Introduction to Alcohols and Carboxylic Acids

Welcome to one of the most practical chapters in Chemistry! We are going to explore two important "families" of chemicals: alcohols and carboxylic acids. You encounter these every day—alcohols are used in fuels and hand sanitisers, while carboxylic acids like vinegar give salad dressings their tang. In this guide, we will break down their structures, how they react, and how we make them.

1. The Alcohol Family

Alcohols are a homologous series of organic compounds. This just means they follow the same general formula and have similar chemical properties because they all contain the same functional group.

The Functional Group

The functional group for all alcohols is the hydroxyl group: \( -OH \). This is the "business end" of the molecule where most reactions happen.

The First Four Alcohols

You need to know the names and formulas for the first four alcohols. A simple way to remember the order of the prefixes (Meth-, Eth-, Prop-, But-) is the mnemonic: "Mothers Eat Proper Butter".

  • Methanol: \( CH_3OH \)
  • Ethanol: \( CH_3CH_2OH \) (or \( C_2H_5OH \))
  • Propanol: \( C_3H_7OH \)
  • Butanol: \( C_4H_9OH \)

Typical Reactions of Alcohols

Alcohols react in predictable ways. One of the most important is combustion (burning). When alcohols burn in plenty of air, they undergo complete combustion to produce carbon dioxide and water:

\( \text{Ethanol} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} \)

\( C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O \)

Quick Tip: Alcohols are often used as fuels because they burn very cleanly!

2. Producing Ethanol

There are two main ways to make ethanol, but for this chapter, we focus on fermentation and how to purify the result.

Fermentation

This is a natural process used for thousands of years to make bread and drinks. It involves using yeast to convert sugars into ethanol and carbon dioxide.

The Recipe for Fermentation:

  1. Mix a sugar solution with yeast.
  2. Keep it in warm conditions (usually around \( 30^{\circ}C \) to \( 40^{\circ}C \)). If it's too cold, the yeast is inactive; if it's too hot, the enzymes in the yeast are denatured.
  3. Ensure anaerobic conditions (no oxygen). This is vital because oxygen would turn the ethanol into vinegar!

\( \text{Glucose} \xrightarrow{\text{Yeast}} \text{Ethanol} + \text{Carbon Dioxide} \)

Concentrating Ethanol by Fractional Distillation

The ethanol produced by fermentation is usually in a mixture with water and yeast, with a concentration of only about \( 15\% \). To get pure or concentrated ethanol, we use fractional distillation. This works because ethanol has a lower boiling point (\( 78^{\circ}C \)) than water (\( 100^{\circ}C \)). When the mixture is heated, the ethanol evaporates first, is cooled in a condenser, and collected as a pure liquid.

3. The Carboxylic Acid Family

If you leave wine open too long, it goes sour. This is because the alcohol is reacting with oxygen to become a carboxylic acid.

The Functional Group

The functional group for carboxylic acids is the carboxyl group: \( -COOH \). When drawing these, you must show a double bond between the carbon and one oxygen atom (\( C=O \)) and a single bond to an \( -OH \) group.

The First Four Carboxylic Acids

  • Methanoic acid: \( HCOOH \)
  • Ethanoic acid: \( CH_3COOH \) (the main ingredient in vinegar)
  • Propanoic acid: \( C_2H_5COOH \)
  • Butanoic acid: \( C_3H_7COOH \)

Properties and Reactions

Carboxylic acids behave like other acids you have studied, but they are usually weak acids. This means:

  • They react with metal carbonates to produce a salt, water, and carbon dioxide (you would see fizzing).
  • They have a pH typically between 3 and 5.
  • They react with magnesium to produce hydrogen gas.

4. Oxidation of Alcohols

As mentioned before, alcohols can be oxidised to form carboxylic acids. This is a chemical reaction where the alcohol gains oxygen and loses hydrogen atoms.

General Rule: \( \text{Alcohol} + \text{Oxidising Agent} \rightarrow \text{Carboxylic Acid} + \text{Water} \)

Example: Ethanol can be oxidised to become Ethanoic acid.

Note: You do not need to know the specific chemical reagents used for this oxidation for the exam, just that the process occurs!

5. Core Practical 9.28C: Investigating Combustion

In this practical, you investigate how much energy is released when different alcohols burn. You burn a known mass of an alcohol (like ethanol, propanol, or butanol) to heat a fixed volume of water.

  • The Goal: To measure the temperature rise in the water.
  • The Trend: Generally, as the number of carbon atoms in the alcohol chain increases, the amount of energy released per gram of fuel also increases.
  • Common Mistake: Forgetting to weigh the spirit burner before and after the experiment to find the exact mass of alcohol burned!

6. Higher Tier Only: Polyesters

(Higher Tier Students Only)

Alcohols and carboxylic acids can join together to make polymers. Specifically, they form polyesters through condensation polymerisation.

  • This requires two different monomers: a dicarboxylic acid (a molecule with a \( -COOH \) group at both ends) and a diol (a molecule with an \( -OH \) group at both ends).
  • Every time a link is made between these two monomers, a small molecule of water (\( H_2O \)) is lost. This is why it is called "condensation" polymerisation.

Summary: Key Takeaways

1. Functional Groups: Alcohols have \( -OH \); Carboxylic acids have \( -COOH \).
2. Naming: Use the prefixes Meth (1C), Eth (2C), Prop (3C), But (4C).
3. Making Ethanol: Fermentation requires yeast, warmth, and no oxygen. Fractional distillation concentrates it.
4. Reactions: Alcohols burn to make \( CO_2 \) and \( H_2O \). Alcohols can be oxidised to make carboxylic acids.
5. Practical: Longer-chain alcohols usually release more heat energy when burned.