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While providing the reason for the higher melting point of trans molecules, everyone says that trans molecules are more symmetric than cis molecules. An example can be found over here, where the OP states that:

trans-2-Butene has more symmetry than its cis isomer which results in better packing and hence higher melting point.

But to my (uneducated) eyes there doesn't seem any more symmetry (if any) in trans form than cis form of but-2-ene. I mean cis form is vertically and horizontally symmetric, whereas trans form is not.

Why do we call trans form more symmetric than cis form?

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  • $\begingroup$ When comparing the melting points, the most important point is not the symmetry of the molecule. It is the symmetry of the spatial arrangement of the molecules in the solid state. l $\endgroup$ – Maurice Jun 14 at 12:14
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    $\begingroup$ "More symmetric" is not a thing at all. However, trans isomers tend to have central symmetry, which is not the case with cis. $\endgroup$ – Ivan Neretin Jun 14 at 12:17
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Trans alkenes have a $C_\mathrm{2h} $ symmetry, identified as such because it has a $C_2$ rotational axis (you need to rotate 180° to have an identical molecule) and a mirror plane perpendicular to that axis $\sigma_\mathrm h$. You need to follow the symmetry flowchart to assign symmetry groups.

A $C_\mathrm{2h}$ group is characterised by having:

  1. $E$: the identity operation
  2. $C_2$: a twofold rotational symmetry axis
  3. $i$: a center of inversion
  4. $σ_\mathrm h$: a horizontal mirror plane

trans-2-butene

Cis alkanes have a $C_\mathrm{2v}$ symmetry; therefore they have:

  1. $E$: the identity operation
  2. $C_2$ : a twofold rotational symmetry axis
  3. $2σ_\mathrm v$: two mirror planes that include the rotation axis

cis-2-butene

As you see a $C_\mathrm{2h}$ group has more symmetry elements and therefore sometimes people say "more symmetry" but this is not a technical way of describing it. To say which is more symmetric you need to consider the possible symmetry operations you can do.

Easier answer:

A symmetric object is an object that looks the same if you rotate, reflect etc. The more operations you can do keeping the object identical, the higher the degree of symmetry. For example a sphere is "more symmetric" than a cube because you have much more symmetry operations you can do (you can have infinite rotation axis and infinite mirror planes etc.). In the case of alkenes the main difference between the trans and cis ones is the so-called inversion point (blue drawing in first figure) that only the trans alkene has and therefore is "more symmetric".

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  • $\begingroup$ Please don't mix bold symbols with MatJax. When people see bold italic serif, they immediately recognize it as a vector. Also, <br> are unnecessary if you are writing in Markdown and pollute the code. Please visit this page, this page and this one on how to format your future posts better with MathJax and Markdown. $\endgroup$ – andselisk Jun 14 at 20:25

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