How will you convert benzene into (i) p-nitrobromo – Class 11 Chemistry

Exercises · 9.13

Q. How will you convert benzene into

(i) p-nitrobromobenzene

(ii) m- nitrochlorobenzene

(iii) p - nitrotoluene

(iv) acetophenone?

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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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The key idea is to use the directing effects of substituents on the benzene ring to install the nitro group at the desired position. For (i) and (iii), a bromo or methyl group (ortho/para-directing) is introduced first, then nitrated. For (ii), a chloro group (ortho/para-directing) is introduced first, but to get the meta product, the nitro group must be installed before the chloro group. For (iv), a Friedel-Crafts acylation directly gives acetophenone.

The Concept: The Director's Cut

Benzene is a symmetrical, flat molecule. To make a specific disubstituted product, you can't just throw reagents at it randomly — the order matters enormously. This is because the first substituent you put on the ring becomes a "director" for the second one. Some groups (like $-\text{Br}$ , $-\text{CH}_3$ , $-\text{Cl}$ ) are ortho/para-directing — they push the next group to the positions next to them (ortho) or directly across (para). Others (like $-\text{NO}_2$ ) are meta-directing — they force the next group to the position in between.

The trick is to choose the sequence so that the director points exactly where you want the nitro group to land.

Step-by-Step Synthesis

(i) p-Nitrobromobenzene

We want a bromine atom and a nitro group opposite each other (para). Both bromine and the nitro group are deactivating, but bromine is still ortho/para-directing.

Introduce the director first. We want the nitro group to go para to the bromine. So, we first brominate benzene.

\text{C}_6\text{H}_6 + \text{Br}_2 \xrightarrow{\text{FeBr}_3} \text{C}_6\text{H}_5\text{Br} + \text{HBr}

This gives bromobenzene. The $-\text{Br}$ group is now the director.

2. Nitrate at the para position. Now we treat bromobenzene with a nitrating mixture (conc. $\text{HNO}_3$ + conc. $\text{H}_2\text{SO}_4$ ). The $-\text{Br}$ group directs the incoming $-\text{NO}_2$ group to the ortho and para positions. Because the ortho positions are sterically hindered by the large bromine atom, the major product is the para isomer.

\text{C}_6\text{H}_5\text{Br} + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} p\text{-Br-C}_6\text{H}_4\text{-NO}_2 + \text{H}_2\text{O}

Watch out

A common mistake is to try nitrating first and then brominating. If you nitrate benzene first, you get nitrobenzene. The $-\text{NO}_2$ group is a strong meta-director. Brominating nitrobenzene would then give m-bromonitrobenzene, not the para product we want.

(ii) m-Nitrochlorobenzene

This is the trickiest one. We want a chlorine atom and a nitro group meta to each other. Chlorine is ortho/para-directing. So if we put chlorine on first, nitration will give the ortho and para products, not the meta product we need.

Install the meta-director first. To get a meta relationship, we must put the nitro group on first. So, we nitrate benzene.

\text{C}_6\text{H}_6 + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2 + \text{H}_2\text{O}

This gives nitrobenzene. The $-\text{NO}_2$ group is now the director.

2. Chlorinate at the meta position. Now we treat nitrobenzene with chlorine gas in the presence of a Lewis acid catalyst like $\text{FeCl}_3$ or anhydrous $\text{AlCl}_3$ . The $-\text{NO}_2$ group is a strong meta-director, so the incoming chlorine atom goes to the meta position.

\text{C}_6\text{H}_5\text{NO}_2 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} m\text{-Cl-C}_6\text{H}_4\text{-NO}_2 + \text{HCl}

Tip

This is a classic example of "reverse the order to get the meta product." If you want a meta-disubstituted benzene where one group is ortho/para-directing and the other is meta-directing, you must install the meta-directing group first.

(iii) p-Nitrotoluene

We want a methyl group ( $-\text{CH}_3$ ) and a nitro group para to each other. The methyl group is a strong activator and an ortho/para-director. This is straightforward.

Introduce the methyl group. We alkylate benzene using the Friedel-Crafts reaction.

\text{C}_6\text{H}_6 + \text{CH}_3\text{Cl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{CH}_3 + \text{HCl}

This gives toluene. The $-\text{CH}_3$ group is now the director.

2. Nitrate at the para position. We treat toluene with the nitrating mixture. The methyl group strongly activates the ring and directs the nitro group to the ortho and para positions. The para product is the major one, especially if we control the temperature (around 30°C) to minimise ortho substitution. …

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Q.How will you convert benzene into

The Concept: The Director's Cut

Step-by-Step Synthesis

(i) p-Nitrobromobenzene

(ii) m-Nitrochlorobenzene

(iii) p-Nitrotoluene

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