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NEET CHEMISTRYMedium

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).
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