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Hydrocarbons question

2019 · Shift 2 · Q8
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Hydrocarbons question

2019 · Shift 2 · Q8

JEE AdvancedChemistryHydrocarbonsMultiple correct+4 / −1
Which of the following reactions produce(s) propane as a major product?
  1. A
    JEE Advanced 2019 Paper 2 Offline Chemistry - Hydrocarbons Question 26 English Option 1
  2. B
    JEE Advanced 2019 Paper 2 Offline Chemistry - Hydrocarbons Question 26 English Option 2
  3. C
    JEE Advanced 2019 Paper 2 Offline Chemistry - Hydrocarbons Question 26 English Option 3
  4. D
    JEE Advanced 2019 Paper 2 Offline Chemistry - Hydrocarbons Question 26 English Option 4
View written solutionFree

Correct answer: A, C

Step-by-step analysis of each option:

Option A: CH3CH2CH2Cl→Zn,dil.HClCH_3CH_2CH_2Cl \xrightarrow{Zn, dil. HCl}CH3​CH2​CH2​ClZn,dil.HCl​

  1. Identify the reaction type: This is the reduction of an alkyl halide (1-chloropropane) using a metal and acid (ZnZnZn in dilute HClHClHCl).
  2. Determine the reagents' function: The reaction of zinc with dilute HCl produces nascent hydrogen ([H][H][H]), which is a powerful reducing agent. The reaction is: Zn+2HCl→ZnCl2+2[H]Zn + 2HCl \rightarrow ZnCl_2 + 2[H]Zn+2HCl→ZnCl2​+2[H].
  3. Write the chemical equation for the reduction: The alkyl halide is reduced to an alkane. The chlorine atom is replaced by a hydrogen atom. CH3CH2CH2Cl+2[H]→Zn,dil.HClCH3CH2CH3+HClCH_3CH_2CH_2Cl + 2[H] \xrightarrow{Zn, dil. HCl} CH_3CH_2CH_3 + HClCH3​CH2​CH2​Cl+2[H]Zn,dil.HCl​CH3​CH2​CH3​+HCl
  4. Identify the product: The product formed is propane (CH3CH2CH3CH_3CH_2CH_3CH3​CH2​CH3​).
  5. Conclusion: Since propane is the only organic product formed, it is the major product. Therefore, option A is correct.

Option B: CH3CH2COONa→NaOH,CaO,ΔCH_3CH_2COONa \xrightarrow{NaOH, CaO, \Delta}CH3​CH2​COONaNaOH,CaO,Δ​

  1. Identify the reaction type: This is the soda-lime decarboxylation of a sodium salt of a carboxylic acid (sodium propanoate).
  2. Determine the reagents' function: Soda-lime (NaOH+CaONaOH + CaONaOH+CaO) is a reagent used to remove the carboxylate group (–COONa–COONa–COONa) from the molecule.
  3. Write the chemical equation: The reaction results in an alkane with one fewer carbon atom than the parent carboxylate salt. The R−COONaR-COONaR−COONa group is converted to R−HR-HR−H. CH3CH2COONa+NaOH→CaO,ΔCH3CH3+Na2CO3CH_3CH_2COONa + NaOH \xrightarrow{CaO, \Delta} CH_3CH_3 + Na_2CO_3CH3​CH2​COONa+NaOHCaO,Δ​CH3​CH3​+Na2​CO3​
  4. Identify the product: The starting material is sodium propanoate (3 carbons). The product is ethane (CH3CH3CH_3CH_3CH3​CH3​, 2 carbons).
  5. Conclusion: The product is ethane, not propane. Therefore, option B is incorrect.

Option C: CH3CH2Cl+CH3Cl+Na→dry etherCH_3CH_2Cl + CH_3Cl + Na \xrightarrow{dry \ ether}CH3​CH2​Cl+CH3​Cl+Nadry ether​

  1. 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.
  2. 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: 2CH3CH2Cl+2Na→CH3CH2CH2CH32 CH_3CH_2Cl + 2Na \rightarrow CH_3CH_2CH_2CH_32CH3​CH2​Cl+2Na→CH3​CH2​CH2​CH3​ (n-butane)
    • Self-coupling of methyl chloride: 2CH3Cl+2Na→CH3CH32 CH_3Cl + 2Na \rightarrow CH_3CH_32CH3​Cl+2Na→CH3​CH3​ (ethane)
    • Cross-coupling of ethyl chloride and methyl chloride: CH3CH2Cl+CH3Cl+2Na→CH3CH2CH3CH_3CH_2Cl + CH_3Cl + 2Na \rightarrow CH_3CH_2CH_3CH3​CH2​Cl+CH3​Cl+2Na→CH3​CH2​CH3​ (propane)
  3. 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 (CH3∙+∙CH2CH3CH_3\bullet + \bullet CH_2CH_3CH3​∙+∙CH2​CH3​) for every one possibility of self-coupling (CH3∙+∙CH3CH_3\bullet + \bullet CH_3CH3​∙+∙CH3​ or CH3CH2∙+∙CH2CH3CH_3CH_2\bullet + \bullet CH_2CH_3CH3​CH2​∙+∙CH2​CH3​).
  4. 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: CH3CH2COONa(aq)→electrolysisCH_3CH_2COONa(aq) \xrightarrow{electrolysis}CH3​CH2​COONa(aq)electrolysis​

  1. Identify the reaction type: This is the Kolbe's electrolysis of an aqueous solution of a sodium salt of a carboxylic acid (sodium propanoate).
  2. Predict the products: In Kolbe's electrolysis, the carboxylate ion is oxidized at the anode, loses CO2CO_2CO2​, and forms an alkyl radical. These radicals then dimerize.
    • At the anode: 2CH3CH2COO−→2CH3CH2∙+2CO2+2e−2 CH_3CH_2COO^- \rightarrow 2 CH_3CH_2\bullet + 2CO_2 + 2e^-2CH3​CH2​COO−→2CH3​CH2​∙+2CO2​+2e−
    • Dimerization: 2CH3CH2∙→CH3CH2CH2CH32 CH_3CH_2\bullet \rightarrow CH_3CH_2CH_2CH_32CH3​CH2​∙→CH3​CH2​CH2​CH3​ (n-butane)
  3. 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.
  4. 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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