Welcome to the Chemistry of Medicine!

In this chapter, we are stepping into the shoes of a pharmaceutical chemist. We’ll be exploring the section "What’s in a medicine? (WM)". Using aspirin as our main example, we will learn how to identify organic molecules, how to transform them through reactions, and how to make sure the medicine we produce is pure and safe. Don't worry if some of the names sound long—we’ll break them down piece by piece!

1. The Organic Toolbox: Functional Groups

To understand medicines, we first need to recognize the "active parts" of a molecule. These are called functional groups. They determine how a molecule behaves.

Key Homologous Series in WM:

  • Carboxylic Acids: Contain the \( -COOH \) group. Think of vinegar (ethanoic acid).
  • Phenols: An \( -OH \) group attached directly to a benzene ring. Example: Salicylic acid.
  • Alcohols: Contain the \( -OH \) group attached to a standard carbon chain.
  • Esters: Contain \( -COO - \). These often smell fruity!
  • Aldehydes & Ketones: Both contain the carbonyl group \( C=O \). Aldehydes have it at the end of a chain; Ketones have it in the middle.
  • Acid Anhydrides: Reactive molecules used to make esters (like aspirin).
  • Ethers: An oxygen atom "sandwiched" between two carbon groups (\( R-O-R \)).

The Alcohol Family

Not all alcohols are the same! We classify them by how many "carbon friends" the carbon holding the \( -OH \) group has:

  • Primary (1°): The \( C-OH \) carbon is attached to 1 other carbon.
  • Secondary (2°): The \( C-OH \) carbon is attached to 2 other carbons.
  • Tertiary (3°): The \( C-OH \) carbon is attached to 3 other carbons.

Quick Review Box:
If you find this confusing, just count the carbon atoms directly touching the carbon that holds the \( -OH \).
0 or 1 = Primary
2 = Secondary
3 = Tertiary

2. The Special Chemistry of Phenols

Phenols look like alcohols, but they are more "moody" (acidic) because of the benzene ring they sit on.

Phenol vs. Carboxylic Acid

Students often mix these up! Here is how to tell them apart in the lab:

  • Acidity: Both are acidic and will react with alkalis (like \( NaOH \)).
  • The Carbonate Test: Carboxylic acids are strong enough to react with carbonates (like \( Na_2CO_3 \)) to produce \( CO_2 \) bubbles. Phenols are NOT strong enough to do this. No bubbles = Phenol.
  • The Iron(III) Test: If you add neutral iron(III) chloride solution to a phenol, it turns a beautiful purple. This is a classic test for the presence of a phenol group in medicines like aspirin precursors.

Key Takeaway: Phenols react with alkalis and acid anhydrides (to make esters), but they never react with carbonates!

3. Transforming Alcohols: Key Reactions

Medicines are built by changing one functional group into another. Here are the four main reactions you need for alcohols:

A. Oxidation (The "Loss of Hydrogen" reaction)

We use acidified dichromate(VI) (\( Cr_2O_7^{2-}/H^+ \)) as the oxidizing agent. It turns from orange to green when it works.

  • Primary Alcohols: Can be oxidized to Aldehydes (using distillation to stop the reaction early) or all the way to Carboxylic Acids (using reflux).
  • Secondary Alcohols: Are oxidized to Ketones (using reflux).
  • Tertiary Alcohols: Don't react! They are "stubborn" because they have no hydrogen atom on the central carbon to lose.

B. Esterification

To make an ester, you react an alcohol (or phenol) with a carboxylic acid (needs a strong acid catalyst like \( H_2SO_4 \)) or an acid anhydride. Making aspirin involves reacting salicylic acid (a phenol) with ethanoic anhydride.

C. Dehydration (Elimination)

This removes a water molecule from an alcohol to create a C=C double bond (an alkene). You need a catalyst like heated \( Al_2O_3 \) or concentrated \( H_2SO_4 \).

D. Substitution

Replacing the \( -OH \) group with a halogen (like \( Cl \) or \( Br \)) to make a haloalkane. This is often done using halide salts and acid.

Did you know? Reflux is like a "circular" boiling system. It allows us to heat organic reactions for a long time without the volatile chemicals evaporating away into the room!

4. Lab Skills: Making and Purifying Medicines

In the "What's in a medicine?" section, you must know how to actually make a solid like aspirin and ensure it's pure.

Step-by-Step: Purifying a Solid (Recrystallisation)

  1. Dissolve the impure solid in the minimum amount of hot solvent.
  2. Filter hot to remove any insoluble impurities.
  3. Cool the solution slowly. The pure product will form crystals, while soluble impurities stay dissolved.
  4. Filter under reduced pressure (using a Buchner funnel) to separate the crystals.
  5. Wash with a tiny bit of ice-cold solvent and dry.

How do we know it's pure?

  • Melting Point Determination: A pure solid has a sharp melting point that matches the data book. If it’s impure, it will melt over a wide range and at a lower temperature.
  • Thin Layer Chromatography (TLC): This separates components in a mixture. We compare the \( R_f \) value of our product to a known pure sample.

Common Mistake: Using too much solvent during recrystallisation. If you use too much, your product will stay dissolved and you won't get any crystals back!

5. Analytical Techniques: Fingerprinting Molecules

How do we prove we've made the right medicine? We use two main "eyes":

Infrared (IR) Spectroscopy

Molecules vibrate when they hit IR light. Different bonds vibrate at different frequencies. We look for "peaks" on a graph:

  • A broad "U" shape at \( 3200-3600 cm^{-1} \) is an O-H group.
  • A sharp "spike" at \( 1650-1750 cm^{-1} \) is a C=O (carbonyl) group.

Mass Spectrometry

This tells us the mass of the molecule.

  • Molecular Ion Peak (\( M^+ \)): The peak furthest to the right (ignore tiny \( M+1 \) peaks). This tells you the Relative Molecular Mass (\( M_r \)).
  • Fragmentation: The molecule breaks into pieces. If you see a loss of 15, it probably lost a \( CH_3 \) group!

Memory Aid: IR is for Functional Groups (what parts does it have?), Mass Spec is for Mass/Formula (how heavy is it?).

6. Green Chemistry

Modern medicine must be sustainable. There are 12 principles, but for your exam, focus on these big ideas:

  • Atom Economy: Try to make sure all the atoms from your starting materials end up in your product, not as waste.
  • Renewable Feedstocks: Use plant-based chemicals instead of oil-based ones where possible.
  • Catalysts: Use them to lower energy needs.
  • Prevention: It is better to prevent waste than to clean it up later.

Quick Summary:
To succeed in the WM section, master the Iron(III) test for phenols, understand why reflux is used in oxidation, and be able to describe the recrystallisation process for purifying aspirin!