Suggest a route for the preparation of nitrobenzene starting from acetylene?

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Q. Suggest a route for the preparation of nitrobenzene starting from acetylene?

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Concept understanding — Organic Synthesis

Organic Synthesis: Building Molecules from Scratch

Imagine you're a chef who wants to make a complex dish like biryani. You don't just throw rice, chicken, and spices into a pot and hope for the best. You follow a recipe: first marinate the meat, then fry the onions, layer everything, and cook on a slow flame. Each step transforms simple ingredients into something more complex, and the order matters.

Organic synthesis is exactly that — but for molecules. It's the art and science of building a desired organic compound (the "target molecule") from simpler, readily available starting materials, using a sequence of chemical reactions.

The Core Intuition

Nature gives us simple molecules: methane ( $CH_4$ ), ethene ( $C_2H_4$ ), benzene ( $C_6H_6$ ), ethanol ( $C_2H_5OH$ ). But we need complex ones: medicines like paracetamol, polymers like nylon, dyes, pesticides, and plastics. Organic synthesis is how we bridge that gap.

Think of it like Lego. You have basic bricks (functional groups like -OH, -COOH, -NH₂). You have connectors (reagents like $H_2SO_4$ , $KMnO_4$ , $NaBH_4$ ). And you have instructions (reaction conditions: temperature, solvent, catalyst). Your job is to click the right bricks in the right order to build the exact structure you want.

The Precise Statement

Organic synthesis is the deliberate construction of organic compounds through a planned sequence of chemical reactions, where each step transforms a starting material into an intermediate, ultimately yielding the target molecule with the desired structure and stereochemistry.

The Two Big Challenges

1. Selectivity — You want only one product, not a mixture. For example, if you want to convert an alcohol ( $R-OH$ ) to an aldehyde ( $R-CHO$ ), you must stop the reaction before it over-oxidises to a carboxylic acid ( $R-COOH$ ). This requires choosing the right reagent (e.g., PCC instead of $K_2Cr_2O_7$ ).

2. Yield — Every reaction loses some material. If you have 10 steps, each with 90% yield, your final yield is only $0.9^{10} \approx 35\%$ . Good synthesis minimises steps and maximises yield per step.

How It Actually Works: Retrosynthesis

Chemists don't start from the beginning. They start from the target molecule and work backwards, asking: "What simpler molecule could I make this from?" This reverse-thinking is called retrosynthesis.

Tip

Retrosynthesis is like solving a maze backwards — you start at the cheese and find the path to the entrance.

Example: Suppose you want to make paracetamol (acetaminophen). The target has a benzene ring with an -OH group and an -NHCOCH₃ group. Working backwards:

The -NHCOCH₃ group can come from reacting an amine ( $-NH_2$ ) with acetic anhydride ( $(CH_3CO)_2O$ ).
The -OH group can come from a diazonium salt (made from an amine).
The amine can come from reducing a nitro group ( $-NO_2$ ).
The nitro group can come from nitrating phenol.

So the forward synthesis becomes: Phenol → Nitration → Reduction → Acetylation → Paracetamol.

Why It Matters

Every medicine you take, every plastic bottle you use, every synthetic fabric you wear exists because someone figured out how to synthesise it. The 2010 Nobel Prize in Chemistry went to Heck, Negishi, and Suzuki for developing palladium-catalysed cross-coupling reactions — tools that let chemists join carbon atoms together with precision, revolutionising how we make complex molecules. …

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To prepare nitrobenzene from acetylene, first cyclize three molecules of acetylene to form benzene, then nitrate benzene using a mixture of concentrated nitric and sulphuric acids. The final product is nitrobenzene.

Preparing nitrobenzene from acetylene involves two main conceptual transformations: first, building an aromatic ring from an alkyne, and second, introducing a nitro group onto that aromatic ring. Acetylene ( $\text{C}_2\text{H}_2$ ) is a simple alkyne, while nitrobenzene is an aromatic compound with a $-\text{NO}_2$ group. The most straightforward approach is to first synthesize benzene from acetylene, and then perform an electrophilic aromatic substitution reaction (nitration) on benzene.

Here is the step-by-step route:

Formation of Benzene from Acetylene (Cyclic Polymerisation/Aromatisation)

The first step is to convert acetylene into benzene. This is achieved through a cyclic polymerisation reaction, also known as trimerisation. Three molecules of acetylene combine to form one molecule of benzene.
- Reasoning: Acetylene is a highly unsaturated compound. Under specific conditions, its triple bonds can rearrange to form a stable aromatic ring structure. This reaction is a classic method for synthesizing benzene from alkynes.
- Conditions: This reaction requires passing acetylene through a red-hot iron tube (or copper tube) heated to approximately $400^\circ\text{C}$ .
- Reaction:

3\text{CH}\equiv\text{CH} \xrightarrow{\text{Red hot iron tube}, 400^\circ\text{C}} \text{C}_6\text{H}_6

    (Acetylene) (Benzene)

2. Nitration of Benzene to Nitrobenzene

Once benzene is formed, the next step is to introduce a nitro group onto the benzene ring. This is a classic example of an electrophilic aromatic substitution reaction.

*   **Reasoning:** Benzene is an electron-rich aromatic system, making it susceptible to attack by electrophiles. The nitronium ion ($\text{NO}_2^+$) acts as the electrophile in this reaction.
*   **Reagents:** A mixture of concentrated nitric acid ($\text{HNO}_3$) and concentrated sulphuric acid ($\text{H}_2\text{SO}_4$) is used. This mixture is often called the "nitrating mixture."
*   **Role of Sulphuric Acid:** Concentrated sulphuric acid acts as a catalyst. It protonates nitric acid, leading to the formation of the highly electrophilic nitronium ion ($\text{NO}_2^+$).
    $$ \text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightleftharpoons \text{NO}_2^+ + \text{H}_3\text{O}^+ + 2\text{HSO}_4^- $$ …

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Q.Suggest a route for the preparation of nitrobenzene starting from acetylene?

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