15:38 Jan 23, 2011 

"Hindered" implies energetically unfavourable but may occur.
"No free rotation" not an energetic impossibility (never say never) but without extreme circumstances or a significant, externally applied 'energy push' it's not goign to happen.

15:32 Jan 23, 2011 

2 (or 3 or 4) physically large groups attached to a centre at one end of A NORMAL, SINGLE, COVALENT BOND, impede, hinder, get in the way of, obstruct, but do not necessarily completely prevent the free rotation, of said bond.
This is a special case of "hindered rotation"; but do you remember studying translation and something about 'generalisation and particularisation'? "Hindered rotation" would be a generalisation - an unnecesary generalisation because we have the exact, literal equivalent, - "no free rotation". As you said yourself, you remember from high school that free rotation about a C=C is impossible , not hindered but impossible.
In short, it is simply not necessary, and also incorrect becuase detail is lost from the original source text, to generalise from "rotación impedida (PREVENTED) alrededor de uniones dobles carbono carbono", to "hindered rotation"
Hope that clarifies it a bit.

15:03 Jan 23, 2011 

Yes, I think I do still remember my high school chemistry—rotation is *impossible* without disrupting the bond. But how is this not a special case of hindered rotation?

I will gladly defer to experience on this one, but I am simply not seeing the point.

14:55 Jan 23, 2011 

but can also mean 'prevent' (see ref.)
http://www.wordmagicsoft.com/dictionary/es-en/impedir.php
IN this case that is the correct interpretation; the C=C double bond PREVENTS (totally and completely, not just hinders) any free rotation.....see explanation below and further references.