Reduction Pathways & Synthesis Design
Reduction reactions represent the reverse of oxidation pathways. You must know the reagents and conditions for the reduction of carbonyl compounds and nitrobenzene, and combine reactions into multi-step synthesis routes.
Reduction of Carbonyl Compounds
Aldehydes, ketones, and carboxylic acids can all be reduced back to alcohols. Because hydrogen is added to the molecule, we represent the reducing agent with the symbol [H] in balanced equations.
Reducing Agents
The choice of reducing agent depends on the strength required to reduce the specific functional group:
- Sodium borohydride (\(\text{NaBH}_4\)): A milder reducing agent. It is safe to use in aqueous or alcoholic solutions (like ethanol). It is strong enough to reduce aldehydes and ketones, but is not strong enough to reduce carboxylic acids.
- Lithium aluminium hydride (\(\text{LiAlH}_4\)): A much more powerful reducing agent. It reacts violently with water and alcohols, so it must be dissolved in a completely dry organic solvent like dry ether (ethoxyethane) at room temperature. The reaction is followed by the addition of a dilute acid (such as \(\text{H}_2\text{SO}_4\)) to release the alcohol. It is strong enough to reduce carboxylic acids, as well as aldehydes and ketones.
Carbonyl Reducing Agents (NaBH₄ vs LiAlH₄)
- \(\text{NaBH}_4\) (Sodium borohydride): Milder reducing agent, used in aqueous/alcoholic solution. Reduces aldehydes → 1° alcohols and ketones → 2° alcohols (cannot reduce carboxylic acids).
- \(\text{LiAlH}_4\) (Lithium aluminium hydride): Strong reducing agent in dry ether followed by aqueous acid. Reduces carboxylic acids → 1° alcohols, aldehydes, and ketones.
Reduction of Nitrobenzene
Nitrobenzene (\(\text{C}_6\text{H}_5\text{NO}_2\)) can be reduced to form phenylamine (\(\text{C}_6\text{H}_5\text{NH}_2\), also known as aniline). Phenylamine is an important starting material in the manufacture of dyes and pharmaceuticals. This reduction is carried out in a two-stage process:
Stage 1: Acidic Reduction of Nitrobenzene
Stage 1 (Reflux with Sn and conc. HCl):
\[\text{C}_6\text{H}_5\text{NO}_2(\text{l}) + 6\text{H}^+(\text{aq}) + 6\text{e}^- \rightarrow \text{C}_6\text{H}_5\text{NH}_3^+(\text{aq}) + 2\text{H}_2\text{O}(\text{l})\]In strongly acidic conditions, the protonated phenylammonium ion is formed.
Stage 2: Alkaline Liberation of Phenylamine
Stage 2 (Deprotonation with NaOH):
\[\text{C}_6\text{H}_5\text{NH}_3^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{C}_6\text{H}_5\text{NH}_2(\text{l}) + \text{H}_2\text{O}(\text{l})\]Hydroxide removes a proton, releasing oily, insoluble phenylamine (aniline).
Organic Synthesis & Reaction Pathways
Organic synthesis involves designing a sequence of reactions to convert a starting material into a desired target molecule. In the IB exam, you may be asked to outline a multi-step pathway, stating the reagents and conditions for each step.
The Reaction Pathway Map
The diagram below summarises all the key reaction pathways you must know for both Standard Level and Higher Level organic chemistry:
Worked Example: Designing a Synthesis Route
Multi-Step Organic Synthesis Pathway
Problem: Synthesize ethyl ethanoate starting from ethene.
- Step 1: \(\text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}(\text{g}) \xrightarrow{\text{H}_3\text{PO}_4, 300^\circ\text{C}} \text{CH}_3\text{CH}_2\text{OH}\) (ethanol)
- Step 2: \(\text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \xrightarrow{\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}^+, \text{reflux}} \text{CH}_3\text{COOH}\) (ethanoic acid)
- Step 3: \(\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightleftharpoons{\text{conc. }\text{H}_2\text{SO}_4, \Delta} \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}\)
Organic Reduction Pitfalls
- Reducing agent choice: Never specify \(\text{NaBH}_4\) for reducing carboxylic acids (it is too weak; must use \(\text{LiAlH}_4\) in dry ether).
- Nitrobenzene two-stage sequence: Always include both \(\text{Sn}/\text{conc. HCl}\) (Stage 1) followed by \(\text{NaOH}\) (Stage 2) to obtain phenylamine.
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