[FredTT]

Okay Guys,

I'm new (as you can tell by my post count).  I've been working all night on a few OChem Problems for class because my group ditched me.  Any help would be greatly appreaciated.  I've managed to do a great portion of the problems, but some are still givinging me headaches...

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Morphine, shown below, can be used to separate a racemic mixture of the amino acid, phenylalanine, also shown below. Briefly describe how this is done, showing the chemical reactions at appropriate places. Explain why you are able to separate the enantiomers when enantiomers have identical physical properties, with the exception of their behavior with plane polarized light.

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Can this process be used to separate a racemic mixture of 2-chlorobutane? Why or why not?

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Iodoalkanes are readily prepared from the corresponding chloro compounds by SN2 reaction with sodium iodide in acetone. This particular procedure is especially useful because the inorganic bi-product, sodium chloride, is insoluble in acetone; its precipitation drives the equilibrium in the desired direction. Thus, it is not necessary to use excess NaI, and the process goes to completion in a very short time. Because of its great convenience, this method is named after its developer (the Finkelstein reaction). In an attempt to synthesize optically pure (R)-2-iodoheptane, a student prepared a solution of (S)-2-chloroheptane in acetone. In order to ensure success, he added excess sodium iodide and allowed the mixture to stir over the weekend. His yield of 2-iodoheptane was high, but to his dismay, he found that his product was racemic. Explain.
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Any help on these would be greatly appreciated.

~Fred

[Yggdrasil]

As a general rule, you should always try to post the work you've done on the problem or thoughts you have about the solution when you post questions.  If you post a question without showing us first that you've thought about it, you shouldn't expect very many responses.

However, I will give you some hints to guide your thinking about the questions:

1) One way to separate enantiomers is to convert them to diastereomers.  Since diastereomers have different physical and chemical properties in an achiral environment, the diastereomers can be easily separated.

2)  How do SN2 reactions affect stereochemistry?  Another way to approach the explain the the results would involve thinking about alternative pathways which yield the same product.

If you come up with some possible answers post them and we'll be glad to check them.

[FredTT]

1) One way to separate enantiomers is to convert them to diastereomers.  Since diastereomers have different physical and chemical properties in an achiral environment, the diastereomers can be easily separated.

The problem is that I don't see it.  I understand what you are saying, but I don't see how to get it done.  I can see point A and point B but there is a giant fog inbetween.  (I'm not trying to mooch information to get my homework done.  I've done 18 out of the 20 problems)

2)  How do SN2 reactions affect stereochemistry?  Another way to approach the explain the the results would involve thinking about alternative pathways which yield the same product.

SN2 reactions work by attaching a nucleophile on one side of a substrate, forcing the halide on the opposite side to break it's bond.  Does the answer to this one lay anywhere in the fact that SN2 reactions occur only at relativly unhindered sites?

~Fred

[Yggdrasil]

Since these are pretty difficult questions, I don't feel guilty giving you the answers now:

1)  Answering this question requires recognizing three things: that phenylalanine is an acid, that morphine is a base, and that morphine is chiral.  Therefore, when you react phenylalanine with morphine in a simple acid/base reaction you form a diastereomeric salt.  The salt is diastereomeric because it contains two stereocenters, the morphine's stereocenter and the phenylalanine's stereocenter.  Assuming you start with optically pure morphine (say L morphine), you will get two salts (L, R)-morphonium phenylalanate and (L,L)-morphonium phenylalanate.  These two diastereomers have different physical and chemical properties and can be separated easily (for example, by fractional crystalization).

2)  You are correct when you say that an SN2 reaction will invert the configuration about the stereocenter.  So therefore the reaction you want is:

(S)-2-chloroheptane + I^- --> (R)-2-iodoheptane + Cl^-

But since there's excess iodide ions in solution what's stopping the following SN2 reaction from occuring?

(R)-2-iodoheptane + I^- --> (S)-2-iodoheptane+ I^-

The excess NaI results in the racemization of the product.