Chemical Properties of Alkanes | Hydrocarbons Class 11

Chemical Properties of Alkanes

Reactivity, Mechanisms & Major Reactions | Hydrocarbons Class 11

1. Introduction (Paraffins)

Alkanes are generally inert towards acids, bases, and oxidizing agents under normal conditions. This is due to the strong non-polar $C-C$ and $C-H$ bonds. Hence, they are called Paraffins (Latin: parum = little, affinis = affinity).

However, under specific conditions (heat, light, catalyst), they undergo substitution and thermal reactions.

2. Substitution Reactions (Halogenation)

One or more hydrogen atoms of an alkane are replaced by halogens ($Cl, Br, I$).

$$ R-H + X_2 \xrightarrow{h\nu / \Delta} R-X + HX $$

Reactivity Order of Halogens: $F_2 > Cl_2 > Br_2 > I_2$.

Fluorination: Too violent/explosive (controlled).
Iodination: Very slow and reversible. Requires oxidizing agents ($HIO_3, HNO_3$) to remove HI formed.

Mechanism: Free Radical Substitution

Initiation: $Cl-Cl \xrightarrow{h\nu} 2\dot{Cl}$ (Homolytic fission).
Propagation:
$CH_4 + \dot{Cl} \rightarrow \dot{C}H_3 + HCl$
$\dot{C}H_3 + Cl_2 \rightarrow CH_3Cl + \dot{Cl}$
Termination: Radicals combine ($\dot{C}H_3 + \dot{C}H_3 \rightarrow C_2H_6$).

3. Combustion

Alkanes burn in the presence of air or oxygen to produce carbon dioxide, water, and heat.

A. Complete Combustion

$$ C_nH_{2n+2} + \left( \frac{3n+1}{2} \right)O_2 \rightarrow nCO_2 + (n+1)H_2O + \Delta H $$

Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ (Exothermic).

B. Incomplete Combustion

In limited supply of air, Carbon Black ($C$) or Carbon Monoxide ($CO$) is formed.

4. Controlled Oxidation

Alkanes give different products depending on the catalyst:

$2CH_4 + O_2 \xrightarrow{Cu/523K/100atm} 2CH_3OH$ (Methanol)
$CH_4 + O_2 \xrightarrow{Mo_2O_3/\Delta} HCHO + H_2O$ (Methanal)
$2C_2H_6 + 3O_2 \xrightarrow{(CH_3COO)_2Mn} 2CH_3COOH + 2H_2O$ (Ethanoic Acid)

                    Note: Normally alkanes resist oxidation, but tertiary hydrogens can be oxidized to alcohols by $KMnO_4$.
                

5. Isomerization

n-Alkanes convert into branched isomers when heated with anhydrous $AlCl_3$ and $HCl$ gas.

$$ \text{n-Butane} \xrightarrow{AlCl_3/HCl} \text{Isobutane} $$

6. Aromatization

n-Alkanes having 6 or more carbons, when heated to 773K at 10-20 atm with oxide catalysts ($V_2O_5, Cr_2O_3, Mo_2O_3$ supported on Alumina), get dehydrogenated and cyclized to Benzene or its homologues.

$$ n\text{-Hexane} \xrightarrow{Cr_2O_3/773K} \text{Benzene} $$

7. Pyrolysis (Cracking)

Decomposition of higher alkanes into smaller fragments (alkanes, alkenes, hydrogen) by the application of heat.

Example: $C_6H_{14} \xrightarrow{773K} C_6H_{12} + H_2$ OR $C_4H_8 + C_2H_6$, etc.

This process is the basis of obtaining petrol from kerosene/heavy oil.

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