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Step Up Reactions (Ascent of Series) | chemca

Organic Synthesis

Step Up Reactions (Ascent of Series)

Methods to increase the length of the Carbon Chain.

By chemca Team • Updated Jan 2026

In organic synthesis, it is often necessary to prepare a compound with more carbon atoms than the starting material. This process is called Ascent of Series or Step Up Reaction.

1. Wurtz Reaction

Doubling the Carbon Chain

Alkyl halides react with metallic sodium in the presence of dry ether to form alkanes with double the number of carbon atoms.

$$ 2R-X + 2Na \xrightarrow{\text{Dry Ether}} R-R + 2NaX $$

Example: Methyl bromide to Ethane.

$$ 2CH_3Br + 2Na \rightarrow CH_3-CH_3 + 2NaBr $$

Limitation: This method is suitable only for preparing symmetrical alkanes. Using two different alkyl halides leads to a mixture of products.

2. Cyanide Synthesis (Nitrile Method)

Adding One Carbon

Reaction of alkyl halides with alcoholic KCN introduces one extra carbon atom via the nitrile group ($-\text{CN}$). The nitrile can then be converted to various functional groups.

$$ R-X + KCN \xrightarrow{\text{alc.}} R-C \equiv N + KX $$

Applications:

To Acid: Hydrolysis gives Carboxylic Acid ($RCOOH$).
$$ RCN \xrightarrow{H_3O^+} RCOOH $$
To Amine: Reduction gives Primary Amine ($RCH_2NH_2$).
$$ RCN \xrightarrow{LiAlH_4 \text{ or } Na/EtOH} RCH_2NH_2 $$

3. Using Grignard Reagent

Versatile Carbon Addition

Grignard reagents ($RMgX$) are powerful nucleophiles used to increase carbon chain length.

A. Reaction with $CO_2$ (Carboxylation): Yields carboxylic acid with one extra carbon.

$$ R-MgX + O=C=O \rightarrow R-COOMgX \xrightarrow{H_3O^+} R-COOH $$

B. Reaction with Methanal (HCHO): Yields primary alcohol with one extra carbon.

$$ R-MgX + HCHO \rightarrow R-CH_2-OMgX \xrightarrow{H_3O^+} R-CH_2OH $$

4. Terminal Alkyne Alkylation

Alkynide Method

Terminal alkynes react with $NaNH_2$ to form sodium acetylides, which then react with alkyl halides to form higher alkynes.

$$ HC \equiv CH \xrightarrow{NaNH_2} HC \equiv C^-Na^+ \xrightarrow{R-X} HC \equiv C-R $$

This is excellent for extending the chain of alkynes.

5. Corey-House Synthesis

For Unsymmetrical Alkanes

Overcomes the limitation of Wurtz reaction. An alkyl halide reacts with Lithium dialkyl copper (Gilman reagent) to give specific alkanes.

$$ R_2CuLi + R'X \rightarrow R-R' + RCu + LiX $$

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