[sammyjo06]

I am not sure if this is organic chemistry or if I should have learned it back in general, so I apologize if this is in the wrong forum.

I failed organic chemisty 2 at my home institution so I am taking it at a university near me for the summer in a horrible 4 week MTWThF course beginning at 8:30 am an hour and a half from my house =(. Appropriate punishment, I suppose.

At any rate, the instructor keeps using the letter D in the chemical equations she writes on the board, and despite coming home and trying to google it and looking all through my chemistry book to find an explanation for this, I can't seem to find anything about this letter.

Searching for D organic chemistry just leads me to forums with kids complaining about the Ds they got in organic chem, so of course that does not help .

If anyone coud help me out with this and then tell me what priority this mysterious "D" is in determining the priorities for functional groups, this would be VERY much appreciated.

[spirochete]

D is for deuterium: http://en.wikipedia.org/wiki/Deuterium

[Borek]

Can you show us a full equation with this D?

[sammyjo06]

Thank you, Spiro.

Although I am still confused as to how HDC=CDCH3 is a cis molecule. I don't know if I wrote that correctly, but what I mean is the C=C is in the center and the H and CH3 are on the same side and the 2 Ds are on the same side.

If the D has the same priority as 2 Hydrogens, then it would have higher priority than the H, but lower priority than the methyl group, right? Yet my textbook says that is a cis molecule.

[spirochete]

Your textbook is correct, that molecule is cis. 

Deuterium is a hydrogen with an extra neutron.  Since atomic weight is the final criteria used to define priorities, this means D is higher priority than H. 

But in this case it sounds like you are confusing Cis/Trans and E/Z.  Both of these describe geometry around a double bond, but we need to assign Cahn Ingold priorities only when figuring out E/Z.  E/Z can be used to describe any molecule with the possibility of pi bond isomerizm, whereas Cis and trans can only be applied to certain situations as described below.

With Cis and trans you only need to know whether a substituent is different or the same.  This is only useful when you have two or three different subsituents and those substituents are on different "ends" of the pi bond.  I attached a file that shows what I mean.

Also in the file there is an example where stereoisomerism is not possible and also a separate example where the cis/trans system is no longer useful for describing stereochemistry.

[sammyjo06]

^ Thanks for the *delete me*

You were right; I saw it was E and thought that meant trans, too.