Problem 2 - Basic Principles of Organic Chemistry

Question

Balance each of the following equations. You may need to add \(\mathrm{H}_{2} \mathrm{O}\) to one side or the other of the equations. a. \(\stackrel{\oplus}{\mathrm{K}}^{\ominus} \mathrm{M} \mathrm{O}_{4}+\mathrm{RCH}=\mathrm{CH}_{2} \rightarrow \mathrm{RCO}_{2} \stackrel{\oplus}{\mathrm{K}}+\mathrm{CH}_{2}=\mathrm{O}+\mathrm{MnO}_{2}\) b. \(\mathrm{CrO}_{3}+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{3} \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}+\mathrm{Cr}^{3 \oplus}\)

Step-by-Step Solution

Answer

Both reactions were balanced by adding water and adjusting for charges.

Step 1: Understanding the Reaction Components

In the first reaction, a compound containing potassium permanganate reacts with a hydrocarbon (represented as \( \mathrm{RCH}=\mathrm{CH}_2 \)) to form a carboxylate, carbonyl compound, and manganese dioxide. Note that \( \stackrel{\oplus}{\mathrm{K}}^{\ominus} \mathrm{M}\mathrm{O}_4 \) represents potassium permanganate. Similarly, the second reaction involves chromium trioxide reacting with a hydrocarbon to form a carboxylic acid and a chromium ion.

Step 2: Balancing Potassium Permanganate Reaction

The unbalanced reaction is: \( \stackrel{\oplus}{\mathrm{K}}^{\ominus} \mathrm{MnO}_4 + \mathrm{RCH}=\mathrm{CH}_2 \rightarrow \mathrm{RCO}_2\stackrel{\oplus}{\mathrm{K}} + \mathrm{CH}_{2}=\mathrm{O} + \mathrm{MnO}_2 \). Balance the equations by adding water to the reactants side to provide hydrogen and oxygen as needed.- Add 2 water molecules to the reactants side: \[ \stackrel{\oplus}{\mathrm{K}}^{\ominus} \mathrm{MnO}_4 + \mathrm{RCH}=\mathrm{CH}_2 + 2\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{RCO}_2\stackrel{\oplus}{\mathrm{K}} + \mathrm{CH}_{2}=\mathrm{O} + \mathrm{MnO}_2 + 2\mathrm{H}^+ \]- Now check elements: K, Mn, and the rest are balanced by the addition of water.

Step 3: Balancing the Chromium(VI) Oxide Reaction

For the second reaction, chromium trioxide (\( \mathrm{CrO}_3 \)) reacts as follows:- Write the unbalanced equation: \( \mathrm{CrO}_3 + \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{CH}_3 \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{CO}_2 \mathrm{H} + \mathrm{Cr}^{3+} \).- Add 3 water molecules to the reactants side to balance the oxygens: \[ \mathrm{CrO}_3 + \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{CH}_3 + 3\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{CO}_2 \mathrm{H} + 2 \mathrm{Cr}^{3+} + 6 \mathrm{H}^+ \]- It's now balanced in acidic conditions by equating electron exchanges between oxidation and reduction terms.

Key Concepts

Potassium Permanganate

Potassium permanganate, denoted as \( KMnO_4 \), is a powerful oxidizing agent frequently used in chemical reactions, particularly in redox processes. Its formula reflects a potassium ion \( K^+ \) combined with a permanganate ion \( MnO_4^- \). The permanganate ion is a deep purple color, making potassium permanganate easily identifiable in laboratory settings.
In reactions where potassium permanganate is involved, it acts as an electron acceptor. This means it gets reduced while causing other species to be oxidized. During redox reactions, potassium permanganate converts into manganese compounds of lower oxidation states, such as manganese dioxide \( MnO_2 \).
One common application involves its reaction with organic compounds. For example, in the provided equation, potassium permanganate reacts with a hydrocarbon to result in a series of products including manganese dioxide, a key by-product.

• Potassium permanganate provides oxygen to react with other chemicals.
• It undergoes a reduction process for balancing chemical equations.

Understanding the role of potassium permanganate is essential for mastering its use in chemical processes and its inclusion in reactions often necessitates balancing chemical equations to ensure mass and charge conservation.

Chromium Trioxide

Chromium trioxide \( CrO_3 \) is a significant oxidizing agent primarily used in organic oxidations. It's composed of chromium in its +6 oxidation state and three oxygen atoms. This compound appears as a dark red or maroon crystalline solid.
In chemical reactions, chromium trioxide is known for transforming alcohols and hydrocarbons into carboxylic acids or ketones, depending on the reaction conditions. It is recognized for altering the oxidation state of chromium from +6 to +3, a key transformation in redox chemistry.
In the mentioned exercise, chromium trioxide reacts with a hydrocarbon to produce a carboxylic acid \( C_6H_5CH_2CO_2H \) and a reduced chromium ion \( Cr^{3+} \). This transformation follows a typical reduction where chromium is reduced from its +6 oxidation state to +3. During this reaction:

• Chromium trioxide acts as an electron acceptor, facilitating the oxidation of the hydrocarbon.
• Balancing the reaction requires consideration of \( H_2O \) molecules and protons \( H^+ \) to achieve stoichiometric balance.

Mastering the utilization of chromium trioxide in equations is crucial for chemists engaged in synthesis involving oxidation-reduction mechanisms.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, are chemical processes involving the transfer of electrons between two substances. This fundamental concept underpins many chemical reactions, including those involving potassium permanganate and chromium trioxide.
In redox reactions, one substance loses electrons (oxidized) while another gains electrons (reduced). The substance that loses electrons is the reducing agent, and the one that gains electrons is the oxidizing agent. Understanding these electron exchanges is key to predicting reaction outcomes.

• Oxidizing agents, like \( KMnO_4 \) and \( CrO_3 \), facilitate electron transfer by accepting electrons and undergoing reduction.
• The balancing of redox reactions requires ensuring that the number of electrons lost and gained is equal. This often involves adding molecules like \( H_2O \) or \( H^+ \) ions to maintain equilibrium.

Mastering these concepts is essential for students to comprehend how different species interact and transform during chemical reactions. Proficiency in balancing equations and understanding electron flow in redox reactions are vital skills in chemistry.