How do you tell the difference between an L form of a molecule and a D form?

Question

This has been bugging me for a while now please help me.

Answer 1

The D/L assignment system relates the molecule to derivatives of D- or L-glyceraldehyde. From Wikipedia: "A rule of thumb for determining the D/L isomeric form of an amino acid is the 'CORN' rule. The groups: COOH, R, NH2 and H (where R is a variant carbon chain) are arranged around the chiral center carbon atom. Sighting with the hydrogen atom away from the viewer, if these groups are arranged clockwise around the carbon atom, then it is the D-form. If counter-clockwise, it is the L-form."



Note that if you want to assign R versus S chirality, you need to use the Cahn-Ingold-Prelog priority rules. If you want to assign dextrorotatory versus levorotatory behavior, that can't be done just by looking at the molecule, you have to measure the sign of the rotation (or perhaps predict it theoretically). Likewise, a measurement of optical activity (+ or -, or small d and small l for the dextrorotatory and levorotatory isomers) doesn't tell you whether the isomer is big D or big L.

Answer 2

You can tell by determining the priorities of each functional group attached to the chiral center using sequence rules. For a carbon to be a chiral center, it must have four different groups or atoms attached. Each group / atom is assigned a priority based on atomic numbers for single atoms, and the atomic number of the highest priority atom in a polyatomic substituent.



Now you can determine the rotation of the molecule. The chiral center is tetrahedral in shape, and the lowest priority group is placed in "back" or away from the observer. That leaves three groups in a triangle shape, with priorities ordered 1, 2, and 3. The numbers will be either clockwise, or counterclockwise, and will correspond to the D or L rotation respectively.

Answer 3

Ketone is right it's the CORN rule (or Law as I like to call it because I'm a sad geeky type and remember the corn laws from school too).



Basically look at your chiral carbon (the one with your functional groups attached) and it depends on the way you read CORN. The experimental way is by using plane polarised light which will be rotated right (clockwise) by a dextro-enantiomer and left (anticlockwise) by a laevo-enantiomer. You can work this out by using two polaroid film on either side of the sample and rotating the one in front of your eye in order to locate the direction of rotation.



Hope that helped.

Answer 4

the only distinction for a cap. i and l is that some fonts has the cap. i with a line on the backside of the i and exact of the i. for persons to appreciate that's that's some thing you learn as you circulate. such as you already know L does no longer be on my own in a sentence it might have some thing with it like the artwork Love, Like. at school you learn in a sentience i, while on my own is capitalized or while mixed like i've got, or i will.. i will tell the version between darn and dam, between the r and the n there is that smaaaaalll area.

Answer 5

As I understand it (and I probably don't)Laevo means that plane polarised light is rotated anticlockwise when it travels through a solution of the molecule.The other,otherwise.

Answer 6

omg are you doing salters chemistry a level?



Basically the functional groups should go in clockwise or anticlockwise direction. Unfortunately i can't remember which way is for which.



Fat lot of good I turned out to be!