Question

During oxidation of \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) with \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} / \mathrm{H}^{\oplus}, \Delta\) species which does not oxidises is: (A) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{OH}\) (B) \(\mathrm{Cr}(\mathrm{IV})\) (C) \(\operatorname{Cr}(\mathrm{V})\) (D) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH})_{2}\)

Short answer

(D) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH})_{2}\)

Step-by-step solution

Identify the species involved in the reaction

The reactants and products involved are: Reactants: C2H5OH, K2Cr2O7, and H^+ Products: Possible species CH3-CH2OH, Cr(IV), Cr(V), and CH3CH(OH)2

Write the reaction

The balanced reaction of C2H5OH (ethanol) with K2Cr2O7/H^+ is given by: \[ C_2H_5OH + K_2Cr_2O_7 + 4H^+ \longrightarrow CH_3COOH + 2Cr^{3+} + 2K^+ + 4H_2O \]

Analyze the change in oxidation state of the chromium species

In K2Cr2O7, the oxidation state of chromium is +6. When it reacts with C2H5OH, it is reduced to Cr(III) as shown in the balanced equation. Therefore, the chromium species that does not oxidize during the reaction is not Cr(IV). However, we still need to analyze the other species: CH3-CH2OH and CH3CH(OH)2.

Identify if CH3-CH2OH can be oxidized

CH3-CH2OH is ethanol, which is the reactant itself and has already been oxidized by the K2Cr2O7. As it is a reactant in the oxidation process, it cannot be the species that does not oxidize. Thus, the remaining species to analyze is CH3CH(OH)2.

Analyze CH3CH(OH)2 in the reaction context

CH3CH(OH)2 is ethane-1,2-diol, which is not part of the reaction process. There is no change in the oxidation states of the carbons in CH3CH(OH)2. Thus, it does not oxidize during the reaction. So, the species that does not oxidize during the oxidation of C2H5OH with K2Cr2O7/H^+ is CH3CH(OH)2. #Answer#: (D) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH})_{2}\)

Key concepts

Potassium Dichromate

Potassium dichromate, with the chemical formula \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \), is a well-known oxidizing agent in chemistry. This compound is composed of potassium ions \( (\mathrm{K}^{+}) \) and dichromate ions \( (\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}) \). The chromium ion in this dichromate compound commonly has an oxidation state of +6, making it quite potent in accepting electrons, or oxidizing other substances.
This capability of accepting electrons is frequently employed in chemical reactions to transform alcohols, such as ethanol, into carboxylic acids. Potassium dichromate is also favored due to its relatively stable nature compared to other oxidizing agents, providing consistency and predictability in reactions.
It's crucial to exercise caution while handling potassium dichromate, as it is a toxic substance. It requires proper safety protocols during experimentation and storage. This agent is a strong contributor to the redox reactions used in laboratories, combining safety measures with chemical accuracy.

Ethanol Oxidation

Ethanol oxidation refers to the chemical reaction in which ethanol, \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \), is converted into acetic acid, \( \mathrm{CH}_{3} \mathrm{COOH} \). This process involves the transfer of electrons from the ethanol molecule to the oxidizing agent, which in this discussion is potassium dichromate.
In the balanced reaction:

• Ethanol interacts with dichromate ions and protons, resulting in acetic acid.
• Dichromate ions are reduced to chromium ions with an oxidation state of +3.

This oxidation process is common in both laboratory and industrial settings, useful for producing vinegar—where acetic acid is a main component—from alcoholic solutions.
Understanding the mechanism of ethanol oxidation is crucial, as it showcases the fundamental concepts of how electrons transfer from one species to another. This process is both fascinating and practical as it applies widely in fields such as biochemistry and environmental science.

Oxidation States

Oxidation states, sometimes called oxidation numbers, indicate the degree of oxidation of an atom in a chemical compound. It represents the hypothetical charge an atom would have if all bonds to atoms of different elements were fully ionic. This is crucial for understanding redox reactions, where electrons are transferred between species.
For example, in potassium dichromate \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \), chromium is in the +6 oxidation state. During the reaction with ethanol, chromium ions are reduced to an oxidation state of +3. However, ethanol itself starts with a neutral overall charge, with its carbon atoms being at different oxidation states within the reaction.
Recognizing changes in oxidation states is vital for determining which species is oxidized and which is reduced during the reaction. In sum, keeping track of oxidation states allows chemists to ensure mass and electron balance in chemical equations.

Reaction Analysis

Reaction analysis is the process of breaking down a chemical reaction to understand each component and its role. This involves identifying reactants, intermediates, and products, as well as any changes they undergo.
Let's analyze the oxidation of ethanol by potassium dichromate.

• The major reactants: ethanol, dichromate ions, and protons.
• The main product: acetic acid, alongside reduced chromium ions and water.
• Intermediate species may form during reaction progression, although in this simple equation, they are often not isolated.

Through reaction analysis, we can determine that the unreacted compound \( \mathrm{CH}_3\mathrm{CH}(\mathrm{OH})_2 \), also known as ethane-1,2-diol, does not participate in the oxidation process as it does not undergo any change.
Comprehending each part of the reaction helps in predicting and manipulating conditions to achieve the desired chemical transformations efficiently. It's important for developing new methods in chemistry and optimizing existing ones.