Heat of hydrogenation is directly proportional to no of pi bond and inversely proportional to stability of alkene

as you know here no of pi bond are same so go with the stability of alkene so ans is ethene > propene> butene> pentene

Ranking of Compounds in Increasing Order of Heat Evolved upon Hydrogenation

To determine the increasing order of heat evolved when 1 mole of each compound is hydrogenated using a platinum catalyst, we need to consider the stability of the intermediates formed during the hydrogenation process. The more stable the intermediate, the less heat will be evolved during the reaction. Let's analyze each compound in detail:

1. Ethene
- Ethene contains a double bond between carbon atoms.
- During hydrogenation, one mole of hydrogen adds to the double bond, resulting in the formation of ethane.
- The intermediate formed is stabilized by the presence of two alkyl groups, which reduces the overall heat of the reaction.
- Therefore, ethene will evolve the least heat upon hydrogenation.

2. Propene
- Propene also contains a double bond between carbon atoms.
- During hydrogenation, one mole of hydrogen adds to the double bond, forming propane.
- The intermediate formed has one alkyl group, which is less stabilized compared to ethene.
- As a result, propene will evolve more heat than ethene but less heat than the remaining compounds.

3. Butene
- Butene is an unsaturated hydrocarbon with a double bond between carbon atoms.
- Upon hydrogenation, one mole of hydrogen adds to the double bond, resulting in the formation of butane.
- The intermediate formed has two alkyl groups, providing more stability than propene.
- Hence, butene will evolve less heat than the remaining compound.

4. Pentene
- Pentene is also an unsaturated hydrocarbon with a double bond between carbon atoms.
- When hydrogenated, one mole of hydrogen adds to the double bond, forming pentane.
- The intermediate formed has three alkyl groups, which offers greater stability compared to the previous compounds.
- Therefore, pentene will evolve the most heat among the given compounds upon hydrogenation.

Ranking of Compounds:
Based on the stability of the intermediates formed during hydrogenation, the compounds can be ranked in increasing order of heat evolved as follows:

1. Ethene
2. Propene
3. Butene
4. Pentene

The stability of the intermediates can be attributed to the increasing number of alkyl groups attached to the carbon atoms involved in the double bond. The presence of more alkyl groups provides greater electron density and reduces the reactivity of the double bond, leading to the evolution of less heat during hydrogenation.