Organic synthesis

Welcome to Organic Synthesis!

In this chapter, we are going to learn how to become "molecular architects." Organic synthesis is all about the clever ways chemists build complex molecules from simpler ones. Whether it's making a life-saving medicine or a new type of plastic, the rules are the same. We will look at the tools of the trade (the lab equipment) and the "blueprints" (the reaction routes) you need to master for your OCR A Level exams.

Don't worry if this seems tricky at first! Synthesis is like learning to cook; once you know what your "ingredients" (functional groups) do, you can follow any recipe.

1. The Lab Toolkit: Preparing and Purifying Liquids

Before we can build molecules, we need to know how to handle them. Most organic reactions don't just happen instantly—they need help.

Heating Under Reflux

Organic reactions are often slow and the chemicals are often volatile (they evaporate easily). If we just heated them in an open beaker, they would disappear into the air! Reflux is a technique where we boil a mixture in a flask attached to a vertical condenser. This way, any vapor turns back into a liquid and drips back into the flask.

Analogy: Think of reflux like a boomerang. You throw the molecules up as vapor, but the cold condenser catches them and sends them right back down so they can keep reacting.

Quick Review: Reflux allows for prolonged heating without losing any of your reactants or products.

Distillation

Once the reaction is done, you often have a mixture of things. Distillation separates liquids based on their different boiling points. We heat the mixture, and the liquid with the lowest boiling point evaporates first, travels down a condenser, and is collected in a separate flask.

Purification: The Separating Funnel

Sometimes your product is mixed with water. We use a separating funnel to get rid of the "aqueous" (water-based) layer.
1. Pour the mixture into the funnel.
2. Add water (or a wash solution).
3. Put the stopper in and shake, then invert and open the tap to release pressure.
4. Let the layers settle. The more dense layer (usually water) sits at the bottom and can be run off through the tap.

Drying the Product

Even after using a funnel, your organic liquid might still have tiny traces of water. We add anhydrous salts (like \(MgSO_{4}\) or \(CaCl_{2}\)) which act like chemical sponges to soak up the water. When the liquid goes from cloudy to clear and the salt doesn't "clump" anymore, it's dry!

Key Takeaway: Liquid synthesis follows a "React -> Separate -> Dry -> Redistill" pattern.

2. The Lab Toolkit: Preparing and Purifying Solids

When your product is a solid, the rules change slightly. This is common in the "Nitrogen compounds" section, like when making certain amides or amino acids.

Filtration Under Reduced Pressure

This is a fast way to separate a solid from a liquid. We use a Buchner funnel and a vacuum pump. The vacuum "sucks" the liquid through the filter paper, leaving the dry solid behind much faster than normal gravity filtration.

Recrystallisation: The "Golden Technique"

This is how we get 100% pure crystals.
1. Dissolve the impure solid in the minimum volume of hot solvent.
2. Cool the solution slowly. The pure product will form crystals, while impurities stay dissolved.
3. Filter the pure crystals again and wash with a tiny bit of cold solvent.

Checking Purity: Melting Point

How do you know it's pure? A pure substance has a sharp melting point that matches the data book value. If it's impure, the substance will melt over a wide range and at a lower temperature than expected.

Did you know? This is exactly how pharmaceutical companies ensure your medicine is safe and doesn't contain dangerous by-products!

3. Extending the Carbon Chain

One of the hardest things in synthesis is making the molecule bigger. In OCR Chemistry, there are a few "Carbon-Carbon bond builders" you must know.

Nitriles: The Secret Weapon

Adding a \(CN\) group is the easiest way to add exactly one carbon atom.
- From Haloalkanes: React with \(NaCN\) or \(KCN\) in ethanol. This is a nucleophilic substitution reaction.
- From Carbonyls (Aldehydes/Ketones): React with \(HCN\) (usually made in situ from \(NaCN/H_{2}SO_{4}\)). This is nucleophilic addition and creates a hydroxynitrile.

What do we do with Nitriles?

Once you have a nitrile (\(R-C \equiv N\)), you can turn it into two very useful nitrogen-containing groups:
1. Reduction: Use \(H_{2}\) and a Nickel catalyst to turn it into an Amine (\(R-CH_{2}NH_{2}\)).
2. Hydrolysis: Heat with dilute aqueous acid (e.g., \(HCl\)) to turn it into a Carboxylic Acid (\(R-COOH\)).

Friedel-Crafts: Adding to Benzene

To add a carbon chain to a benzene ring, we use Friedel-Crafts Alkylation (using a haloalkane) or Acylation (using an acyl chloride). Both require a halogen carrier catalyst like \(AlCl_{3}\).

Memory Aid: "CN" stands for Carbon Next—it’s your go-to for making a chain longer!

Key Takeaway: To grow a molecule, look for ways to use Nitriles or Friedel-Crafts reactions.

4. Designing a Synthetic Route

In the exam, you might be asked to show how to get from "Compound A" to "Compound B" in two or three steps. This is where you combine everything you've learned.

The Strategy: "Mind the Gap"

1. Look at the Carbon Skeleton: Did the number of carbons change? If yes, you need a nitrile or Friedel-Crafts step.
2. Look at the Functional Groups: What do I have, and what do I want?
3. Work Backward (Retrosynthesis): If I want an Amine, I could make it from a Nitrile. To get a Nitrile, I need a Haloalkane. To get a Haloalkane, I could start from an Alkane or Alkene.

Common 2-Step Bridges to Remember:

- Alkane -> Haloalkane -> Nitrile: Adds a carbon.
- Nitrobenzene -> Phenylamine -> N-phenylethanamide: Essential for nitrogen synthesis. (Uses \(Sn/HCl\) then an acyl chloride).
- Alcohol -> Aldehyde -> Carboxylic Acid: Oxidation steps using \(K_{2}Cr_{2}O_{7}/H_{2}SO_{4}\).

Common Mistakes to Avoid:

- Wrong Reagent: Using \(HCN\) for haloalkanes (it must be \(NaCN\) in ethanol).
- Missing Conditions: Forgetting to write "reflux," "distil," or "catalyst."
- Wrong Functional Group: Confusing amides (\(CONH_{2}\)) with amines (\(NH_{2}\)). Amides have that "O" next door!

Quick Review Box:
- Nitration of Benzene: Conc \(HNO_{3}\) / Conc \(H_{2}SO_{4}\)
- Reduction of Nitrobenzene: \(Sn\) / Conc \(HCl\)
- Reduction of Carbonyls: \(NaBH_{4}\)
- Acid to Acyl Chloride: \(SOCl_{2}\)

Summary: Synthesis is about knowing your reaction pathways like a map. If you know how to move between functional groups and how to add carbons, you can solve any synthetic puzzle!