[orgo814]

Why would a carbonyl (with deuterium on one side of the carbonyl carbon and CH3 on the other side) be considered achiral? It's certainly not symmetric so I can't figure out why my professor called it achiral. Thanks.

[kriggy]

It is symetrical. If you draw it on the paper, the plane of symetry is the paper. SOrry for quite bad analogy, I dont have any drawing software at my pc to draw it for you 

[orgo814]

Oh I see what you mean though. So this can apply to any stand alone carbonyl?

[Borek]

I don't understand what you are asking about. Chirality requires an sp³ atom with four bonds sticking out in 3D, carbonyl contains sp² carbon that is flat. How can it be chiral?

[clarkstill]

Chirality requires an sp³ atom with four bonds sticking out in 3D

That's a dangerous definition to use. All chirality requires is that the mirror image is not super-imposable on the original molecule. Your definition would discount e.g. axially and helically chiral molecules.

In answer to the original question, yes - if you take any carbonyl as a stand-alone (outside the larger context of the molecule) then you couldn't describe it as chiral. However, this is almost a meaningless statement - chirality is a molecular quality, not an atomic one, so you can't really talk about the chirality of a section of a molecule, only of the molecule as a whole.

Perhaps a more useful way of saying it is that the carbonyl carbon is not a stereogenic centre, since interchanging the groups attached to it doesn't lead to stereoisomers.

[Borek]

Chirality requires an sp³ atom with four bonds sticking out in 3D

That's a dangerous definition to use. All chirality requires is that the mirror image is not super-imposable on the original molecule. Your definition would discount e.g. axially and helically chiral molecules.

Yes. I ignored them to not confuse things further.

[SirReal]

The reason it is achiral is because the carbon in carbonyl is not sp3 hybridized; rather it is sp2 hybridized.  Therefore, it is not 3-Dimensional in nature.  It is a "flat" structure, from my understanding.  In order for a carbon to be a stereocenter, it must be sp3 hybridized (tetrahedral) and contain 4 different substituents.  The double bond is what prevents this from happening.