Question

How is ethyl alcohol prepared from starch? Give equation.

Short answer

Question: Explain the process of preparing ethyl alcohol (ethanol) from starch. Answer: The preparation of ethanol from starch involves four main steps: 1) conversion of starch to glucose through hydrolysis, 2) conversion of glucose to pyruvate via glycolysis, 3) conversion of pyruvate to acetaldehyde through decarboxylation, and 4) reduction of acetaldehyde to ethanol. This process starts with the hydrolysis of starch in the presence of water and an acid or enzyme, followed by glycolysis to produce pyruvate. Pyruvate is then converted to acetaldehyde through decarboxylation under anaerobic conditions, and finally, acetaldehyde is reduced to ethanol using NADH and hydrogen ions.

Step-by-step solution

Conversion of starch to glucose

Starch is a polysaccharide, which means it is a long chain of glucose molecules bonded together. To produce ethanol, we first need to convert this starch into individual glucose units. This can be done by hydrolysis, in which starch is treated with water (H2O) in the presence of an acid or enzyme (such as amylase). The reaction can be shown as: (C6H10O5)n + nH2O → nC6H12O6

Conversion of glucose to pyruvate

After obtaining glucose from starch, the next step is to convert it to pyruvate through the process of glycolysis. Glycolysis is a series of chemical reactions that occurs in cells and produces pyruvate and energy. We will show one final equation for the whole glycolysis process: C6H12O6 → 2C3H4O3 + 2NADH + 2ATP Here, NADH and ATP are used to represent energy in cells.

Conversion of pyruvate to acetaldehyde

Next, the produced pyruvate must be converted to acetaldehyde. This process takes place in the absence of oxygen (anaerobic conditions) and is called decarboxylation. A decarboxylation reaction results in the loss of carbon dioxide (CO2) and the formation of acetaldehyde. C3H4O3 → C2H4O + CO2

Conversion of acetaldehyde to ethanol

Finally, acetaldehyde is reduced to ethanol in the presence of NADH, a reducing agent, and hydrogen ions. This reaction is known as the reduction of acetaldehyde. C2H4O + NADH + H+ → C2H5OH + NAD+

Summary of the overall reaction

In summary, the overall process of preparing ethyl alcohol (ethanol) from starch can be represented as follows: Starch + H2O (in the presence of acid/enzyme) → Glucose → Pyruvate (via glycolysis) → Acetaldehyde (decarboxylation) → Ethanol (reduction)

Key concepts

Hydrolysis of Starch

Understanding the initial phase in ethyl alcohol preparation starts with hydrolysis of starch. Starch, primarily found in plants like potatoes and corn, is a complex carbohydrate formed by the bonding of numerous glucose units. The hydrolysis process is akin to unlocking these units from their chain. Under the influence of water, acid, or enzymes like amylase, these bonds break effectively transforming starch into free-standing glucose molecules. This reaction can be expressed with the equation:
\[ (C_6H_{10}O_5)_n + nH_2O \rightarrow nC_6H_{12}O_6 \]
Through hydrolysis, we capitalize on the ample energy stored in starch for subsequent conversion into alcohol, highlighting the significance of this first step in the overall biochemical transformation.

Glycolysis

Once we have glucose from the hydrolysis of starch, it undergoes glycolysis, a crucial energy-releasing pathway used by all forms of life. During glycolysis, glucose is methodically disassembled through a series of ten reactions. These reactions take place in the cytoplasm of cells and do not require oxygen, which qualifies glycolysis as an anaerobic process. The result of this series of chemical reactions is the formation of pyruvate.
The simplified equation representing glycolysis is: \[ C_6H_{12}O_6 \rightarrow 2C_3H_4O_3 + 2NADH + 2ATP \]
Energy is conserved in the form of NADH and ATP molecules, vital for the energy economy of the cell. By converting one molecule of glucose into two molecules of pyruvate, glycolysis sets the stage for the subsequent steps required to produce ethanol.

Decarboxylation

The journey from glucose to ethyl alcohol continues with the decarboxylation of pyruvate. This is where a carboxyl group, consisting of one carbon atom and two oxygen atoms, is elegantly removed from the molecule of pyruvate, releasing carbon dioxide in the process.
The relevant chemical equation is: \[ C_3H_4O_3 \rightarrow C_2H_4O + CO_2 \]
This step is significant in the transition from pyruvate to acetaldehyde, a simpler compound that is closer in structure to ethanol. Decarboxylation is primarily an anaerobic process which means it occurs in the absence of oxygen, typically in microbes during fermentation. It is an essential bridge between the sugar breakdown in glycolysis and the final reduction that will yield alcohol.

Reduction of Acetaldehyde

The final transformation in the synthesis of ethyl alcohol entails the reduction of acetaldehyde. In this stage, acetaldehyde acts as an acceptor of electrons and is reduced to ethanol, thanks to the presence of NADH, which functions as a reducing agent along with hydrogen ions.
The equation reflecting this process is: \[ C_2H_4O + NADH + H^+ \rightarrow C_2H_5OH + NAD^+ \]
This reduction reaction is the heart of the alcohol production phase, turning the somewhat reactive acetaldehyde into the more stable, energy-rich molecule of ethanol (ethyl alcohol). This entire process, from the initial hydrolysis of starch to the reduction of acetaldehyde, is fundamental in industries like brewing and biofuel production, showcasing the versatile application of biochemical pathways.