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Step Down Reactions (Descent of Series) | chemca

Organic Synthesis

Step Down Reactions (Descent of Series)

Methods to decrease the length of the Carbon Chain.

By chemca Team • Updated Jan 2026

When transforming a higher member of a homologous series to a lower member (decreasing the number of carbon atoms), the process is called Descent of Series or Step Down Reaction.

1. Decarboxylation

Removing $CO_2$ from Acids

Sodium salts of carboxylic acids, when heated with Soda-lime ($NaOH$ + $CaO$ in 3:1 ratio), lose a molecule of $CO_2$ to form an alkane with one less carbon atom.

$$ R-COONa + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3 $$

Example: Sodium Ethanoate ($C_2$) to Methane ($C_1$).

$$ CH_3COONa + NaOH \xrightarrow{CaO} CH_4 + Na_2CO_3 $$

2. Hoffmann Bromamide Degradation

Amide to Primary Amine

This is the most important laboratory method for stepping down a series. An amide reacts with Bromine and alcoholic KOH to form a primary amine with one carbon less than the parent amide.

$$ R-CONH_2 + Br_2 + 4KOH \rightarrow R-NH_2 + K_2CO_3 + 2KBr + 2H_2O $$

Example: Acetamide ($C_2$) to Methylamine ($C_1$).

$$ CH_3CONH_2 \xrightarrow{Br_2/KOH} CH_3NH_2 $$

Key Feature: The carbonyl carbon is lost as carbonate. The alkyl group migrates from Carbon to Nitrogen.

3. Hunsdiecker Reaction

Acid Salt to Alkyl Halide

The silver salt of a carboxylic acid reacts with Bromine in $CCl_4$ to form an alkyl bromide with one less carbon atom.

$$ R-COOAg + Br_2 \xrightarrow{CCl_4, \Delta} R-Br + CO_2 + AgBr $$

This involves a free-radical mechanism involving decarboxylation.

4. Ozonolysis

Cleavage of Double Bonds

Strong oxidative cleavage of alkenes using Ozone ($O_3$) followed by Zinc/Water breaks the carbon skeleton at the double bond, often producing smaller aldehydes or ketones.

$$ R-CH=CH-R' \xrightarrow{1. O_3, 2. Zn/H_2O} R-CHO + R'-CHO $$

Example: 2-Butene ($C_4$) to Ethanal ($C_2$).

$$ CH_3-CH=CH-CH_3 \xrightarrow{O_3, Zn/H_2O} 2CH_3CHO $$

5. Schmidt Reaction

Direct Acid to Amine

Carboxylic acids react with Hydrazoic acid ($N_3H$) in the presence of conc. $H_2SO_4$ to yield primary amines with one less carbon.

$$ R-COOH + N_3H \xrightarrow{H_2SO_4} R-NH_2 + CO_2 + N_2 $$

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