stereo chemistry help?

Question

how can i know if the molecule is chiral or achiral?
or how can i know if the molecule has a plane of symmetry or not?
e.g the Fischer formula of
COOH
H _________OH
HO _________ H
COOH
and the molecule
CH3 CH3
H ________ OH H_________OH
H ________ OH or H_________OH
H _________OH HO_________H
CH3 CH3

Answer 1

If you take a look at a single carbon in the molecule, see if all four substiuents are different from one another. Remember, all four substiuents must be different to make it chiral. Chiral molecule are non-superimposable on its mirror image too.

Example, if you look at a single carbon that has a Fl, Cl, Br, and H attached to it, you know it's a chiral because they are different from one another.

Now, if you see a carbon that has Fl, Cl, Br, and Cl attached to it, that's not a chiral since it has two Cl's.


Symmetry is when one half of the object appears to be a reflection (like a "mirror") of the other half. So if you drew a line down the middle, one side will look like the other side. For example,

H3 - C - C - H3 (is the same as CH3-CH3). This molecule is symmetry because if you drew a line between the carbons, one side will look like the other.

Hope that helps you.

Answer 2

Wow! You have asked two toughies! The first is tartaric acid. Yahoo makes it hard to draw molecules. There is a vertical line from the top COOH to the bottom COOH. You have to draw out the configurations of the two carbon atoms in three dimensions. I make it that the top and bottom carbons of the tartaric acid are of the S-configuration. Therefore, this is one enantiomer of racemic tartaric acid. In my reading, this is not the meso compound, in which one would be R and the other S.So in the first case, there is no plane of symmetry.

The second molecule has a vertical line from the top CH2CH3 to the bottom CH2CH3. Likewise in your second column after "or." If you move the first molecule to right over the second one, they fit, except for the bottom H---OH and HO---H. Also, in the second molecule, there is a pseudoasymmetric center. It has the property of doubling the number of meso compounds (like meso tartaric acid). Meso compounds have planes of symmetry.

Please pardon me. You seem to be in grad school. I also was once. Maybe e-mail me.