Production (HSC SSCE Chemistry): Revision Notes

Production

Alcohols can be made in several different ways, depending on their intended use. For alcohols in beverages and biofuels, fermentation of carbohydrates is the primary method. Industrial alcohol production typically uses either hydration of alkenes or substitution of haloalkanes. Increasingly, biofuel plants are also producing industrial alcohol, which can be further purified through distillation to achieve 99.9% purity.

Production from alkenes

When water is added to an alkene with sulfuric acid as a catalyst, the double bond breaks and a hydroxyl group ($\text{-OH}$) attaches to the molecule. This process is called hydration.

Production from halogenated organic compounds

When water is added to a haloalkane (an organic compound containing a halogen atom), a substitution reaction occurs. The halogen atom is replaced by a hydroxyl group to form an alcohol.

Why haloalkanes react more easily

Haloalkanes undergo substitution reactions much more readily than regular alkanes. This is because the carbon-halogen bond is easier to break than carbon-carbon or carbon-hydrogen bonds.

The ease of breaking a bond depends on its bond energy, which is the energy required to break a chemical bond. For a substitution reaction to occur, the $\text{C-X}$ bond (where $\text{X}$ is a halogen) must break so the hydroxyl group can substitute in.

Bond energies for important bonds in haloalkanes:

Bond	Bond energy ($\text{kJ} \cdot \text{mol}^{-1}$)
$\text{C-H}$	$412$
$\text{C-C}$	$348$
$\text{C-Cl}$	$338$
$\text{C-Br}$	$276$
$\text{C-I}$	$238$

Classification of haloalkanes

Like alcohols, haloalkanes can be classified as primary, secondary, or tertiary based on the number of carbon atoms bonded to the carbon carrying the halogen:

• A primary haloalkane has one carbon bonded to the carbon atom carrying the halogen
• A secondary haloalkane has two carbons bonded to the carbon atom carrying the halogen
• A tertiary haloalkane has three carbons bonded to the carbon atom carrying the halogen

Reactivity of different haloalkanes

In substitution reactions with water, tertiary haloalkanes are the most reactive, followed by secondary haloalkanes. Primary haloalkanes do react, but only very slowly as they are the least reactive.

Production from fermentation

Fermentation is the process of converting simple sugars like glucose into ethanol in an anaerobic (oxygen-free) environment. This is the traditional method for producing alcohol for beverages and is increasingly important for biofuel production.

The overall equation for fermentation is:

$\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} \rightarrow 2\text{C}_2\text{H}_5\text{OH(aq)} + 2\text{CO}_2\text{(g)}$

The role of yeast

Yeast carries out fermentation when oxygen is absent. Glucose molecules pass through the yeast cell wall, where the organism uses them to produce energy through its normal metabolic pathways. Under oxygen-free conditions, yeast produces ethanol as a by-product of these processes, which it then excretes along with carbon dioxide.

Types of sugars

Yeast requires simple sugars to produce ethanol effectively. Understanding the different types of sugars is important for successful fermentation.

Monosaccharides

Monosaccharides are simple sugars with a single ring structure containing four, five, or six carbon atoms. They also contain multiple hydroxyl groups ($\text{-OH}$). Common examples include:

• Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$)
• Fructose ($\text{C}_6\text{H}_{12}\text{O}_6$)

Glucose and fructose are isomers of each other (they have the same molecular formula but different structures). Many fruits, including grapes, contain these simple monosaccharides.

Disaccharides

A disaccharide consists of two carbon rings joined together. Two monosaccharides join in a condensation reaction to produce disaccharides. The most common example is:

• Sucrose (table sugar): $\text{C}_{12}\text{H}_{22}\text{O}_{11}$

Polysaccharides

Many grains and vegetables used to produce ethanol contain polysaccharides (also called carbohydrates). These are large molecules formed when multiple monosaccharides and disaccharides join together. Important examples include:

• Cellulose: found in plant cell walls
• Starch: found in grains and vegetables

Both have the general formula $(\text{C}_6\text{H}_{10}\text{O}_5)_n$, where $n$ indicates multiple units are present in the polymer chain.

Breaking down complex sugars

Yeast is most efficient at breaking down simple monosaccharide sugars. Most types of yeast cannot break down polysaccharides at all, and can only break down sucrose in a limited way. Therefore, more complex sugars must be broken down before fermentation can occur.

Enzymes present in the fermentation mixture break down complex sugars into glucose through hydrolysis reactions:

Hydrolysis of polysaccharides to monosaccharides: $(\text{C}_6\text{H}_{10}\text{O}_5)_n\text{(aq)} + n\text{H}_2\text{O(l)} \rightarrow n\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)}$

Hydrolysis of disaccharides to monosaccharides: $\text{C}_{12}\text{H}_{22}\text{O}_{11}\text{(aq)} + \text{H}_2\text{O(l)} \rightarrow 2\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)}$

Once glucose has formed, it can be converted to ethanol by the yeast in the fermentation mixture.

Conditions for fermentation

Very specific conditions are required for successful fermentation. Without these conditions, it is nearly impossible to produce drinkable wine or spirits, or efficient biofuel production.

Temperature control

The temperature of the fermentation mixture must be kept within a narrow range:

• Red wine production: maximum $29°\text{C}$
• White wine production: maximum $18°\text{C}$

pH control

Enzymes and yeast are also very sensitive to pH. The ideal range for fermentation is pH 6.1-6.8 (slightly acidic). Enzymes will denature outside this range and fermentation will stop.

Anaerobic conditions

Fermentation must occur under anaerobic conditions, meaning no oxygen must be present. If oxygen is present, it will oxidise the ethanol to produce ethanal and ethanoic acid, giving an unpleasant vinegar taste.

A simple airlock is used to keep oxygen out of the fermentation mixture. The liquid in the airlock (usually water) allows the carbon dioxide produced during fermentation to escape, preventing pressure buildup inside the reaction vessel. At the same time, the water prevents oxygen from entering the mixture.

Dilution requirements

The mixture must be kept dilute, usually by periodically adding water. The ethanol produced by yeast will poison the yeast cells if the concentration becomes too high.

Keeping the mixture dilute is not a problem for the final product, as the ethanol can be distilled off and concentrated at the end of the process.

Investigation 11.3: Measuring the rate of fermentation

Aim

To create a fermentation mixture and measure the rate of reaction.

Materials

• $5\text{ mL}$ of $0.1\text{ mol} \cdot \text{L}^{-1}$ sodium dichromate solution ($\text{Na}_2\text{Cr}_2\text{O}_7$)
• $5\text{ mL}$ of $6\text{ mol} \cdot \text{L}^{-1}$ sulfuric acid ($\text{H}_2\text{SO}_4$)
• $250\text{ mL}$ conical flask
• $5\text{ g}$ yeast (dried packet yeast for bread-making)
• $10\text{ g}$ glucose (sucrose or table sugar will also work)
• $40\text{ mL}$ warm water ($30-40°\text{C}$)
• Spatula
• Electronic balance
• Controlled environment (incubator) set at approximately $25-30°\text{C}$
• Plastic hose
• One-hole rubber stopper
• Limewater (calcium hydroxide)
• Test tube
• Thermometer
• $5\text{ mL}$ measuring cylinder
• Disposable pipettes

Risk assessment

Method

1. Place $5\text{ g}$ of yeast in the conical flask
2. Add $10\text{ g}$ glucose and $40\text{ mL}$ warm water to the flask. Swirl to mix contents
3. Stopper the conical flask and attach a simple airlock created by making a circle of a length of plastic tube and filling with water

1. Measure the mass of the set-up and record
2. Measure the mass of the fermentation set-up over the next $2-3$ days, recording against the number of hours since set-up. If possible, store the mixture in a temperature-controlled environment at approximately $25°\text{C}$. Generally a classroom window will be sufficient to see results
3. During one mass measurement, unwind the plastic tubing and bubble the gas produced through a solution of limewater, recording any observations
4. After completion of mass measurements, extract approximately $5\text{ mL}$ of the fermentation mixture and add acidified dichromate solution. Record any observations

Results

Record the mass of the fermentation mixture against the number of hours of fermentation in a suitable table. Record observations of limewater and acidified dichromate ion tests.

Discussion

1. Explain, using an equation, why there was a decrease in mass over time. Explain how this can be used to measure rate of fermentation
2. Construct a graph of mass lost against hours of fermentation. Describe and explain any trends seen
3. Explain the purpose of the limewater test. Use a balanced equation in your answer
4. Explain the purpose of the acidified dichromate ion test. Use a balanced equation in your answer

Conclusion

Write a conclusion summarising any trends found in this investigation. You should observe that mass decreases over time as fermentation progresses, with the rate of mass loss potentially slowing as fermentation approaches completion or as conditions become less favourable for the yeast.