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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
  4. 1,3-butadiene

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?

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Monosubstituted alkene is more reactive than disubstituted because directly attached alkyl groups stabilize the system. Therefore 1 butene is more reactive.

Cis is more reactive than trans because Cis compounds are unstable. As they have similar groups on same side, it results in stearic crowding.

Thirdly, conjugated dienes are very stable beacuse they are resonance stabilized.

Thus order will be 1 butene will be most reactive, then Cis, then trans and then 1,3 butadiene..

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Correct reason for 2nd question is heat of hydrogenation is inversely proportional to the stability of the compound. And (D) is most stable due to conjugation.

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  • $\begingroup$ Good answer, I know a classic example of this is to compare the heat of hydrogenation of hydrogen with that of cyclohexene. The heat of hydrogenation of benzene is much less than three times that cyclohexene. $\endgroup$ – Nuclear Chemist May 11 '18 at 15:57

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