alkene symmetry

[Noory86]

---

Members do not see this ad.

why are trans-alkenes considered more symmetrical?

 

[HawkeyePostOp]

I don't think they necessarily are. (Think cis-1,2-dibromoethene)

What was the problem/statement?

 

[Noory86]

Well in the Kaplan premier book, they mention the fact that trans-alkene have higher melting points than cis-alkenes due to "higher symmetry"...

 

[Phlame217]

Trans alkenes have more symmetry in the fact that the larger groups are on opposite sides so they inherently have less sterichs. Less sterichs means more stability. More stability = higher boiling boint.

 

[amar21]

Trans alkenes have more symmetry in the fact that the larger groups are on opposite sides so they inherently have less sterichs. Less sterichs means more stability. More stability = higher boiling boint.

Actually, trans alkenes have a higher Melting point. CIS alkenes, due to polarity (net dipole) have a higher boiling point.

Essentially, in part.. sterics decide melting points, where as Polarity decides boiling points. Trans alkenes are less polar than Cis alkenes.

 

[BerkReviewTeach]

Less sterichs means more stability. More stability = higher boiling boint.

This is such a common misconception that it might be good to reconsider this statement.

Stability refers to the chemical reactivity of a compound as it reacts with other molecules. An unstable molecule will undergo energetically favorble chemical reactions. This is different than undergoing a phase change.

Boiling point, on the other hand, depends on intermolecular forces. It is a physical process that depends on the molecular mass of the compound (heavier molecules require more energy to be lifted into the gas phase) and intermolecular forces.

As mentioned by a previous poster, the polarity of the alkene impacts the boiling point as that affects its intermolecular forces. Although the subsitution affects both molecular stability and boiling point, it cannot be stated that strutural instability has a predictable effect on the boiling or melting points of a compound.