Structure, Isomerism, and Physical Properties
The carbon atoms in alkanes are $sp^3$ hybridized, leading to a tetrahedral geometry around each carbon atom with a bond angle of approximately $109.5^\circ$.
Because of free rotation about single $\text{C-C}$ bonds, alkanes exist in various spatial arrangements known as conformations or conformational isomers.
Ethane, for instance, exhibits staggered and eclipsed conformations, where the staggered conformer is more stable due to minimal torsional strain.
As the molecular weight increases within the homologous series of straight-chain alkanes, physical properties change systematically. The first four members ($\text{CH}_4$ to $\text{C}_4\text{H}_{10}$) are gases at room temperature, $\text{C}_5$ to $\text{C}_{17}$ are liquids, and higher members are solids.
Because molecules are non-polar, intermolecular forces are restricted to weak London dispersion forces (Van der Waals forces).
Branching in alkanes plays a critical role in determining melting and boiling points.
Straight-chain isomers possess a larger surface area, leading to stronger intermolecular forces and higher boiling points compared to their branched counterparts.
Conversely, branched isomers are more compact, spherically symmetrical, and pack less efficiently in the crystal lattice, which often results in higher melting points due to high symmetry.
Methods of Preparation
Preparing alkanes in the laboratory and industrial settings involves reducing carbon chains, hydrogenating unsaturated systems, or coupling alkyl halides.
One of the most classic methods for extending the carbon chain is the Wurtz Reaction.
In this reaction, an alkyl halide (typically an alkyl chloride or bromide) reacts with sodium metal in the presence of dry ether to yield a symmetrical alkane containing twice the number of carbon atoms present in the starting alkyl halide.
Wurtz Reaction General Equation: $2\text{R-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R-R} + 2\text{NaX}$
Other vital preparation methods include the catalytic hydrogenation of alkenes and alkynes using finely divided metals like platinum, palladium, or nickel (Sabatier-Senderens reduction).
Another method is the decarboxylation of carboxylic acid salts.
Decarboxylation involves heating a sodium salt of a carboxylic acid with soda lime (a mixture of $\text{NaOH}$ and $\text{CaO}$), which releases carbon dioxide and produces an alkane with one carbon atom less than the parent acid.
Additionally, the reduction of alkyl halides using zinc and dilute acid or lithium aluminum hydride ($\text{LiAlH}_4$) yields alkanes.
For large-scale synthesis, the Kolbe’s Electrolysis method involves the electrolytic decarboxylation of aqueous sodium or potassium salts of monocarboxylic acids, yielding symmetrical alkanes at the anode.
Chemical Reactions: Free Radical Halogenation
Alkanes are generally inert towards acids, bases, oxidizing agents, and reducing agents under normal conditions, earning them the historical name paraffins (Latin: parum affinis, meaning little affinity).
However, they readily undergo substitution reactions under high temperature or UV light exposure, most notably free radical halogenation.
Chlorination and bromination of alkanes proceed via a free radical chain mechanism comprising three distinct steps: initiation, propagation, and termination.
Fluorination is explosively violent and uncontrollable, whereas iodination is reversible and slow unless conducted in the presence of an oxidizing agent like iodic acid ($\text{HIO}_3$) to consume the byproduct hydrogen iodide ($\text{HI}$).
- Initiation: Homolytic cleavage of a halogen molecule ($\text{Cl}_2$) by thermal energy or UV light to generate chlorine free radicals ($\text{Cl}^\bullet$).
- Propagation: A chlorine radical abstracts a hydrogen atom from the alkane to form an alkyl radical ($\text{R}^\bullet$) and $\text{HCl}$. The alkyl radical then reacts with another halogen molecule to form the alkyl halide and regenerate a chlorine radical.
- Termination: Combination of any two free radicals to bring the chain reaction to a close (e.g., $\text{R}^\bullet + \text{Cl}^\bullet \rightarrow \text{R-Cl}$).
Regioselectivity during halogenation depends heavily on the stability of the intermediate free radical.
The stability order of carbon radicals is $3^\circ > 2^\circ > 1^\circ > \text{methyl}$.
Consequently, chlorination is less selective than bromination; bromination is highly selective and predominantly yields the tertiary alkyl halide due to the endothermic nature of the transition state for tertiary hydrogen abstraction.
Key Points to Remember
- Alkanes have the general formula $\text{C}_n\text{H}_{2n+2}$ and feature only $sp^3$ hybridized carbon centers.
- Branched alkanes possess lower boiling points than straight-chain isomers due to decreased surface area.
- Wurtz reaction fails to produce odd-numbered alkanes cleanly because it yields a complex statistical mixture of products.
- Soda lime decarboxylation involves $\text{NaOH}$ as the reactive base and $\text{CaO}$ to keep the mixture dry and workable at high temperatures.
- Free radical halogenation follows the radical stability order: tertiary ($3^\circ$) > secondary ($2^\circ$) > primary ($1^\circ$).
- Combustion of alkanes is highly exothermic, serving as a primary source of heat and energy.
- Controlled catalytic oxidation of alkanes yields alcohols, aldehydes, or carboxylic acids depending on the catalysts and reaction conditions.
Important Facts / Formulas
| Reaction / Concept | Key Reagents / Conditions | Major Product |
|---|---|---|
| Wurtz Reaction | Alkyl halide + $\text{Na}$ in Dry Ether | Symmetrical Alkane ($2\text{R}$) |
| Decarboxylation | Sodium salt of acid + Soda Lime ($\text{NaOH} + \text{CaO}$, $\Delta$) | Alkane with $n-1$ carbons |
| Kolbe’s Electrolysis | Concentrated aqueous solution of sodium/potassium carboxylate | Alkanes at Anode |
| Halogenation | $\text{Cl}_2$ or $\text{Br}_2$ in presence of UV light / Heat | Haloalkane mixture |
Previous Year Question Hints
Question 1: Why is the Wurtz reaction not suitable for the preparation of propane?
Hint: Focus on the statistical mixture of products formed when a mixture of chloromethane and chloroethane is treated with sodium in dry ether (yielding ethane, propane, and butane).
Question 2: Arrange the following free radicals in order of decreasing stability: $\text{(CH}_3)_3\text{C}^\bullet$, $\text{(CH}_3)_2\text{CH}^\bullet$, $\text{CH}_3\text{CH}_2^\bullet$.
Hint: Use hyperconjugation and inductive effects associated with alkyl substituents on the radical carbon.
Quick Revision Summary
- Alkanes are saturated hydrocarbons containing strong $\sigma$-bonds, imparting chemical inertness under normal laboratory conditions.
- Conformational isomerism arises due to free rotation around $\text{C-C}$ single bonds, with staggered conformations being the most stable.
- Melting points depend strongly on molecular symmetry, whereas boiling points depend on molecular weight and chain straightness.
- Wurtz reaction and Kolbe electrolysis are standard methods used to build higher symmetrical alkanes from lower starting materials.
- Soda-lime decarboxylation effectively steps down the carbon chain by removing one carbon atom as $\text{CO}_2$.
- Halogenation proceeds via a free-radical chain mechanism, showing high regioselectivity for tertiary hydrogen abstraction during bromination.
- Combustion reactions are strongly exothermic and are utilized extensively for energy generation.
- Isomerization of straight-chain alkanes to branched alkanes occurs in the presence of anhydrous aluminum chloride to boost octane ratings.
