Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

Potassium salt of 2,3 -dimethyl maleic acid on electrolysis gives (a) ethyene (b) propyne (c) 2 -butyne (d) 1 -butyne

Step-by-Step Solution

Answer

The product formed is (c) 2-butyne.

Step 1: Understanding the Problem

The question asks about the product formed during the electrolysis of the potassium salt of 2,3-dimethyl maleic acid. This is a type of organic reaction often referred to as Kolbe electrolysis or Kolbe's reaction, where the electrolysis of a salt leads to dimerization of carboxylic acid groups.

Step 2: Identifying the Process

In Kolbe electrolysis, the carboxylate ions are oxidized at the anode to form radicals. These radicals often dimerize. With the compound 2,3-dimethyl maleic acid, after the removal of the carboxyl groups ( ext{CO}_2 is released), the remaining part is subject to dimerization.

Step 3: Determining the Structure of Radicals

When the carboxyl groups are removed from 2,3-dimethyl maleic acid, we are left with methyl-ethyl radicals, as the structure of 2,3-dimethyl maleic acid is CH₃-CH(CO₂)-CH(CO₂)-CH₃. Removing ext{CO}_2 from the middle positions after electrolysis gives CH₃-C* and *CH-CH₃ radicals.

Step 4: Dimerization of Radicals

The methyl-ethyl radicals formed after ext{CO}_2 release will dimerize to form 2-butene. However, during the Kolbe reaction, further transformation or rearrangement occurs to stabilize the structure. This results in the formation of an alkyne structure, specifically an internal alkyne due to the symmetry of the radicals.

Step 5: Finalizing the Product

The internal alkyne formed by the dimerization and rearrangement of the radicals during electrolysis of the potassium salt of 2,3-dimethyl maleic acid is 2-butyne. This is because the symmetric nature of radicals allows for this specific structure to form.

Key Concepts

Organic Reactions

Organic reactions are chemical processes that involve organic compounds, primarily composed of carbon atoms, along with elements like hydrogen, oxygen, and nitrogen. These reactions often involve fundamental changes in the structure of these organic molecules. They are classified into several types, such as addition, elimination, substitution, and rearrangement reactions. Organic reactions are pivotal in creating a wide range of compounds including pharmaceuticals, plastics, and synthetic materials.

Understanding organic reactions opens the door to advancing chemical innovation and discovering new materials. Key factors to consider in organic reactions are the reagents used, reaction conditions, and the mechanisms involved. Kolbe electrolysis, which is the focus here, is an excellent example of an organic redox reaction, where the electrolyte plays a crucial role. It not only involves the breakdown and formation of bonds but also the transformation of one type of molecule into another, which has significant practical applications in synthesis.

Dimerization of Carboxylic Acids

Dimerization refers to the chemical process where two molecules combine to form a single dimer. In the context of carboxylic acids, this happens under specific conditions, such as in Kolbe electrolysis.

During the electrolysis of carboxylic acids or their salts, carboxylate ions are oxidized at the anode. This process removes carbon dioxide and forms radicals. These radicals subsequently pair up to form dimers. The 2,3-dimethyl maleic acid produces methyl-ethyl radicals upon removal of carboxyl groups. These radicals specifically undergo dimerization to form more stable compounds.

• The newly formed dimer stabilizes through bond formation.
• This process results in internal alkynes like 2-butyne, due to symmetrical arrangement.

Dimerization is crucial because it allows for the synthesis of larger organic structures from smaller ones, offering a simple pathway to create complex molecules from basic precursors.

Electrolysis in Chemistry

Electrolysis is a powerful chemical process that uses electric current to drive a non-spontaneous chemical reaction. In chemistry, it's used to break apart compounds into their individual elements or to bring about transformations that would not occur under normal conditions.

Key aspects of electrolysis include:

• Application of an electric current through a conducting solution (electrolyte).
• Breaking down chemical bonds in the process called redox reactions.
• Production of useful chemical changes, such as in Kolbe electrolysis.

During the electrolysis of carboxylic salts, the anode reactions result in the liberation of carbon dioxide and formation of reactive radicals. These radicals are crucial for further reactions such as dimerization.

Electrolysis serves multiple applications from industrial to laboratory settings, including refining metals and producing important chemical compounds, showcasing its immense utility in both experimentation and practical applications.

Reaction Mechanisms

Understanding a reaction mechanism is about knowing how and why a reaction occurs at the molecular level. It involves identifying the changes that take place as reactants transform into products, covering each step of the reaction sequence.

Mechanisms provide insights into:

• Step-by-step changes that take place in the reaction.
• The nature and behavior of intermediates, such as radicals in Kolbe electrolysis.
• Factors affecting reaction rates and outcomes.

For the electrolysis of 2,3-dimethyl maleic acid, the mechanism starts with oxidation at the anode, leading to the formation of methyl-ethyl radicals. These radicals guide the progression of the reaction as they pair up during dimerization.

Exploring reaction mechanisms is crucial for chemists to alter and optimize conditions for desired chemical outcomes, enabling them to develop better synthetic methods and to innovate new chemical processes.