Introduction to Organic Functional Groups in Medicines

Welcome! In this chapter, we are diving into the world of What’s in a Medicine? (WM). Have you ever wondered why aspirin helps a headache or how scientists design new drugs? It all comes down to functional groups—specific clusters of atoms that give a molecule its unique "personality" and chemical powers. By the end of these notes, you’ll be able to spot these groups in complex molecules and understand how they react.

Don't worry if this seems like a lot to memorize at first! We will break it down into small, manageable patterns that you'll start to recognize everywhere.

1. Meet the Families: Homologous Series

In organic chemistry, we group molecules into "families" called homologous series. Molecules in the same family have the same functional group and similar chemical properties.

For your AS Level (Salters B), you need to recognize these groups. You don't need to learn the complex naming rules (nomenclature) for all of them yet, just be able to spot them!

Key Functional Groups to Recognize:

  • Carboxylic Acids: Contain the \(–COOH\) group. These are the "sour" groups found in vinegar (ethanoic acid) and aspirin.
  • Phenols: An \(–OH\) group attached directly to a benzene ring (a hexagon with a circle or alternating double bonds). Important: They aren't just alcohols!
  • Acid Anhydrides: Think of these as two carboxylic acids joined together with a water molecule removed. Formula: \(R-CO-O-CO-R\).
  • Esters: Contain \(–COO–\). These usually smell amazing—like pear drops or glue! They are made by reacting an alcohol with an acid.
  • Aldehydes: Contain a \(–CHO\) group at the end of a carbon chain.
  • Ketones: Contain a \(C=O\) (carbonyl) group in the middle of a carbon chain.
  • Ethers: An oxygen atom sandwiched between two carbon atoms (\(C–O–C\)).

Quick Review Box:
Carboxylic Acid: \(–COOH\)
Ester: \(–COOR\)
Aldehyde: \(–CHO\) (on the end)
Ketone: \(–C=O\) (in the middle)

Key Takeaway: Functional groups are the "reactive centers" of a molecule. If you know the group, you can predict how the whole medicine will behave!

2. The Alcohol Hierarchy: Primary, Secondary, and Tertiary

Not all alcohols (compounds with an \(–OH\) group) are the same. We classify them based on how many "carbon neighbors" the carbon attached to the \(–OH\) has.

  • Primary (\(1^{\circ}\)) Alcohols: The \(–OH\) carbon is attached to one other carbon. (Example: Ethanol).
  • Secondary (\(2^{\circ}\)) Alcohols: The \(–OH\) carbon is attached to two other carbons.
  • Tertiary (\(3^{\circ}\)) Alcohols: The \(–OH\) carbon is attached to three other carbons.

Memory Aid: Look at the carbon holding the \(–OH\) group. Count how many "friends" (other carbons) it has. 1 friend = Primary; 2 friends = Secondary; 3 friends = Tertiary.

Key Takeaway: This classification is vital because it determines if the alcohol can be oxidized (turned into something else).

3. Phenols: The Special Cases

A phenol is an \(–OH\) group joined directly to an aromatic benzene ring. Even though they have an \(–OH\), they behave very differently from normal alcohols.

How to spot a Phenol vs. a Carboxylic Acid:

Both are acidic, but they have different "strengths":

  • Phenols are weakly acidic. They react with strong bases like sodium hydroxide (\(NaOH\)), but they are not strong enough to react with carbonates (like \(Na_2CO_3\)). No bubbles of \(CO_2\) will appear!
  • Carboxylic acids are stronger. They react with both sodium hydroxide and carbonates (they will fizz/effervesce).

The Purple Test:

To identify a phenol in the lab, add neutral iron(III) chloride solution (\(FeCl_3\)). If a phenol is present, you will see a distinct purple colouration.

Did you know? Salicylic acid, which comes from willow bark, has both a phenol group and a carboxylic acid group. This is the precursor to aspirin!

Key Takeaway: Phenols = Purple with \(FeCl_3\). They react with alkalis but never with carbonates.

4. Alcohol Reactions: Changing One Group into Another

In medicine synthesis, we often need to transform alcohols into other functional groups. Here are the four key reactions you must know:

A. Oxidation (The Big One)

We use acidified dichromate(VI) (\(H^+ / Cr_2O_7^{2–}\)) as the oxidizing agent. It turns from orange to green if oxidation happens.

  • Primary Alcohols: Can be oxidized twice.
    1. Distillation \(\rightarrow\) Aldehyde
    2. Reflux \(\rightarrow\) Carboxylic Acid
  • Secondary Alcohols: Oxidized once (under reflux) \(\rightarrow\) Ketone.
  • Tertiary Alcohols: Cannot be oxidized. The solution stays orange.

B. Esterification

Alcohol + Carboxylic Acid \(\rightleftharpoons\) Ester + Water.
This requires a concentrated sulfuric acid (\(H_2SO_4\)) catalyst. You can also make esters by reacting an alcohol with an acid anhydride.

C. Dehydration (Elimination)

This removes a water molecule (\(H_2O\)) from an alcohol to create a \(C=C\) double bond (an alkene).
Conditions: Use heated aluminum oxide (\(Al_2O_3\)) or reflux with concentrated \(H_2SO_4\).

D. Substitution

Replacing the \(–OH\) group with a halogen (like \(Cl\) or \(Br\)) to make a haloalkane. This usually involves reacting the alcohol with halide ions in the presence of acid.

Common Mistake to Avoid: When writing oxidation equations, remember to use \([O]\) to represent the oxidizing agent. For example: \(R-CH_2OH + [O] \rightarrow R-CHO + H_2O\).

Key Takeaway:
Primary \(\rightarrow\) Aldehyde/Acid.
Secondary \(\rightarrow\) Ketone.
Tertiary \(\rightarrow\) No reaction.
Alcohol + Acid \(\rightarrow\) Ester.

Quick Review: Summary Table

Test for Phenol: Neutral \(FeCl_3 \rightarrow\) Purple color.
Test for Carboxylic Acid: \(Na_2CO_3 \rightarrow\) Effervescence (fizzing).
Oxidation of Alcohol: Orange dichromate \(\rightarrow\) Green (except for Tertiary).
Making an Ester: Alcohol + Acid + \(H_2SO_4\) catalyst \(\rightarrow\) Sweet smell.