[OrganicH2O]

I was wondering about the relative stability of 1-methylCyclohexene and 1-ethylCyclopentene, because 1-cyclopentyl-1-ethanol supposedly rearranged in H2SO4/heat to 1-methylhexene through a surprising carbocation rearrangement. Then, I found heats of formation and saw that 1-methylcyclohexene is indeed more stable. In fact, even cyclohexene is slightly more stable than 1-methylcyclopentene, also as shown by heat of formation data ( https://webbook.nist.gov/cgi/cbook.cgi?Source=1961LAB%2FROS476-480&Units=SI&Mask=1 ). This was more surprising because 1-methylcyclohexene is trisubstituted.

My question is, would this typically always be true with alkenes? I recall reading somewhere that 5 membered ring alkenes could be more stable than 6. But apparently I was thinking of something else, or I misread something. Are there any examples of equilibria that favor 5 over 6?

[Enthalpy]

The heats of formation at Nist are given for the liquids. If comparing strain energies, it's better to take gases.

Gaseous 1-methylcyclopentene
-36.4+32.6 = -3.8 kJ/mol

Gaseous cyclohexene
-38.2+33.6 = -4.6 kJ/mol

The difference is too small to draw any conclusion. It's much smaller than the discrepancies between the authors.

Daily life in thermochemistry. Heats of formation are measured through combustions so they aren't accurate enough. Hydrogenolysis would be more accurate but it is impractical.

Whether 2kJ/mol (=0.8*RT)  drive a reaction, I won't risk any opinion. Aren't reaction mechanisms more important?

[pgk]

The ring strain of the chair cyclohexane ring, is ≈ 0 kcal/mol and the ring strain of the envelope cyclopentane ring, is ≈ 6 kcal/mol, which is about equal to the energy of a hydrogen bond. Thus, a intramolecular rearrangement that leads to a cyclopentane ring with intramolecular H-bonding, is theoretically possible.
Besides, the ring strain difference of the corresponding cycloalkenes, is even lower because cyclohexene has lost the chair conformation, in contrast to the cyclopentene ring that keeps the envelope conformation.

[OrganicH2O]

Thank you both for the thoughtful answers:

To enthalpy: It did seem like the numbers were very close for those alkenes you mentioned. This question was specifically inspired by the dehydration of 1-cyclopentyl-1-ethanol in H2SO4. While looking over problems on practice exams for two different undergraduate organic chemistry classes, I saw two different answers for this question over a period of three days.

The first answer said hydride shift to form a tertiary carbocation, followed by elimination to create 1-ethylcyclopentene. This makes sense kinetically, because hydride has higher migratory aptitude, and also because it immediately makes a tertiary carbocation. This is the answer I predicted. The second answer was ring expansion to make a 6 atom ring secondary carbocation, which would lead to 1-methylcyclohexene. This makes sense thermodynamically, because the 6 membered ring is significantly more stable. The energy difference between those two products is very large. Based on heats of formation on NIST the difference is ~60 kJ/mol.

I could speculate that one product is kinetic and one is thermodynamic. Or, the one without the ring expansion is just non-sense, and it never forms. Certainly under some conditions it could eventually equilibrate to the larger ring.


To PGK: That's an interesting point about hydrogen bonding. I have wondered why in the monosaccharide form of fructose, it favors the pyranose form in water solvent (~70%/30%), and when fructose is part of the diasaccharide sucrose, fructose favors the furanose form. But I this could also happen just because the enzyme fits them together that way.