Problem 74 - Chemistry and Chemical Reactivity

Question

A Write balanced equations for the following reduction half-reactions involving organic compounds. (a) \(\mathrm{HCO}_{2} \mathrm{H} \longrightarrow \mathrm{CH}_{2} \mathrm{O} \quad\) (acid solution) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3} \quad\) (acid solution) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \quad\) (acid solution) (d) \(\mathrm{CH}_{3} \mathrm{OH} \longrightarrow \mathrm{CH}_{4} \quad\) (acid solution)

Step-by-Step Solution

Answer

(a) \(\mathrm{HCO}_{2}\mathrm{H} + 2\mathrm{e}^- + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_{2}\mathrm{O} + \mathrm{H}_2\mathrm{O}\); (b) \(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H} + 2\mathrm{e}^- + 3\mathrm{H}^+ \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{CH}_3 + \mathrm{H}_2\mathrm{O}\); (c) \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO} + 2\mathrm{e}^- + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\); (d) \(\mathrm{CH}_3\mathrm{OH} + 2\mathrm{e}^- + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_4 + \mathrm{H}_2\mathrm{O}\)."}

Step 1: Identify Reduction Half-Reaction (a)

The reduction half-reaction involves formic acid, \(\mathrm{HCO}_{2}\mathrm{H}\), reducing to formaldehyde, \(\mathrm{CH}_{2}\mathrm{O}\). We need to balance the equation in an acidic solution.

Step 2: Balance Atoms other than O and H for Reaction (a)

Formic acid and formaldehyde both contain one carbon atom, so the carbon is balanced. No changes needed in this step for carbon atoms.

Step 3: Balance Oxygen Atoms for Reaction (a)

There are two oxygen atoms in \(\mathrm{HCO}_{2}\mathrm{H}\) and one oxygen atom in \(\mathrm{CH}_{2}\mathrm{O}\). Add one water molecule:\[\mathrm{HCO}_{2}\mathrm{H} \rightarrow \mathrm{CH}_{2}\mathrm{O} + \mathrm{H}_2\mathrm{O}\]

Step 4: Balance Hydrogen Atoms for Reaction (a)

Add \(\mathrm{H}^+\) to the left side. Include 2 hydrogens for the right side and one \(\mathrm{H}_2\mathrm{O}\) on the left to balance all hydrogens.\[\mathrm{HCO}_{2}\mathrm{H} + \mathrm{H}^+ \rightarrow \mathrm{CH}_{2}\mathrm{O} \]

Step 5: Balance Charge for Reaction (a)

Add two electrons \(\mathrm{e}^-\) to the left to balance the charge. The balanced equation for the reduction half-reaction in acidic solution is:\[\mathrm{HCO}_{2}\mathrm{H} + 2\mathrm{e}^- + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_{2}\mathrm{O} + \mathrm{H}_2\mathrm{O}\]

Step 6: Identify Reduction Half-Reaction (b)

The reduction half-reaction involves benzoic acid, \(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H}\), converting to toluene, \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_3\).

Step 7: Balance Atoms for Reaction (b)

The carbon and hydrogen atoms are balanced because both sides involve the same benzene ring framework. Oxygen imbalance is handled next.

Step 8: Balance Oxygen Atoms for Reaction (b)

Add a water molecule on the right side to match the left side's 2 oxygen atoms:\[\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H} \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{CH}_3 + \mathrm{H}_2\mathrm{O}\]

Step 9: Balance Hydrogen Atoms for Reaction (b)

Add \(\mathrm{H}^+\) to the left to balance hydrogens. This adjustment balances hydrogens added through \(\mathrm{H}_2\mathrm{O}\).\[\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H} + 3\mathrm{H}^+ \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{CH}_3 + \mathrm{H}_2\mathrm{O}\]

Step 10: Balance Charge for Reaction (b)

Introduce two electrons to the left to reach a balanced net charge equation:\[\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H} + 2\mathrm{e}^- + 3\mathrm{H}^+ \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{CH}_3 + \mathrm{H}_2\mathrm{O}\]

Step 11: Identify Reduction Half-Reaction (c)

The reduction half-reaction involves acetaldehyde, \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO}\), reducing to propanol, \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\).

Step 12: Balance Atoms for Reaction (c)

For the one oxygen atom, add one water molecule to the right side. Continue with hydrogen and charge adjustments.\[\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO} \rightarrow \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH} + \mathrm{H}_2\mathrm{O}\]

Step 13: Balance Hydrogen Atoms for Reaction (c)

Add \(\mathrm{H}^+\) to the left side, adjusting for hydrogens added via water.\[\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO} + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\]

Step 14: Balance Charge for Reaction (c)

Include two electrons on the left to achieve charge neutrality. Final balanced equation:\[\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO} + 2\mathrm{e}^- + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\]

Step 15: Identify Reduction Half-Reaction (d)

The reduction half-reaction involves methanol, \(\mathrm{CH}_3\mathrm{OH}\), converting to methane, \(\mathrm{CH}_4\).

Step 16: Balance Atoms for Reaction (d)

Add a water molecule to accommodate all hydrogens and one for oxygen.\[\mathrm{CH}_3\mathrm{OH} \rightarrow \mathrm{CH}_4} + \mathrm{H}_2\mathrm{O}\]

Step 17: Balance Hydrogen Atoms for Reaction (d)

Add 2 \(\mathrm{H}^+\) to the left side to ensure hydrogen balancing. \[\mathrm{CH}_3\mathrm{OH} + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_4 + \mathrm{H}_2\mathrm{O}\]

Step 18: Balance Charge for Reaction (d)

Include two electrons to the left in order to balance charge. Final balanced equation:\[\mathrm{CH}_3\mathrm{OH} + 2\mathrm{e}^- + 2\mathrm{H}^+ \rightarrow \mathrm{CH}_4 + \mathrm{H}_2\mathrm{O}\]

Key Concepts

Organic Chemistry

Organic chemistry is a fascinating branch of chemistry that deals with the structure, properties, composition, reactions, and synthesis of organic compounds. These compounds primarily consist of carbon and hydrogen, and may include other elements such as oxygen, nitrogen, sulfur, and halogens. Organic compounds are classified based on their functional groups like alcohols, ketones, acids, and many more.
Understanding organic chemistry is crucial in reduction reactions, particularly when identifying the changes in structural formulas before and after reactions. In exercises involving reduction reactions of organic compounds, the goal is to simplify the molecular structure, often by adding electrons or hydrogen atoms to the molecule. This field of chemistry is not just about balancing the equations but also gaining insights into how molecules interact and change.

Half-Reaction Balancing

In redox chemistry, a half-reaction is a part of a redox reaction that involves either oxidation or reduction, but not both. When balancing these reactions, it is important to focus on balancing each half-reaction separately for atoms and charges before combining them into a balanced full redox reaction.
The steps generally include:

• Balancing all atoms other than oxygen and hydrogen first.
• Balancing oxygen by adding water molecules.
• Balancing hydrogen atoms by adding hydrogen ions (H⁺).
• Balancing charges by adding electrons.

These steps ensure both mass and charge are balanced. Half-reaction balancing is essential for accurately representing redox reactions, especially in the context of organic compounds undergoing reduction.

Acidic Solutions

Acidic solutions play a significant role in redox reactions, particularly in half-reactions involving organic compounds. An acidic solution provides a medium where hydrogen ions (H⁺) are available to facilitate the reduction process.
In the context of balancing equations in acidic solutions, the presence of free H⁺ ions is used to balance the hydrogen atoms added during reaction steps. This ability makes acidic media quite favorable for many reduction reactions in organic and inorganic chemistry. When balancing half-reactions, always consider hydrogen ions from the acidic environment as part of your strategy to ensure both charge and mass are balanced properly.

Redox Chemistry

Redox chemistry involves reactions where the oxidation states of atoms are changed through the transfer of electrons. The two components of redox reactions are:

• Reduction: the gain of electrons.
• Oxidation: the loss of electrons.

In the context of the given exercise, we focus on reduction half-reactions where organic compounds gain electrons, often simplifying their structure. Redox chemistry provides the basis for understanding electron flow and energy changes during chemical reactions, which is fundamental in both biological systems and industrial processes.
The concepts learned from redox chemistry are applicable in various fields such as energy storage, corrosion prevention, and biosystems functioning, making it an invaluable area of study for students and chemists alike. By grasping these core ideas, students can better balance and predict outcomes in complex organic redox processes.