Preparation of Alkanes
Construct the carbon skeleton. Master the intricate mechanisms, reagent limitations, and anode/cathode products of Wurtz, Kolbe's Electrolysis, and Decarboxylation.
Module Focus: Coupling vs. Reduction
Preparing an alkane generally falls into two strategies: Coupling reactions (which build larger carbon chains by joining smaller ones, like Wurtz or Kolbe) and Reduction/Decarboxylation reactions (which strip away functional groups like halogens, double bonds, or carboxyl groups without increasing the carbon chain). In NEET, you must know exactly which reagent suits which strategy.
1. From Unsaturated Hydrocarbons (Hydrogenation)
Alkenes and alkynes add $H_2$ gas in the presence of finely divided metal catalysts like Palladium (Pd), Platinum (Pt), or Nickel (Ni) to form alkanes. This is also called Sabatier-Senderens reduction (specifically when using Ni at elevated temperatures).
2. From Alkyl Halides ($R-X$)
A. Wurtz Reaction (Chain Doubling)
Alkyl halides react with Sodium metal in dry ether to form higher alkanes containing an even number of carbon atoms. It proceeds primarily via a free radical mechanism ($R^\bullet$).
- Why Dry Ether? Moisture ($H_2O$) reacts explosively with Sodium to form $NaOH$ and $H_2$, destroying the reagent. Ether provides an inert, aprotic solvent.
- Major Limitation: Methane ($CH_4$) cannot be prepared by this method.
If you try to synthesize Propane ($CH_3-CH_2-CH_3$) using a mix of Methyl Chloride and Ethyl Chloride, you get a chaotic mixture of three products that are incredibly difficult to separate due to similar boiling points.
To make unsymmetrical alkanes, we use the Corey-House method. It utilizes the Gilman Reagent ($R_2CuLi$), which selectively reacts with primary and secondary alkyl halides ($R'X$) to yield the target unsymmetrical alkane ($R-R'$).
B. Reduction of Alkyl Halides
Alkyl halides (except fluorides) can be reduced directly to alkanes by replacing the halogen with a hydrogen atom.
- $Zn$ and dilute $HCl$: Forms nascent hydrogen ($[H]$) which reduces the halide.
$R-X + 2[H] \rightarrow R-H + HX$ - $HI$ with Red Phosphorus: The universal reducing agent at high temperatures ($420\text{K}$).
$R-X + 2HI \xrightarrow{\text{Red P}} R-H + I_2 + HX$
3. From Carboxylic Acids
A. Decarboxylation (Step-Down Reaction)
Sodium salts of carboxylic acids ($RCOONa$) are heated with Soda-Lime (a mixture of $NaOH$ and $CaO$ in a 3:1 ratio). The $CaO$ keeps the $NaOH$ dry as it is highly hygroscopic.
B. Kolbe's Electrolytic Method
An aqueous solution of sodium or potassium salt of a carboxylic acid is subjected to electrolysis. It produces symmetrical alkanes containing an even number of carbon atoms. Methane cannot be prepared.
During Kolbe's electrolysis, water is reduced at the cathode, generating $OH^-$ ions (forming $NaOH$ or $KOH$). As the reaction proceeds, the concentration of base increases. Therefore, the pH of the solution continuously increases during the reaction.
4. From Aldehydes and Ketones
The carbonyl group ($>C=O$) of aldehydes and ketones can be completely reduced to a methylene group ($>CH_2$) to form alkanes. There are two primary methods based on the pH conditions required.
Uses Zinc amalgam ($Zn-Hg$) and concentrated $HCl$.
Best for: Compounds stable in strong acids. (Fails for acid-sensitive groups like $-OH$).
Uses Hydrazine ($NH_2NH_2$) followed by heating with $KOH$ or potassium tert-butoxide in a high-boiling solvent like ethylene glycol.
Best for: Compounds stable in strong bases. (Fails for base-sensitive groups like halogens).
NEET Grand Test: Alkane Preparation
15 High-Yield Questions testing reagents, Kolbe's electrolysis products, and reaction limitations.
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