What is the order of enthalpy of hydrogenation of the following compounds?I. II.III. IV.

Hydrogenation of all of the above dienes involve the same bonding change, i.e. two pi-bonds are hydrogenated in each case. Conjugated dienes are more stable, will evolve less heat in hydrogenation than the isolated diene. Also, the least substituted would evolve most heat. I is conjugated but more substituted than II, therefore, I would evolve least heat followed by II. Between II ,IV and III  iii evolves less heat as it is more substituted.