[Ph(i)Dias]

I would be glad to get the following question's answer:
The definition of enantiomers is almost the same in the most sources: two isomers (more precisely, two stereoisomers) that are non-superposable mirror images of each other. This is what I call a theoretical feature. Later, the student discovers that they differ in their ability to rotate plane-polarized light in opposite directions, which could be called a practical feature of enantiomers.
My question is, given that the most definitions of enantiomers ONLY comprise the theoretical feature, why is every pair of non-superposable mirror images of organic compounds' structural formulae NOT considered a pair of enantiomers? I mean, I am aware that, considering the practical feature, each such pair may not be a pair of enantiomers (because the two compounds may not be optically active), but it actually IS by the theoretical one.
For instance, look at the attached pictures of mirror images of trans-2-butene. The two are non-superposable in the same plane, yet not enantiomers, because they have no chiral C atom, right?

It may be true, but then should enantiomers not be primarily defined also by this criterion (the presence of asymmetric Cs), not only by the chirality of their molecules, as we see that every two enantiomers are chiral molecules, but not every two chiral reflected molecules are enantiomers? I mean, since the set of chiral molecules includes the set of enantiomers, defining enantiomers only by chirality sounds to me like only identifying the genus, not the differentia. I know that there are some enantiomers which have no assymetric C atoms, but choosing this criterion as differentia would still lead us to a proper definition for most of them, right?
So would "two stereoisomers that are non-superposable mirror images of each other and that contain at least one asymmetric C atom" not be a better theoretical definition for THE MOST OF the enantiomers (unlike the initial definition, which is confusing and may allow many non-enantiomeric compounds be considered enantiomers)?
Thanks all.

[thetada]

Your " two" butene molecules are superimposable. If the right hand molecule is rotated by 180 degs in the y axis it is superimposable. With enantiomers, no orientation is possible where they become superimposable.

[Ph(i)Dias]

Thanks a lot.
I was hugely mistaken from the beginning by the definition, because I thought that the condition for enantiomers' existence was minimal, i.e. any molecule could admit an enantiomer if it had at least one axis by rotation about which a non-superposable mirror image is obtained. Moreover, I explained to myself the fact that, for instance, meso-2,3-dibromobutane has no enantiomer by introducing the theory that, for two molecules to be enantiomers, the non-superposability must occur in any position of the two in the same plane ("mediate non-superposability"), not just in the initial position obtained due to reflection ("immediate non-superposability"). Applying the same theory to trans-2-butene, since by rotation about the x and z axes two (identical) non-superposable (in the same plane) images are obtained, I thought the pair would be enantiomeric even though by rotation about y the images would be immediately superposable.
Now I realise that the condition for existence is maximal (non-superposable by all axes, i.e. asymmetric) and that the non-superposability must occur in the initial reflected position and any change in orientation, either in the same plane or in different ones, is not allowed as a criterion.

[thetada]

Glad I could help. If you get a chance to use a molecular modeling kit it'll really help to further consolidate your understanding. Or you could use modeling software like ACD Chem sketch