[Rutherford]

What else could cause chirality beside sp³ C atoms that have four different substituents and the assymetry caused by a ring?

[discodermolide]

Take a look at BINAP.

[Rutherford]

Thanks. That is axial chirality. The one with the sp³ C atom is central chirality?

How is called the chirality caused by a ring (attached)?
Is it planar chirality?

[willug]

Do you think those structures you've posted are chiral?

[discodermolide]

These rings are not flat. Make models.
They are pseudochiral centers. See Hirschmann etal Tetrahedron 1974, 30, 3648-3656 (PROCHIRAL AND PSEUDOASYMMETRIC CENTERS: IMPLICATIONS OF RECENT DEFINITIONS)
[caps not mine copied from title]

Have a look here. http://faculty.concordia.ca/muchall/chem325/1bchiralcenter.pdf

[willug]

Ah I see (sort of) Thanks for pointing this out disco... my models are still confusing me a bit! It just seems like a conformational thing; i.e. are the methyl groups 'axial' or 'equatorial'. I think those representations are pretty misleading then, don't you think?

I mean, they are superimposable on their mirror images if you don't change the conformation.

[Rutherford]

Can pseudochiral be called as a subgroup of chiral?

[Dan]

Can pseudochiral be called as a subgroup of chiral?

No. Chirality is a property of a molecule as a whole. A molecule is either chiral or achiral.

For a molecule to be chiral, it must have a non-superimposable mirror image - that is the definition.

Pseudoasymmetric is a property of a point - i.e. you can use it to describe a centre, but it is not descriptive of the molecule as a whole.

A molecule can have multiple asymmetric centres (a.k.a. chiral centres) and still be achiral (see: meso compounds) and some molecules are chiral despite having no asymmetric centres (see: axial chirality, for example). This is why I personally think the term "chiral centre" should not be used.

Asymmetric centres need not be at C atoms or involve sp³ hybridisation. There are plenty of chiral metal complexes where the asymmetric centre is the metal (e.g. Ru(bipy)₃), and chiral phosphines and sulfoxides with asymmetric centres at P and S feature in many ligands and auxiliaries used in asymmetric synthesis.

The two cyclobutane structures you posted are both achiral.

[Rutherford]

They are two different compounds, but they aren't optically active.

"For a molecule to be chiral, it must have a non-superimposable mirror image - that is the definition."

This tears away any confusion which the other definitions gave. Thanks for the help.