Chemistry Study Notes: Important Compounds in Our Daily Lives

Hey everyone! Ready to dive into some really cool and practical chemistry? In this chapter, we're going to explore four amazing compounds that you definitely use or encounter every single day: aspirin, detergents, nylon, and polyesters. You'll learn what they're made of, how they work, and why they're so important. It might sound like a lot, but we'll break it down into simple, easy-to-understand parts. Let's get started!


1. Aspirin: The Wonder Drug

What is Aspirin?

You've probably heard of aspirin. It's one of the most common medicines in the world, used to treat headaches, pain, and fevers. Its proper chemical name is acetylsalicylic acid (or 2-ethanoyloxybenzenecarboxylic acid).

The Structure of Aspirin

To understand how aspirin works, we first need to look at its molecule. The key to any organic molecule's function is its functional groups:

  • Carboxylic Acid Group (\(-\text{COOH}\)): This group makes aspirin acidic. It can react with bases to form water-soluble salts.
  • Ester Group (\(-\text{OCOCH}_3\)): Formed by the acetylation of the phenolic \(-\text{OH}\) group in salicylic acid.
  • Benzene Ring: A stable six-membered aromatic carbon ring.
Quick Review: Aspirin's Functional Groups

1. Carboxylic Acid (\(-\text{COOH}\))
2. Ester (\(-\text{OCOCH}_3\))
Memory Aid: Think "Aspirin is an ACid with an Ester" -> ACE

Preparation (Synthesis) of Aspirin

In the laboratory, aspirin is prepared by reacting salicylic acid (2-hydroxybenzenecarboxylic acid) with ethanoic anhydride (or ethanoyl chloride) in the presence of an acid catalyst (such as concentrated \(\text{H}_2\text{SO}_4\) or \(\text{H}_3\text{PO}_4\)):

\(\text{C}_6\text{H}_4(\text{OH})\text{COOH} + (\text{CH}_3\text{CO})_2\text{O} \rightarrow \text{C}_6\text{H}_4(\text{OCOCH}_3)\text{COOH} + \text{CH}_3\text{COOH}\)

Hydrolysis of Aspirin

When exposed to moisture or inside the digestive system, the ester linkage of aspirin can be hydrolysed back into salicylic acid and ethanoic acid:

  • Acid hydrolysis: Warming with dilute acid yields salicylic acid and ethanoic acid.
  • Alkaline hydrolysis: Boiling with dilute \(\text{NaOH}\) forms the water-soluble sodium salicylate and sodium ethanoate salts.

Medical Applications of Aspirin

What do these functional groups allow aspirin to do? It's like a multi-tool for your body's minor problems:

  • Relieves pain (It's an analgesic).
  • Reduces fever (It's an antipyretic).
  • Reduces inflammation and swelling (It's an anti-inflammatory drug).
  • Reduces the risk of heart attack / stroke by inhibiting blood clotting (acting as an anti-platelet agent).

Key Takeaway: Aspirin

Aspirin (acetylsalicylic acid) has two key functional groups: a carboxylic acid and an ester. It is synthesized by esterifying salicylic acid and acts as an analgesic, antipyretic, and anti-inflammatory drug.


2. Detergents: The Science of Cleaning

The Big Problem: Oil and Water Don't Mix

Ever tried to wash a greasy dish with just water? Grease and oil are non-polar and hydrophobic (water-repelling). Detergents act as surfactants to bridge the gap between water and oil.

The Structure of a Detergent Molecule

Every detergent molecule has a dual-nature structure:

  • The Hydrophobic Tail: A long non-polar hydrocarbon chain that repels water and readily dissolves in oils and grease.
  • The Hydrophilic Head: An ionic or polar head group that interacts strongly with water molecules via ion-dipole interactions or hydrogen bonding.

How Detergents Clean (The Cleansing Action)

  1. Detergent is added to water. The hydrophobic tails dissolve into the grease droplet, while the hydrophilic heads remain in water.
  2. Agitation (scrubbing or machine motion) pulls the grease layer off the surface and breaks it into tiny droplets.
  3. Detergent molecules surround each grease droplet to form spherical aggregates called micelles.
  4. The negatively charged heads on the outer surface repel each other, keeping the droplets suspended as a stable emulsion and preventing them from coalescing.
  5. Rinsing with water flushes the micelles and trapped dirt away.

Soaps vs. Soapless Detergents

Soaps
  • Structure: Sodium or potassium salts of long-chain fatty acids (e.g. \(\text{R-COO}^-\text{Na}^+\)).
  • How they're made: Through saponification (alkaline hydrolysis of triglycerides/fats with concentrated \(\text{NaOH}\) or \(\text{KOH}\)), producing soap and glycerol (propane-1,2,3-triol).
  • The Problem: In hard water (containing \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions), soaps form insoluble grey precipitates called scum:
    \(2\text{RCOO}^-\text{(aq)} + \text{Ca}^{2+}\text{(aq)} \rightarrow (\text{RCOO})_2\text{Ca(s)}\)
Soapless Detergents (Synthetic Detergents)
  • Structure: Usually alkylbenzenesulphonates or alkyl sulphates (e.g. \(\text{R-SO}_3^-\text{Na}^+\) or \(\text{R-OSO}_3^-\text{Na}^+\)).
  • How they're made: Synthesised from petroleum derivatives.
  • The Advantage: Calcium and magnesium salts of synthetic detergents are water-soluble, so they do not form scum in hard water.

Environmental Concerns of Detergents

  • Non-biodegradability: Detergents with heavily branched hydrocarbon chains cannot be easily broken down by bacteria, causing persistent foam in rivers and sewage plants. Modern detergents use unbranched (linear) chains that are biodegradable.
  • Eutrophication: Phosphate builders added to detergents increase nutrient levels in waterways, leading to algal blooms, depletion of dissolved oxygen, and harm to aquatic life.

Key Takeaway: Detergents

Detergents have a hydrophobic tail and a hydrophilic head that trap oil in micelles. Soaps form scum in hard water, while soapless detergents do not. Linear chains reduce foaming issues, and phosphate-free builders help prevent eutrophication.


3. Nylon and Polyesters: Man-Made Fibres

First, a Quick Review: Polymers

Polymers are giant macromolecules formed by linking many small repeating monomer units together.

Condensation Polymerisation

Nylon and polyesters are produced via condensation polymerisation: monomers join together with the simultaneous elimination of small molecules such as \(\text{H}_2\text{O}\) or \(\text{HCl}\).

Nylon: The Strong and Silky Polyamide

  • Monomers: Typically made from a dicarboxylic acid (or dioyl chloride) and a diamine (e.g., hexanedioic acid and hexane-1,6-diamine for Nylon-6,6).
  • The Linkage: Monomers react to form repeating amide links (\(-\text{CONH}-\)).
  • Properties: Strong intermolecular hydrogen bonds between adjacent polymer chains provide high tensile strength, elasticity, and abrasion resistance.
  • Uses: Ropes, textiles, fishing lines, parachutes, and gears.
  • Equation:
    \(n\,\text{HOOC-R-COOH} + n\,\text{H}_2\text{N-R'-NH}_2 \rightarrow -[\text{OC-R-CONH-R'-NH}]_n- + (2n-1)\,\text{H}_2\text{O}\)

Polyesters: The Versatile Polymer

  • Monomers: Made from a dicarboxylic acid (e.g., benzene-1,4-dicarboxylic acid) and a diol (e.g., ethane-1,2-diol for PET).
  • The Linkage: Monomers join to form repeating ester links (\(-\text{COO}-\)).
  • Properties: Strong dipole-dipole attractions and dispersion forces make polyesters durable, wrinkle-resistant, and quick-drying.
  • Uses: Synthetic fabrics (Terylene/Dacron), plastic drink bottles (PET), sails, and packaging films.
  • Equation:
    \(n\,\text{HOOC-R-COOH} + n\,\text{HO-R'-OH} \rightarrow -[\text{OC-R-COO-R'-O}]_n- + (2n-1)\,\text{H}_2\text{O}\)
Common Mistake to Avoid!

In addition polymerisation, unsaturated monomers join without losing any atoms. In condensation polymerisation, bifunctional monomers join with the elimination of small molecules such as water.

Key Takeaway: Nylon and Polyesters

Nylon (a polyamide) and polyesters are formed by condensation polymerisation with the elimination of small molecules.
- Nylon: Made from dicarboxylic acids and diamines (contains amide links).
- Polyester: Made from dicarboxylic acids and diols (contains ester links).