Chemistry (0620) | Alcohols

🔬 Organic Chemistry: The Wonderful World of Alcohols (Topic 11.6)

Hello Chemists! Welcome to the section on Alcohols. This is a really important group of organic compounds because the most famous member, Ethanol, is used everywhere—from the fuel in racing cars to cleaning products and even some beverages!

In these notes, we'll dive into how we make ethanol in two very different ways, explore its chemical reactions, and understand why it's so useful. Don't worry if organic structures seem tricky; we'll break down the concepts step-by-step!

1. What is an Alcohol? The Basics

Alcohols belong to a special family of organic compounds called a homologous series (remember that term from the previous section?). They all share a defining feature.

Key Terminology

• Functional Group: The specific atom or group of atoms that determines the chemical properties of a compound. For alcohols, this group is the hydroxyl group, $\mathbf{-\text{OH}}$.
  (It is attached to an alkyl chain, e.g., $\text{CH}_3\text{CH}_2\text{OH}$)

• General Formula: Alcohols have the general formula: $\mathbf{\text{C}_n\text{H}_{2n+1}\text{OH}}$.

• Naming: The names of alcohols end in $\mathbf{-ol}$.

Focus Compound: Ethanol

Ethanol is the simplest alcohol we need to study in detail (the one with $n=2$).

• Molecular Formula: $\mathbf{\text{C}_2\text{H}_5\text{OH}}$

• Displayed Formula: Showing all atoms and bonds:

  (Imagine a structure with two carbons bonded together. The first carbon has three hydrogens attached, the second has two hydrogens and the $-\text{OH}$ group.)

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Key Takeaway: Alcohols are defined by the presence of the hydroxyl ($\mathbf{-\text{OH}}$) functional group. Our main focus is Ethanol ($\text{C}_2\text{H}_5\text{OH}$).

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2. Manufacture of Ethanol: Method 1 – Fermentation (The Natural Way)

Fermentation is the traditional method, often used to produce alcoholic drinks, and it relies on living organisms.

What is Fermentation? (Core 11.6.1a)

Fermentation is the process where aqueous glucose (sugar solution) is converted into ethanol and carbon dioxide using yeast.

Step-by-Step Process & Conditions

1. A sugary solution (like fruit juice or sugar cane extract, which contains glucose, $\text{C}_6\text{H}_{12}\text{O}_6$) is mixed with yeast.

2. Yeast contains enzymes that act as biological catalysts for the reaction.

3. The mixture is kept warm, typically between $\mathbf{25^{\circ}\text{C}}$ and $\mathbf{35^{\circ}\text{C}}$. (If it's too cold, the yeast is slow; if it's too hot, the enzymes in the yeast are denatured and die).

4. The reaction must occur in the absence of oxygen (anaerobic conditions). If oxygen is present, the yeast would simply respire and produce water and $\text{CO}_2$ instead of ethanol.

5. Once the ethanol concentration reaches about 15%, the yeast dies, and the reaction stops. Fractional distillation is then needed to obtain pure ethanol.

Chemical Equation for Fermentation

$$\text{C}_6\text{H}_{12}\text{O}_6(aq) \xrightarrow{\text{yeast, } 25-35^{\circ}\text{C}} 2\text{C}_2\text{H}_5\text{OH}(aq) + 2\text{CO}_2(g)$$

Did you know? This is how brewers make beer and winemakers make wine!

3. Manufacture of Ethanol: Method 2 – Catalytic Addition of Steam to Ethene (The Industrial Way)

This method is fast, continuous, and ideal for producing large quantities of pure ethanol needed for industrial applications. It is sometimes called the Hydration of Ethene.

What is Hydration? (Core 11.6.1b)

It is an addition reaction where ethene ($\text{C}_2\text{H}_4$) reacts with steam ($\text{H}_2\text{O}$) to form ethanol.

Step-by-Step Process & Conditions

1. Ethene gas is manufactured, usually obtained from cracking crude oil fractions.

2. Ethene is reacted with steam (gaseous water).

3. Very specific, harsh conditions are required:

   • Temperature: $\mathbf{300^{\circ}\text{C}}$ (High heat)

   • Pressure: $\mathbf{6000\text{ kPa}}$ ($\mathbf{60 \text{ atm}}$) (Very high pressure)

   • Catalyst: An acid catalyst (often phosphoric acid) is used to speed up the reaction.

4. The product, ethanol, is then condensed and collected.

Remember the name: This is a catalytic addition reaction because the steam is added across the double bond of the ethene, and it requires a catalyst.

Chemical Equation for Catalytic Addition

$$\text{C}_2\text{H}_4(g) + \text{H}_2\text{O}(g) \xrightarrow{\text{acid catalyst, } 300^{\circ}\text{C}, 60 \text{ atm}} \text{C}_2\text{H}_5\text{OH}(g)$$

4. Comparing the Manufacture Methods (Supplement 11.6.4)

Now that you know both methods, you need to be able to compare their advantages and disadvantages. This is a common exam question that tests your understanding of both chemistry and economics/sustainability.

Fermentation (Cores 11.6.4a)

• Advantages (Pros):

  1. Renewable Source: Glucose comes from crops (like sugar cane or corn), which can be grown again. This makes the raw material renewable.

  2. Gentle Conditions: Uses low temperatures ($25^{\circ}\text{C}-35^{\circ}\text{C}$) and normal atmospheric pressure, so energy costs are low.

• Disadvantages (Cons):

  1. Slow Rate: It's a batch process that takes several days.

  2. Impure Product: The ethanol produced is an aqueous solution (up to 15% concentration), requiring expensive fractional distillation for purification.

  3. Takes up Land: Land used to grow crops for ethanol might otherwise be used to grow food.

Catalytic Addition of Steam to Ethene (Supplement 11.6.4b)

• Advantages (Pros):

  1. Fast Rate: It's a rapid, continuous industrial process, meaning high volumes can be produced quickly.

  2. Pure Product: Produces high-purity ethanol directly, reducing purification costs.

• Disadvantages (Cons):

  1. Non-Renewable Source: Ethene is typically made from crude oil (petroleum), which is a finite resource.

  2. High Energy Costs: Requires very high temperatures ($\mathbf{300^{\circ}\text{C}}$) and pressures ($\mathbf{60 \text{ atm}}$), leading to high operating costs.

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Memory Aid: Think of the two methods as "Brewing vs. Factory".

Brewing (Fermentation): Slow, natural, renewable, but impure.
Factory (Hydration): Fast, industrial, non-renewable, but pure.

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5. Chemical Properties of Ethanol: Combustion (Core 11.6.2)

Like most organic compounds, ethanol burns easily in the presence of oxygen. This is why it is often used as a fuel.

The Combustion Reaction

Ethanol undergoes complete combustion (plenty of oxygen) to produce carbon dioxide and water, releasing a large amount of energy (it is an exothermic reaction).

$$\text{C}_2\text{H}_5\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(l)$$

Common Mistake to Avoid: When balancing the combustion of organic compounds, count the atoms in the following order: C, then H, then O.

6. Uses of Ethanol (Core 11.6.3)

Ethanol is extremely versatile, largely due to its molecular structure ($\text{C}_2\text{H}_5\text{OH}$), which contains both a non-polar ethyl part ($\text{C}_2\text{H}_5$) and a polar hydroxyl part ($\text{OH}$).

Use 1: As a Solvent (Core 11.6.3a)

Ethanol is an excellent solvent. It can dissolve substances that water can dissolve (because of the $-\text{OH}$ group) and many organic substances (that water cannot dissolve, because of the carbon chain).

• Real-World Example: It is used in perfumes, cosmetics, tinctures (like iodine solution), and cleaning agents, as it can dissolve oils, fats, and resins.

Use 2: As a Fuel (Core 11.6.3b)

Ethanol burns cleanly (as seen in the combustion equation above) and is used as a fuel, particularly when mixed with petrol (gasoline) to make gasohol or biofuel.

• Real-World Example: Ethanol fuels are often promoted because they are renewable (if made by fermentation) and burn relatively cleanly, reducing certain harmful emissions compared to pure fossil fuels.

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Summary of Alcohols: Alcohols contain the $-\text{OH}$ group. Ethanol can be made via sustainable fermentation (slow, low temp) or via high-volume catalytic addition (fast, high temp/pressure). Its main uses are as a solvent and a clean fuel. Keep practicing those reaction conditions—they are key for the exams!