Chemical Properties of Alkanes
Master the inert "paraffins". Decode the Reactivity vs. Selectivity principle, catalyst-specific controlled oxidations, and extreme-condition aromatization.
Module Focus: The "Paraffins"
Alkanes are known as paraffins (Latin: parum = little, affinis = affinity) because strong $C-C$ and $C-H$ $\sigma$-bonds make them highly unreactive towards acids, bases, oxidizing, and reducing agents under normal conditions. Their reactions strictly require extreme conditions (heat, UV light, or catalysts) and overwhelmingly proceed via Free Radical Mechanisms.
1. Substitution: Halogenation
One or more hydrogen atoms are replaced by halogens in the presence of UV light ($h\nu$) or high heat (520-670 K). The order of reactivity of halogens is: $F_2 > Cl_2 > Br_2 > I_2$.
- Fluorination is dangerously violent and explosive. It cannot be done directly. (We use Swarts reaction instead).
- Iodination is very slow and reversible. It requires an oxidizing agent like $HIO_3$ or $HNO_3$ to consume the $HI$ formed and drive the reaction forward.
Proceeds via free radicals in 3 steps:
- Initiation: $Cl_2 \xrightarrow{h\nu} 2Cl^\bullet$
- Propagation: $CH_4 + Cl^\bullet \rightarrow \mathbf{CH_3^\bullet} + HCl$
$CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$ - Termination: Radicals combine ($Cl^\bullet + Cl^\bullet \rightarrow Cl_2$).
Chlorine is highly reactive ("fast and careless") and will attack $1^\circ$, $2^\circ$, and $3^\circ$ hydrogens somewhat indiscriminately based on probability. Bromine is highly selective ("lazy but picky"). It will almost exclusively attack the most stable $3^\circ$ or $2^\circ$ position because the endothermic nature of its transition state makes it highly sensitive to free radical stability.
2. Combustion & Controlled Oxidation
A. Complete Combustion
Alkanes burn in excess air/oxygen to produce $CO_2$, $H_2O$, and a large amount of heat (highly exothermic).
B. Controlled Oxidation (Must Memorize Catalysts!)
Alkanes mixed with oxygen at high pressure and temperature in the presence of specific catalysts yield different functional groups.
$523\text{K}, 100\text{ atm}$
Yields: ALCOHOLS
$CH_4 \rightarrow CH_3OH$
High $\Delta$
Yields: ALDEHYDES
$CH_4 \rightarrow HCHO$
High $\Delta$
Yields: ACIDS
$C_2H_6 \rightarrow CH_3COOH$
Isobutane ($2\text{-Methylpropane}$) $\xrightarrow{KMnO_4}$ tert-Butyl alcohol
3. Structural Reorganizations
A. Isomerization
Straight-chain alkanes convert into branched-chain isomers when heated with Anhydrous $AlCl_3$ and $HCl$ gas. This is crucial in the petroleum industry to increase the octane rating of fuel.
B. Aromatization (Reforming)
Alkanes having six or more carbon atoms, when heated to $773\text{K}$ at $10-20\text{ atm}$ in the presence of oxides of Vanadium, Molybdenum, or Chromium supported over alumina, undergo simultaneous dehydrogenation and cyclization to yield benzene and its homologues.
*Note: If n-Heptane ($C_7$) is used, the product will be Toluene (Methylbenzene).
4. Pyrolysis (Cracking)
Higher alkanes, on heating to high temperatures ($773-973\text{K}$) in the absence of air, decompose into lower alkanes, alkenes, and hydrogen gas. This relies on the homolytic cleavage of $C-C$ and $C-H$ bonds (free radical mechanism). It is widely used to prepare oil gas or petrol from heavier petroleum fractions.
NEET Grand Test: Chemical Properties
15 High-Yield Questions testing halogenation selectivity, controlled oxidation catalysts, and reaction mechanisms.
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