JEE AdvancedChemistryHydrocarbonsMultiple correct+4 / −1
Which of the following reactions produce(s) propane as a major product?
- A

- B

- C

- D

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Correct answer: A, C
Step-by-step analysis of each option:
Option A:
- Identify the reaction type: This is the reduction of an alkyl halide (1-chloropropane) using a metal and acid ( in dilute ).
- Determine the reagents' function: The reaction of zinc with dilute HCl produces nascent hydrogen (), which is a powerful reducing agent. The reaction is: .
- Write the chemical equation for the reduction: The alkyl halide is reduced to an alkane. The chlorine atom is replaced by a hydrogen atom.
- Identify the product: The product formed is propane ().
- Conclusion: Since propane is the only organic product formed, it is the major product. Therefore, option A is correct.
Option B:
- Identify the reaction type: This is the soda-lime decarboxylation of a sodium salt of a carboxylic acid (sodium propanoate).
- Determine the reagents' function: Soda-lime () is a reagent used to remove the carboxylate group () from the molecule.
- Write the chemical equation: The reaction results in an alkane with one fewer carbon atom than the parent carboxylate salt. The group is converted to .
- Identify the product: The starting material is sodium propanoate (3 carbons). The product is ethane (, 2 carbons).
- Conclusion: The product is ethane, not propane. Therefore, option B is incorrect.
Option C:
- Identify the reaction type: This is a mixed Wurtz reaction involving two different alkyl halides (ethyl chloride and methyl chloride) with sodium metal in dry ether.
- Predict the products: In a Wurtz reaction, alkyl radicals are formed which then couple. In a mixed reaction, three types of coupling can occur:
- Self-coupling of ethyl chloride: (n-butane)
- Self-coupling of methyl chloride: (ethane)
- Cross-coupling of ethyl chloride and methyl chloride: (propane)
- Analyze the product distribution: The reaction produces a mixture of ethane, propane, and n-butane. To determine the major product, we consider the statistical probability of the radical couplings. If we assume equimolar amounts of reactants and equal reactivity, the statistical ratio of products (butane : propane : ethane) would be approximately 1:2:1. This is because there are two possibilities for cross-coupling () for every one possibility of self-coupling ( or ).
- Conclusion: Propane is formed in the highest yield among the alkane products. Thus, it can be considered a major product of this reaction mixture. Therefore, option C is correct.
Option D:
- Identify the reaction type: This is the Kolbe's electrolysis of an aqueous solution of a sodium salt of a carboxylic acid (sodium propanoate).
- Predict the products: In Kolbe's electrolysis, the carboxylate ion is oxidized at the anode, loses , and forms an alkyl radical. These radicals then dimerize.
- At the anode:
- Dimerization: (n-butane)
- Identify the major product: The main organic product from the coupling of ethyl radicals is n-butane. Side products from disproportionation (ethane and ethene) can also form but are generally minor. Propane is not a product of this reaction.
- Conclusion: The major product is n-butane, not propane. Therefore, option D is incorrect.
Final Summary:
- Reaction A produces propane as the sole organic product.
- Reaction B produces ethane.
- Reaction C produces a mixture of alkanes where propane is the most abundant product.
- Reaction D produces n-butane as the major product.
Thus, the reactions that produce propane as a major product are A and C.
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