The following question was asked on myPAT website:

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I started with the solution by counting the hyper-conjugative structures available to each alkene. Based on this concept only, order is:

I > II = III > IV

Then, I noticed that II and III have different carbon count. As, n-hexane (~68 °C) have more thermodynamic stability than n-pentane (~36 °C), therefore the order becomes:

I > II > III > IV

But, option B was incorrect. What I am doing wrong?

The correct option is:

enter image description here

  • $\begingroup$ The queston involves heats of formation. Check here: ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/thermo/… $\endgroup$
    – user55119
    Oct 6 '19 at 17:30
  • $\begingroup$ @user55119: what should the order be? (acc. to you) $\endgroup$ Oct 6 '19 at 17:44
  • $\begingroup$ They only want an answer based on hyperconjugation in which case you have a point. As to heats of formation, II and III should be switched. But the atom count makes a difference. $\endgroup$
    – user55119
    Oct 6 '19 at 18:52
  • $\begingroup$ You are erroneously associating the boiling point of the substance with its thernodynamic stability. As mentioned by user55119, the heats of formation are a better indicator for stability. $\endgroup$ Oct 6 '19 at 23:05
  • $\begingroup$ @TanYongBoon: Okay, I admit that I'm in chaos. I know that lesser the $\ce{\Delta H_f^°}$, greater is the stability. Using this concept and some data from the table (provided by user55119), I can write the stability order based on heat of formation only as: I > II > IV > III. Hence, resultant order comes out as given in option B (as hyperconjugation is more dominant factor). Now what I'm supposed to do with my life? $\endgroup$ Oct 8 '19 at 2:13

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