[Kate]

Hi.

My book says the following: "The faces are enantiotopic in an unsymmetrically substituted ketone, such as 2-butanone, because they are interconverted by a σ plane."

I can't see this, at all. Could someone explain?

Thanks.

[sjb]

Hi.

My book says the following: "The faces are enantiotopic in an unsymmetrically substituted ketone, such as 2-butanone, because they are interconverted by a σ plane."

I can't see this, at all. Could someone explain?

Thanks.

Do you know about re and si faces?

[Kate]

Yes, I do. It employs the same logic as R/S but it's used for trigonal centers, like a carbonyl. 

What I don't really understand is how the faces of an unsymmetrically substituted ketone, such as 2-butanone, are interconverted by a σ plane. If you apply a mirror plane to 2-butanone, the image you get isn't superimposable on the first.

Besides, the book also says this: "The faces are diastereotopic in a structure such as either enantiomer of 3-chloro-2-butanone, because there are no symmetry elements that interconvert the faces."

Edit: Nevermind, I think I understand it now.