# Reactivity order in hydrogenation reaction of but-2-yne and butene isomers

Question 1:

The reactivity order towards hydrogenation of the following compounds is:

1. but-2-yne ($\ce{CH3-C\bond{3}C-CH3}$)
2. cis-but-2-ene ($\ce{CH3-CH=CH-CH3}$)
3. trans-but-2-ene ($\ce{CH3-CH=CH-CH3}$)
4. 2,3-dimethyl-but-2-ene

Question 2:

Which of the following has the lowest heat of hydrogenation per mole:

1. cis-2-butene
2. trans-2-butene
3. 1-butene

In the first question, I understand that 2 > 3 (cis is more reactive than trans generally) and 4th is the least reactive (steric hindrance). But I am not quite sure about the order of (1). I could think that alkynes are more reactive toward hydrogenation because they have more pi electron density because of which its adsorption on the surface of metal catalysts will be easier. Is that the reason? I have also read that alkynes are more reactive because they have less number of substituents around them. Which logic is correct and why?

I am more confused in the second question since there are just so many factors to compare - more no of pi bonds (more electron density) (according to my above logic, it should increase reactivity), steric hindrance, increased reactivity of cis isomers. I can make out that a > b (cis is more reactive than trans) and d > c (more electron density) in terms of reactivity toward hydrogenation but how do we compare the others?