About isomers

[cainnabelspawn]

Ok, I am just now begging to learn about the concept of isomers in chemistry, and there are a few things I need cleared up.

Lets say that MDMA is being synthed, how would different molecular formations come to be? Is one particular synthesis going to yield one isomer or a racemic mixture?

I guess I need that answered before I ask more...

 

[Smyth]

MDMA has one chiral center and is commonly made by reduction of an imine. Every chiral compound in PIHKAL is a racemic mixture in its primary sense as are most of the compounds in TIHKAL (except for the LSD structures). Isomers is quite complicated to just babble about (with no set question). All I can say is, if you are only interested in MDMA then you dont need to worry about isomers.

One of the cases I can think of where isomers ARE important is in the Felkin-Ahn model. This is called 'asymmetric induction'. If you look at how buprenorphine and etorphine are made then you will see what I mean. Other cases where isomers crop up are all over the place and anybody skilled in synthetic chemistry is bound to have encountered it.

The simplest case I can think of is in terpene chemistry. This is where two compounds of the same structure but opposite sterochemistry occur in nature in different plants. One of the isomers smells of spearmint whereas the other smells of 'caraway'.

http://www.scienceiq.com/ShowFact.cfm?ID=287

 

[TheDEA.org]

You are trapped in a bar with fifty drunken angry Marines. Sometimes you get punched in the right side of your face, sometimes you get punched in the left side. Which outcome results from a particular interaction largely depends on whether it was more convenient for a given drunk to hit you from the right or left (which depends largely on your relative positions.)

Whether the R or S isomer of MDMA is formed in a given collision between imine and hydride (for instance) simply depends on the luck of the draw; if the hydrogen being added hits on one side of the imine you get one, if it hits on the other side you get the other. Although this is random, when you average it out over the hugenumbers of molecules involved in a given synthesis, you wind up with as close to 50:50 as makes no difference. There ARE reactions that favor one isomer or the other by making it easier for the reactants to run into each other from one side than the other. Failing that, isomers can be seperated (although usually not very easily) after synthing a racemic product.

'Stereoselective' synths (where one isomer is strongly favored over the other) are a bit on the exotic side as far as basic chemistry goes; none of the 'how to make drugs' synths on the internet are stereoselective. If you wanted one that is, you'd have to go digging into professional chemistry literature (if memory serves, Nichols may have developed a stereoselective MDMA synth.)

 

[cainnabelspawn]

Ok, that makes sense. One more question, I see + isomers are labelled d/R, and - are l/S... What is happening here? What causes a + isomer to be d over R?

Thanks...

 

[phase_dancer]

dextrorotatory and levorotatory are terms used to describe optical[/I] isomers, also often termed (+) or (-) . A polarised light is passed through the molecule and is deflected either in a right [dextrorotatory (+)], or left [levorotatory (-)] direction.

R and S are terms relating to the absolute configuration of a chiral carbon i.e. the arrangement of the 4 different attached groups.

While optical rotation can't affect configuration, for many molecules the R or S assignment may exist in either the (-) or (+) from. However using MDMA as an example, the (-) is the R form and the (+) is the S form.