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

Question

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

Accepted Answer

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 $\propto \frac{1}{	ext{Enthalpy of Hydrogenation}}$.
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.
4. **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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