Chemistry | Alcohols

Hello Future Chemist! Understanding Alcohols

Welcome to the study of Alcohols! If you’ve successfully grasped Alkanes and Alkenes, you’re already halfway there. Alcohols are simply organic molecules where a hydrogen atom has been swapped out for a special group containing oxygen. They are extremely important in everyday life—from the fuel in certain cars to the hand sanitiser you use every day.

Don't worry if Organic Chemistry sometimes feels like learning a new language. We will break down the structure and reactions of alcohols step-by-step, making sure you understand the essential concepts needed for your exams!

Section 1: The Basics of Alcohols

1.1 What Defines an Alcohol? The Functional Group

In organic chemistry, a functional group is the atom or group of atoms that determines how the molecule will react. It’s the ‘action centre’!

• The functional group for all alcohols is the hydroxyl group, which is an oxygen atom bonded to a hydrogen atom: -OH.
• We can think of an alcohol as an alkane chain (\(C_n H_{2n+2}\)) where one of the hydrogen atoms has been replaced by this -OH group.

Key Term: The hydroxyl group (-OH) makes the molecule an alcohol.

General Formula

Since alcohols are based on saturated carbon chains (like alkanes), the general formula is:

$$\text{C}_n \text{H}_{2n+1}\text{OH}$$

Analogy: Imagine a long train (the carbon chain). The -OH group is the special engine (the functional group) that dictates where the train can go and what cargo it can carry (how it reacts chemically).

Section 2: Naming Alcohols (Nomenclature)

Naming alcohols follows the same rules as alkanes and alkenes, but with one key difference:

1. Count the number of carbon atoms in the longest chain (Meth-, Eth-, Prop-, But-).
2. Because it is an alcohol, replace the standard alkane ending -ane with the alcohol ending -ol.

For IGCSE, you must know the first four members of the homologous series:

# Carbons (n)	Name	Molecular Formula	Condensed Formula
1	Methanol	\(\text{CH}_4\text{O}\)	\(\text{CH}_3\text{OH}\)
2	Ethanol	\(\text{C}_2\text{H}_6\text{O}\)	\(\text{C}_2\text{H}_5\text{OH}\)
3	Propanol	\(\text{C}_3\text{H}_8\text{O}\)	\(\text{C}_3\text{H}_7\text{OH}\)
4	Butanol	\(\text{C}_4\text{H}_{10}\text{O}\)	\(\text{C}_4\text{H}_9\text{OH}\)

Memory Trick: Just remember the prefixes (Meth, Eth, Prop, But) and stick -OL on the end!

Section 3: Physical Properties of Alcohols

3.1 Solubility in Water

Unlike longer alkanes, which are non-polar and do not dissolve in water, smaller alcohols dissolve completely in water. Why?

• The -OH group is polar (meaning it has slight positive and negative charges).
• Water (\(\text{H}_2\text{O}\)) is also a polar molecule.
• The rule in chemistry is 'like dissolves like'. The polar -OH group can form strong attractive forces (called hydrogen bonds) with water molecules, allowing them to mix easily.

Important Point: As the carbon chain gets longer (e.g., in Butanol and beyond), the non-polar hydrocarbon part of the molecule becomes dominant, and the solubility decreases.

3.2 Boiling Points

Alcohols have significantly higher boiling points than their corresponding alkanes.

• This is again due to the -OH group, which allows molecules to form strong attractive forces (hydrogen bonds) between each other.
• More energy (higher temperature) is needed to break these strong bonds compared to the weaker forces found in alkanes.
• Trend: As the length of the carbon chain increases (e.g., moving from Methanol to Butanol), the boiling point increases. This is because the molecules get larger, increasing the intermolecular forces that need to be overcome.

Section 4: Chemical Reactions of Alcohols

4.1 Complete Combustion (Burning)

Like other hydrocarbons, alcohols burn easily in a plentiful supply of oxygen, releasing a large amount of energy (exothermic reaction). This makes them good fuels.

The products of complete combustion are always carbon dioxide and water.

Example (Ethanol Combustion):

$$\text{C}_2\text{H}_5\text{OH} \quad + \quad 3\text{O}_2 \quad \rightarrow \quad 2\text{CO}_2 \quad + \quad 3\text{H}_2\text{O}$$

Did you know? Ethanol is often used in ‘flex-fuel’ engines in countries like Brazil, where it is known as a clean-burning biofuel.

4.2 Dehydration (Making Alkenes)

This is a crucial reaction for IGCSE. Dehydration means removing water from a molecule. When water is removed from an alcohol, it forms an alkene.

This reaction is the chemical opposite of the hydration of ethene (which we will see in Section 5).

Step-by-Step Dehydration Process:

1. Take an alcohol (e.g., Ethanol, \(\text{C}_2\text{H}_5\text{OH}\)).
2. Heat it over a catalyst. The IGCSE syllabus usually focuses on hot concentrated sulfuric acid or passing the vapour over heated aluminium oxide (\(\text{Al}_2\text{O}_3\)).
3. The -OH group and a hydrogen atom from an adjacent carbon are removed, forming a molecule of water (\(\text{H}_2\text{O}\)).
4. A double bond forms between the two carbon atoms, creating an alkene.

Equation (Dehydration of Ethanol to Ethene):

$$\text{C}_2\text{H}_5\text{OH} \xrightarrow[\text{Heat}]{\text{Conc. } \text{H}_2\text{SO}_4} \text{C}_2\text{H}_4 \quad + \quad \text{H}_2\text{O}$$

Section 5: Manufacturing Ethanol

Ethanol (\(\text{C}_2\text{H}_5\text{OH}\)) is the most common and useful alcohol. There are two primary methods for its manufacture, which you must know well:

5.1 Method 1: Fermentation (The Bio-Method)

Fermentation is the process used to produce alcoholic drinks. It uses natural, renewable resources like sugar cane or maize (corn).

Process Details:

• Starting Material: Glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)) or starch (which is broken down into glucose).
• Catalyst: Yeast (which contains enzymes).
• Conditions: Anaerobic conditions (absence of air/oxygen) and an optimum temperature of about 30°C – 40°C. If the temperature is too high, the yeast enzymes die.

Chemical Equation for Fermentation:

$$\text{C}_6\text{H}_{12}\text{O}_6 \quad \xrightarrow{\text{Yeast}} \quad 2\text{C}_2\text{H}_5\text{OH} \quad + \quad 2\text{CO}_2$$

Limitation: The maximum concentration of ethanol produced this way is about 15% because the yeast dies once the alcohol concentration gets too high. To achieve higher purity (e.g., 95%), distillation is required.

5.2 Method 2: Hydration of Ethene (The Industrial Method)

This method is used to manufacture massive amounts of industrial ethanol quickly and efficiently.

Process Details:

• Starting Material: Ethene (\(\text{C}_2\text{H}_4\)), which is usually obtained from cracking crude oil fractions (non-renewable).
• Reactant: Steam (\(\text{H}_2\text{O}\)).
• Catalyst: Phosphoric(V) acid (\(\text{H}_3\text{PO}_4\)) supported on silicon dioxide.
• Conditions: High temperature (around 300°C) and very high pressure (around 60–70 atmospheres).

Chemical Equation for Hydration:

$$\text{C}_2\text{H}_4 \quad + \quad \text{H}_2\text{O} \quad \rightleftharpoons \quad \text{C}_2\text{H}_5\text{OH}$$

(Note the reversible arrow, indicating it is an equilibrium reaction.)

Comparison of Methods (IGCSE Requirement)

Feature	Fermentation	Hydration of Ethene
Starting Material	Glucose (Renewable biomass)	Ethene (Crude oil, non-renewable)
Speed	Slow batch process	Very fast continuous process
Purity	Produces dilute ethanol (needs distillation)	Produces purer ethanol
Conditions	Low T, atmospheric P (gentle)	High T, High P (expensive)
Overall Efficiency	Less efficient (low yield)	Highly efficient (high yield)

Section 6: Uses of Alcohols

Alcohols, especially Ethanol and Methanol, have many practical applications:

1. Solvents: Ethanol is an excellent solvent and is used to dissolve substances that do not dissolve well in water, such as perfumes, varnishes, and cleaning agents.
2. Fuels (Biofuels): Ethanol burns cleanly and is used on its own or blended with petrol (gasoline) to power vehicles (e.g., E10 fuel). Since it is derived from crops, it is considered a renewable biofuel.
3. Alcoholic Drinks: Ethanol is the alcohol found in beer, wine, and spirits.
4. Sterilisation: Alcohols are used in medicine for sterilising equipment and in hand sanitisers because they kill bacteria.
5. Methanol Usage: Methanol is toxic and is mainly used as a chemical feedstock (starting material for making other chemicals) and a solvent.