How to Understand the Difference in Alkene Stabilities - Part 1

Stability of an organic compound depends on several factors, and so does that of an alkene!


An organic reaction takes place depending on multiple parameters including (but not limited to) the substrate(s), reagent(s), solvent(s), catalyst, reaction conditions, viz., temperature, pressure, moisture.

For one, an isolated double bond (alkene) is more reactive than an isolated triple bond (alkyne) toward electrophilic addition (AE) reactions. This means saying an alkene is more reactive than an alkyne does not make sense as the reverse might be the case if the reaction conditions were different, say, in nucleophilic addition (AN) reactions.

Conjugated dienes are thermodynamically stable & kinetically unstable.

The enthalpy/heat of combustion/hydrogenation is directly proportional to the number of carbon atoms and inversely proportional to the stability.

Alkyl substitution on a double bond increases its stability by hyperconjugation.

Question:

Arrange the following in the decreasing order of their heats of hydrogenation:



Click/tap to enlarge the image

Solution infographic:




Click/tap to enlarge the image

Sizeable stability❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

‌(1)    Ncert. Chemistry : Textbook for Class XI - Part.II; National Council Of Educational Research And Training: New Delhi.

(2)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

(3)    Graham, T. W.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry.; Hoboken, Nj John Wiley Et Sons, Inc, 2016.

(4)    Libretexts. 16.7 Stability of Conjugated Dienes. Chemistry LibreTexts. https://chem.libretexts.org/Courses/University_of_Illinois_Springfield/UIS%3A_CHE_269_(Morsch_and_Andrews)/Chapters/Chapter_16%3A_Conjugation_Resonance_and_Dienes/16.07_Stability_of_Conjugated_Dienes.

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