Q.Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene.

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

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.

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. …