๐š๐ญ๐ข๐จ๐ง.")`

`color{red}(underset("Sodium ethanoate")(CH_3COO^(-) Na^(+)) + NaOH underset(Delta) overset(CaO)โ†’ CH_4+Na_2CO_3)`

ii) `color{green}("๐Š๐จ๐ฅ๐›๐žโ€™๐ฌ ๐ž๐ฅ๐ž๐œ๐ญ๐ซ๐จ๐ฅ๐ฒ๐ญ๐ข๐œ ๐ฆ๐ž๐ญ๐ก๐จ๐")` An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode.

The reaction is supposed to follow the following path :

i) `color{red}(2CH_3COO^(-) Na^(+) โ‡Œ 2 CH_3 - overset(overset(O)(||))C - O^(-) +2Na^(+))`

ii) `color{green}("๐€๐ญ ๐š๐ง๐จ๐๐ž:")` `color{red}(underset("Acetate ion")(2CH_3 - overset(overset(O)(||))C- O^(-)) + overset(-2e^(-))โ†’ underset("Acetate free radical")( 2CH_3 - overset(overset(O)(||))C - underset(. .) overset(.)O ": ") โ†’ underset("Methyl free radical")( 2 overset(.)CH_3+ 2CO_2 โ†‘))`

iii) `color{red}(H_3 overset(.)C + overset(.)CH_3 โ†’ H_3C - CH_3 โ†‘)`

iv) `color{green}("๐€๐ญ ๐œ๐š๐ญ๐ก๐จ๐๐ž ")`: `color{red}(H_2O + e^(-) โ†’ text()^(-)OH + overset(.)H)`

`color{red}(2 overset(.)H โ†’ H_2 โ†‘)`

Q 3234867752

Sodium salt of which acid will be needed for the preparation of propane ? Write chemical equation for the reaction.


Butanoic acid,

`CH_3CH_2CH_2COO^(-)Na^(+) + NaOH overset(CaO)โ†’ CH_3CH_2CH_3+ Na_2CO_3`

Physical properties

โ€ข Alkanes are almost non-polar molecules because of the covalent nature of `color{red}(C-C)` and `color{red}(C-H)` bonds and due to very little difference of electronegativity between carbon and hydrogen atoms.

โ€ข They possess weak van der Waals forces. Due to the weak forces, the first four members, `color{red}(C_1)` to `color{red}(C_4)` are gases, `color{red}(C_5)` to `color{red}(C_(17))` are liquids and those containing 18 carbon atoms or more are solids at `298 K`.

โ€ข They are colourless and odourless.

โ€ข Petrol is a mixture of hydrocarbons and is used as a fuel for automobiles.

โ€ข Petrol and lower fractions of petroleum are also used for dry cleaning of clothes to remove grease

โ€ข Grease (mixture of higher alkanes) is non-polar and, hence, hydrophobic in nature.

โ€ข It is generally observed that polar substances are soluble in polar solvents, whereas the non-polar ones in non-polar solvents i.e., like dissolves like.

โ€ข Boiling point (b.p.) of different alkanes are given in Table 13.2 from which it is clear that there is a steady increase in boiling point with increase in molecular mass. This is due to the fact that the intermolecular van der Waals forces increase with increase of the molecular size or the surface area of the molecule.

โ€ข With increase in number of branched chains, the molecule attains the shape of a sphere. This results in smaller area of contact and therefore weak intermolecular forces between spherical molecules, which are overcome at relatively lower temperatures. It is observed (Table 13.2) that pentane having a continuous chain of five carbon atoms has the highest boiling point (309.1K) whereas 2,2 - dimethylpropane boils at 282.5K.

Chemical properties

Alkanes undergo the following reactions under certain conditions.

1. `color{green}("๐’๐ฎ๐›๐ฌ๐ญ๐ข๐ญ๐ฎ๐ญ๐ข๐จ๐ง ๐ซ๐ž๐š๐œ๐ญ๐ข๐จ๐ง๐ฌ:")`

One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane is given below:

`color{green}("๐‡๐š๐ฅ๐จ๐ ๐ž๐ง๐š๐ญ๐ข๐จ๐ง")`

`color{red}(CH_4 + Cl_2 overset(hv)โ†’ underset("Chloromethane")(CH_3Cl) + HCl)` ...................(13.10)

`color{red}(CH_3Cl + Cl_2 overset(hv)โ†’ underset("Dichloromethane")(CH_2Cl_2) + HCl)` ..........(13.11)

`color{red}(CH_2Cl_2 + Cl_2 overset(hv)โ†’ underset("Trichloromethane")(CHCl_3) + HCl)` ................(13.12)

`color{red}(CHCl_3 + Cl_2 overset(hv)โ†’ underset("Tetrachloromethane")(C Cl_4) + HCl)` ...............(13.13)

`color{red}(CH_3- CH_3+Cl_2 overset(hv)โ†’ underset("Chloroethane")(CH_3 - CH_2 Cl) + HCl)` ...............(13.14)

โ€ข It is found that the rate of reaction of alkanes with halogens is `color{red}(F_2 > Cl_2 > Br_2 > I_2).`
โ€ข Rate of replacement of hydrogens of alkanes is : `color{red}("3ยฐ > 2ยฐ > 1ยฐ")`.
โ€ข Fluorination is too violent to be controlled.
โ€ข Iodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like `color{red}(HIO_3)` or `color{red}(HNO_3)`.

`color{red}(CH_4 + I_2 โ‡Œ CH_3I + HI)` .....................(13.15)

`color{red}(HIO_3 + 5HI โ†’ 3I_2 + 3H_2O)` .................(13.16)

`color{green}("๐Œ๐„๐‚๐‡๐€๐๐ˆ๐’๐Œ ๐Ž๐… ๐‡๐€๐‹๐Ž๐†๐„๐๐€๐“๐ˆ๐Ž๐:")`

(i) `color{green}("๐ˆ๐ง๐ข๐ญ๐ข๐š๐ญ๐ข๐จ๐ง :")` The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The `color{red}(Clโ€“Cl)` bond is weaker than the `color{red}(Cโ€“C)` and `color{red}(Cโ€“H)` bond and hence, can be easily broken.

`color{red}(Cl - Cl underset("homolysis") overset(hv)โ†’ underset("Chlorine free radicals")(overset(โ€ข)Cl + overset(โ€ข)Cl))`

(ii) `color{green}("๐๐ซ๐จ๐ฉ๐š๐ ๐š๐ญ๐ข๐จ๐ง :")` Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the `color{red}(C-H)` bond to generate methyl free radical with the formation of `color{red}(H-Cl)`.

(a) `color{red}(CH_4 + overset(โ€ข)Cl overset(hv)โ†’ overset(โ€ข)CH_3 + H - Cl)`

(b) `color{red}(overset(โ€ข)CH_3 +Cl - Cl overset(hv)โ†’ underset("Chlorine free radical" )(CH_3 - Cl + overset(โ€ข)Cl))`

The methyl radical thus obtained attacks the second molecule of chlorine to form `color{red}(CH_3 โ€“ Cl)` with the liberation of another chlorine free radical by homolysis of chlorine molecule. The chlorine and methyl free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are those which directly give principal products, but many other propagation steps are possible.

(iii) `color{green}("๐“๐ž๐ซ๐ฆ๐ข๐ง๐š๐ญ๐ข๐จ๐ง:")` The reaction stops after some time due to consumption of reactants and / or due to the following side reactions :
The possible chain terminating steps are :

(a) `color{red}(overset(โ€ข)Cl + overset(โ€ข)Cl โ†’ Cl - Cl)`

(b) `color{red}(H_3 overset(โ€ข)C + overset(โ€ข)CH_3 โ†’ H_3C - CH_3)`

(c) `color{red}(H_3 overset(โ€ข)C + overset(โ€ข)Cl โ†’ H_3C - Cl)`

Though in (c), `color{red}(CH_3 โ€“ Cl)`, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.

๐Ÿ. `color{green}("๐‚๐จ๐ฆ๐›๐ฎ๐ฌ๐ญ๐ข๐จ๐ง:")`

Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.

`color{red}(CH_4(g) + 2O_2(g) โ†’ CO_2(g) +2H_2O (l) ; Delta_cH^(โŠ–) = -890 kJ mol^(-1))` .........(13.17)

`color{red}(C_4H_(10) (g) + 13/2 O_2 (g) โ†’ 4CO_2 (g) + 5H_2O(l) ; Delta_cH^(โŠ–) = -2875.84 kJ mol^(-1))` .........(13.18)

The general combustion equation for any alkane is :

`color{red}(C_n H_(2n+2) + ((3n+1)/2) O_2 โ†’ nCO_2 + (n+1) H_2O) ` .............(13.19)

Due to the evolution of large amount of heat during combustion, alkanes are used as fuels. During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters.

`color{red}(CH_4(g) + O_2(g) underset("combustion") overset("Incomplete")โ†’ C(s) + 2H_2O (l))` ....................(13.20)

๐Ÿ‘. `color{green}("๐‚๐จ๐ง๐ญ๐ซ๐จ๐ฅ๐ฅ๐ž๐ ๐จ๐ฑ๐ข๐๐š๐ญ๐ข๐จ๐ง:")`

Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.

(i) `color{red}(2CH_4 + O_2 overset("Cu / 523K / 100 atm")โ†’ underset("Methanol")(2CH_3OH))` ................(13.21)

(ii) `color{red}(CH_4 + O_2 underset(Delta) overset(Mo_2O_3)โ†’ underset("Methanal")(HCHO) + H_2O)` .............(13.22)

(iii) `color{red}(2CH_3CH_3 + 3O_2 overset((CH_3COO)_2 Mn)โ†’ underset("Ethanoic acid")(2CH_3COOH)+2H_2O)` .................(13.23)

(iv) Ordinarily alkanes resist oxidation but alkanes having tertiary `color{red}(H)` atom.

`color{red}(underset("2-Methylpropane")((CH_3)_3CH) underset("Oxidation") overset(KMnO_4)โ†’ underset("2-Methylpropan-2-ol")((CH_3)_3COH))` ..............(13.24)

๐Ÿ’ . `color{green}("๐ˆ๐ฌ๐จ๐ฆ๐ž๐ซ๐ข๐ฌ๐š๐ญ๐ข๐จ๐ง:")`

n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.

`color{red}(underset("n - Hexane")(CH_3(CH_2)_4 CH_3) overset(" Anhy ." AlCl_3 // HCl)โ†’underset("2-Methylpentane")(CH_3 underset(underset(CH_3)(|))CH - (CH_2)_2 - CH_3) + underset("3-Methylpentane")(CH_3CH_2 - underset(underset(CH_3)(|))CH- CH_2 - CH_3))` ................(13.25)

๐Ÿ“. `color{green}("๐€๐ซ๐จ๐ฆ๐š๐ญ๐ข๐ณ๐š๐ญ๐ข๐จ๐ง:")`

`n`-Alkanes having six or more carbon atoms on heating to `773K` at `10-20` atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as ๐‘Ž๐‘Ÿ๐‘œ๐‘š๐‘Ž๐‘ก๐‘–๐‘ง๐‘Ž๐‘ก๐‘–๐‘œ๐‘› or ๐‘Ÿ๐‘’๐‘“๐‘œ๐‘Ÿ๐‘š๐‘–๐‘›๐‘”.

๐Ÿ”. `color{green}("๐‘๐ž๐š๐œ๐ญ๐ข๐จ๐ง ๐ฐ๐ข๐ญ๐ก ๐ฌ๐ญ๐ž๐š๐ฆ:")`

Methane reacts with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen. This method is used for industrial preparation of dihydrogen gas.

`color{red}(CH_4 + H_2O underset(Delta) overset("Ni")โ†’ CO + 3H_2)` .............(13.27)

๐Ÿ•. `color{green}("๐๐ฒ๐ซ๐จ๐ฅ๐ฒ๐ฌ๐ข๐ฌ:")`

Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.

Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, dodecane, a constituent of kerosene oil on heating to 973K in the presence of platinum, palladium or nickel gives a mixture of heptane and pentene.

`color{red}(underset("Dodecane")(C_(12)H_(26)) underset(973K)overset("Pt/Pd/Ni")โ†’ underset("Heptane")(C_7 H_(16)) + underset("Pentene")(C_5H_(10)) + " other products")` .................(13.29)