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Hydrocarbons question

2020 · 9 Jan · Shift 1 · Q6
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Hydrocarbons question

2020 · 9 Jan · Shift 1 · Q6

JEE MainChemistryHydrocarbonsMCQ+4 / −1
The correct order of heat of combustion for following alkadienes is : JEE Main 2020 (Online) 9th January Morning Slot Chemistry - Hydrocarbons Question 102 English 1 JEE Main 2020 (Online) 9th January Morning Slot Chemistry - Hydrocarbons Question 102 English 2 JEE Main 2020 (Online) 9th January Morning Slot Chemistry - Hydrocarbons Question 102 English 3
  1. A
    (b) < (c) < (a)
  2. B
    (a) < (c) < (b)
  3. C
    (a) < (b) < (c)
  4. D
    (c) < (b) < (a)
View written solutionFree

Correct answer: A

  1. Principle used

For isomeric alkadienes, the heat of combustion is inversely related to stability:

More stable compound⇒lower heat of combustion\text{More stable compound} \Rightarrow \text{lower heat of combustion}More stable compound⇒lower heat of combustion

So, we first compare the stability of the given dienes.

  1. Stability order of dienes

For alkadienes, stability generally follows:

conjugated diene>isolated diene>cumulated diene\text{conjugated diene} > \text{isolated diene} > \text{cumulated diene}conjugated diene>isolated diene>cumulated diene

Reason:

  • Conjugated dienes are stabilized by resonance.
  • Isolated dienes do not get resonance stabilization, but are more stable than cumulated systems.
  • Cumulated dienes (allenes type) are least stable.
  1. Identify the compounds

From the given set, the usual classification is:

  • (a)(a)(a) = cumulated diene
  • (b)(b)(b) = conjugated diene
  • (c)(c)(c) = isolated diene

Therefore, their stability order is:

(b)>(c)>(a)(b) > (c) > (a)(b)>(c)>(a)
  1. Convert stability order to heat of combustion order

Since higher stability means lower heat of combustion:

Heat of combustion: (b)<(c)<(a)\text{Heat of combustion: } (b) < (c) < (a)Heat of combustion: (b)<(c)<(a)
  1. Match with options

This corresponds to:

A: (b)<(c)<(a)\boxed{\text{A: } (b) < (c) < (a)}A: (b)<(c)<(a)​
  1. Comparison with stored answer

Stored correct answer is C: (a)<(b)<(c)(a) < (b) < (c)(a)<(b)<(c).

But this is opposite to the standard stability trend of dienes. Hence, I do not agree with the stored answer.

The correct answer should be:

(b)<(c)<(a)\boxed{(b) < (c) < (a)}(b)<(c)<(a)​
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