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NEET CHEMISTRYHydrocarbonsMedium

Question

Given compounds are as follows: (I) (II) (III). The enthalpy of hydrogenation of these compounds will be in the order as-

A

I > II > III

B

III > II > I

C

II > III > I

D

II > I > III

Step-by-Step Solution

  1. Definition: Enthalpy of hydrogenation is the heat released when one mole of an unsaturated compound is hydrogenated. It is a measure of the stability of the alkene; the more stable the alkene, the lower (less negative/less exothermic) its enthalpy of hydrogenation.
  2. Inverse Relationship: Stability of Alkene ∝1Enthalpy of Hydrogenation\propto \frac{1}{\text{Enthalpy of Hydrogenation}}.
  3. Missing Structures: Although the structures for (I), (II), and (III) are not explicitly provided in the text, this is a standard NEET 2015 question. Usually, it compares the stability of dienes or alkenes based on resonance and hyperconjugation.
  • General Rule: Conjugated dienes are more stable than isolated dienes, which are more stable than cumulated dienes (allenes) due to delocalisation energy.
  • If (I) is most stable: It has the lowest enthalpy of hydrogenation.
  • If (III) is least stable: It has the highest enthalpy of hydrogenation.
  1. Standard Context: In the context of this specific PYQ (NEET 2015), the compounds are often:
  • (I) A conjugated diene (most stable).
  • (II) An isolated diene (less stable).
  • (III) A cumulated diene or less substituted alkene (least stable).
  • Therefore, the heat of hydrogenation order is Inverse of stability: (III) > (II) > (I).

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Hydrocarbons. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYHydrocarbonscompoundsfollowsenthalpyhydrogenationcompounds

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