[harvester of sorrow]

Which of these isomers is the (+) one, and which is the (-) one? How does one tell the difference?

[Dan]

You can't work it out. You have to measure the optical rotation experimentally to determine (+) or (-).

Lucky for you, that's been done already for the enantiomers of lactic acid so you can look it up (google/wikipedia etc.) - much quicker than doing the experiment.

[shalikadm]

In our coursework we can name anyone of them as (+)..then the other one is (-)..there's no need to be confused about it..

[discodermolide]

In our coursework we can name anyone of them as (+)..then the other one is (-)..there's no need to be confused about it..

Of course you can arbitrarily name them but that is not the way. The way is to measure the optical rotation as Dan said.

[fledarmus]

In our coursework we can name anyone of them as (+)..then the other one is (-)..there's no need to be confused about it..

I have to disagree with Discodermolide on this one. You can't name them (+) and (-) arbitrarily. Those symbols have meaning, and are used to distinguish a specific compound of a pair of enantiomers based on the direction it rotates plane polarized light, rather than on its three-dimensional structure. Associating either (+) or (-) with a specific chiral drawing or nomenclature, whether wedges, Fischer projections, or R/S names, indicates that you have an unambiguous structure proof and a measurement of the optical rotation of that structure, either your own work or literature values. If you have measured the optical activity but do not have an unambiguous structure, then you attach a non-specific structure or name and refer to it as "the (+) isomer of xxx" or "the (-) isomer of xxx"; if you have a structure proof but do not have a measure of the optical rotation of that structure, then you do not use (+) or (-) nomenclature.

[discodermolide]

In our coursework we can name anyone of them as (+)..then the other one is (-)..there's no need to be confused about it..

I have to disagree with Discodermolide on this one. You can't name them (+) and (-) arbitrarily. Those symbols have meaning, and are used to distinguish a specific compound of a pair of enantiomers based on the direction it rotates plane polarized light, rather than on its three-dimensional structure. Associating either (+) or (-) with a specific chiral drawing or nomenclature, whether wedges, Fischer projections, or R/S names, indicates that you have an unambiguous structure proof and a measurement of the optical rotation of that structure, either your own work or literature values. If you have measured the optical activity but do not have an unambiguous structure, then you attach a non-specific structure or name and refer to it as "the (+) isomer of xxx" or "the (-) isomer of xxx"; if you have a structure proof but do not have a measure of the optical rotation of that structure, then you do not use (+) or (-) nomenclature.

I was responding to the statement by shalikadm, if he wants to name them arbitrarily that's up to him. I do not condone this action at all because it's not correct as you said. I did qualify my remarks by saying "but that is not the way".