[johngiml]

Hi,
This is quiet a general question.
Sorry about a lack of any good examples.
When you have two similar molecules to compare in order to find whether they are identical, enantiomers, and diastereomers to one another.
To check whether they are identical, would it be possible to rotate a part of the main carbon chain that has different substituent groups attached? Say hydroxy and ethoxy as substituent examples. Also, I'd like to add an assumption that the main chain does not contain any pi bonds and the two molecules have the same connectivity.
Thank you.

[spirochete]

Generally, yes. If bond rotation of one molecule leads to them being identical, then they are the same molecule. Although depending on how the question is asked, they may be considered different conformations, or "conformational isomers". But if that is not an option, then they are the same.

The reality is that the various conformations are rapidly interconverting, so that no single conformation can actually be isolated.

[orgopete]

This is where models can be very useful (providing the structures are not too large). With models in hand, you can change their orientation and/or conformation to make comparisons.