Arrange the following alkene in increasing order of their enthalpy of hydrogenation $(- Δ H)$ :

I I I < I V < I I < I

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I I I < I V < I < I I

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I < I I < I I I < I V

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I < I I < I V < I I I

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Solution

The correct option is B $I I I < I V < I I < I$
Enthalpy of hydrogenation is defined as the change in enthalpy, which occurs when one mole of an unsaturated compound reacts with an excess of hydrogen to become fully saturated at atmospheric pressure and room temperature the reactants and products being in their natural states under these conditions. This is the enthalpy change that occurs during catalytic hydrogenation. Used to compare the stability of pi-bonded molecules, and as a probe of alkene stability.

Order of enthalpy of hydrogenation is reverse of stability of alkenes.

Less substituted alkene evolve more heat in the hydrogenation reaction. I is least substituted, evolve maximum heat while III is most substituted, evolve the least heat. II is disubstituted evolve more heat than trisubstituted IV.
Hence, the order is $I I I < I V < I I < I$ .

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