Methods of Preparation — Revision Notes

⚡ 30-Second Revision

Hydrogenation: — Alkenes/Alkynes + $H_2 \xrightarrow{Ni, Pd, or Pt}$ Alkanes
Reduction of Alkyl Halides: — $R-X \xrightarrow{Zn/HCl,or,LiAlH_4} R-H$
Wurtz Reaction: — $2R-X + 2Na \xrightarrow{Dry,Ether} R-R + 2NaX$ (Symmetrical alkanes, no $CH_4$)
Decarboxylation: — $R-COONa + NaOH \xrightarrow{CaO, Delta} R-H + Na_2CO_3$ (Alkane with one less carbon, $CH_4$ possible)
Kolbe's Electrolysis: — $2R-COONa + 2H_2O \xrightarrow{Electrolysis} R-R + 2CO_2 + H_2 + 2NaOH$ (Symmetrical alkanes, no $CH_4$)
Grignard Reagent Hydrolysis: — $R-MgX + H_2O \rightarrow R-H + Mg(OH)X$

2-Minute Revision

Alkanes, the saturated hydrocarbons, can be prepared through several distinct methods. Catalytic hydrogenation is a straightforward way to convert alkenes and alkynes into alkanes by adding hydrogen gas across their multiple bonds, typically using catalysts like Ni, Pd, or Pt.

Alkyl halides can be reduced to alkanes by replacing the halogen with hydrogen, using reagents such as $Zn/HCl$ or $LiAlH_4$. For synthesizing symmetrical alkanes, two powerful coupling reactions are available: the Wurtz reaction, which involves treating alkyl halides with sodium in dry ether, and Kolbe's electrolytic method, which uses the electrolysis of carboxylic acid salts.

Both Wurtz and Kolbe's methods cannot produce methane and yield mixtures if different starting materials are used. Finally, decarboxylation of carboxylic acid salts with soda-lime ($NaOH/CaO$) provides an alkane with one less carbon atom, and is a viable method for methane synthesis.

Grignard reagents, formed from alkyl halides, also yield alkanes upon hydrolysis.

5-Minute Revision

To master alkane preparation for NEET, focus on the key methods, their specific reagents, conditions, and limitations.

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Hydrogenation of Unsaturated Hydrocarbons: — Alkenes ($R-CH=CH-R'$) and alkynes ($R-C equiv C-R'$) are converted to alkanes ($R-CH_2-CH_2-R'$) by adding $H_2$. Catalysts are crucial: Ni (requires 250-300 °C), Pd or Pt (effective at room temperature). This is a clean, high-yield method.

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From Alkyl Halides:

* Reduction: Alkyl halides ($R-X$) can be directly reduced to alkanes ($R-H$) using reagents like $Zn/HCl$, $LiAlH_4$, or $Red,P/HI$. This method retains the carbon skeleton. * Wurtz Reaction: $2R-X + 2Na \xrightarrow{Dry,Ether} R-R + 2NaX$.

This couples two alkyl groups to form a symmetrical alkane. Key limitations: Only for symmetrical alkanes (mixed halides give mixtures), cannot prepare methane, tertiary halides undergo elimination.

* Grignard Reagent Hydrolysis: $R-MgX + H_2O \rightarrow R-H + Mg(OH)X$. This is a two-step process (R-X $\rightarrow$ R-MgX, then hydrolysis) that also converts alkyl halides to alkanes with the same carbon count.

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From Carboxylic Acids:

* Decarboxylation (Soda-lime): $R-COONa + NaOH \xrightarrow{CaO, Delta} R-H + Na_2CO_3$. This method removes the carboxyl group, yielding an alkane with one less carbon atom. It is suitable for preparing methane (from sodium acetate).

CaO prevents fusion of NaOH. * Kolbe's Electrolytic Method: $2R-COONa + 2H_2O \xrightarrow{Electrolysis} R-R + 2CO_2 + H_2 + 2NaOH$. This is an anodic oxidation and radical coupling process, forming symmetrical alkanes.

Key limitations: Similar to Wurtz, it's best for symmetrical alkanes (mixed salts give mixtures) and cannot prepare methane. Side products like alkenes can also form.

Example: To prepare ethane ($CH_3-CH_3$):

From ethene ($CH_2=CH_2$) via hydrogenation ($H_2/Ni$).
From bromoethane ($CH_3-CH_2-Br$) via reduction ($Zn/HCl$).
From bromomethane ($CH_3-Br$) via Wurtz reaction ($2CH_3Br + 2Na$).
From sodium acetate ($CH_3-COONa$) via Kolbe's electrolysis.
From sodium propanoate ($CH_3-CH_2-COONa$) via decarboxylation ($NaOH/CaO$).

Remember to differentiate between methods that increase, decrease, or maintain the carbon chain length, and always consider the symmetry of the desired alkane.

Prelims Revision Notes

Methods of Preparation of Alkanes (NEET Revision)

1. From Unsaturated Hydrocarbons (Hydrogenation):

Reactants: — Alkenes ($C=C$) or Alkynes ($C equiv C$)
Reagents: — $H_2$ gas
Catalysts & Conditions:

* Nickel (Ni): 250-300 °C, pressure (Sabatier-Senderens reaction) * Palladium (Pd) or Platinum (Pt): Room temperature, atmospheric pressure

Product: — Saturated alkane ($C-C$)
Mechanism: — Syn-addition of hydrogen across the multiple bond.
Example: — $CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$

2. From Alkyl Halides (R-X):

a) Reduction:

* Reagents: $Zn/HCl$, $LiAlH_4$, $NaBH_4$ (milder), $Red,P/HI (Delta)$ * Product: Alkane (R-H) with the same carbon skeleton. * Example: $CH_3-CH_2-Br + Zn/HCl \rightarrow CH_3-CH_3$

b) Wurtz Reaction:

* Reactants: Two molecules of alkyl halide (R-X) * Reagents: Sodium metal (Na) * Conditions: Dry ether (crucial to prevent Na reaction with water) * Product: Symmetrical alkane (R-R) with an even number of carbons. * Limitations: Not for unsymmetrical alkanes (gives mixture), cannot prepare methane, tertiary halides undergo elimination. * Example: $2CH_3-Br + 2Na \xrightarrow{Dry,Ether} CH_3-CH_3 + 2NaBr$

c) From Grignard Reagents:

* Formation: $R-X + Mg \xrightarrow{Dry,Ether} R-MgX$ * Reaction: $R-MgX + H_2O \rightarrow R-H + Mg(OH)X$ * Product: Alkane (R-H) with the same carbon count as R-X. * Example: $CH_3-CH_2-MgBr + H_2O \rightarrow CH_3-CH_3 + Mg(OH)Br$

3. From Carboxylic Acids:

a) Decarboxylation (Soda-lime):

* Reactants: Sodium or potassium salt of carboxylic acid (R-COONa) * Reagents: Soda-lime ($NaOH + CaO$) * Conditions: Heat ($Delta$) * Product: Alkane (R-H) with one less carbon atom than the parent acid. * Role of CaO: Dehydrating agent, prevents fusion of NaOH. * Methane Prep: Yes, from sodium acetate ($CH_3COONa$). * Example: $CH_3-COONa + NaOH \xrightarrow{CaO, Delta} CH_4 + Na_2CO_3$

b) Kolbe's Electrolytic Method:

* Reactants: Aqueous solution of sodium or potassium salt of carboxylic acid (R-COONa) * Conditions: Electrolysis * Product: Symmetrical alkane (R-R) with an even number of carbons. * Mechanism: Free radical coupling at anode. * Limitations: Not for unsymmetrical alkanes (gives mixture), cannot prepare methane. * Byproducts: $CO_2$, $H_2$, NaOH. * Example: $2CH_3-COONa + 2H_2O \xrightarrow{Electrolysis} CH_3-CH_3 + 2CO_2 + H_2 + 2NaOH$

Key Takeaways:

Methane ($CH_4$) preparation: — Only by decarboxylation (from sodium acetate).
Symmetrical alkanes: — Wurtz and Kolbe's methods.
Mixed products: — Wurtz and Kolbe's with different starting alkyl halides/salts.
Carbon chain change: — Hydrogenation/Reduction (same), Decarboxylation ($-1C$), Wurtz/Kolbe ($2 \times R$ group carbons).

Vyyuha Quick Recall

Hydrogenation, Wurtz, Decarboxylation, Kolbe's, Grignard's.

Happy We Do Know Good Alkanes!

Hydrogenation: Add $H_2$ to C=C/C≡C (Ni, Pd, Pt)
Wurtz: $2R-X + 2Na \rightarrow R-R$ (Dry Ether, no $CH_4$, symmetrical)
Decarboxylation: $R-COONa + NaOH/CaO \rightarrow R-H$ (Heat, $-1C$, $CH_4$ possible)
Kolbe's: $2R-COONa \xrightarrow{Electrolysis} R-R$ (Aqueous, no $CH_4$, symmetrical)
Grignard's: $R-MgX + H_2O \rightarrow R-H$ (Same C-count as R-X)