Predicting the Melting Point of Stereoisomers: A Scientific Inquiry

[disneychannel]

The melting point of N-acetyl-DL-alanine is 137◦Cand that of N-acetyl-D-alanine is 125◦C. What
would you expect the melting point of N-acetyl-L-alanine to be or is this impossible to predict?

Why?

I thought that to approach this problem all that needed to be done is take the average of the MPs, however, that doesn't seem correct.

 

[Borek]

DL means mixture, D means a pure product. What about L?

 

[disneychannel]

In your context L would be unpure. But I thought D- dextrorotary prefix refers to + rotation and L =levorotary (l) prefix refers to − rotation. Although I don't see how that has anything to do with the question.

How am I supposed to approach this? Thanks!

 

[epenguin]

Never thought about that. They are either expecting more from you or teaching you more than when I was at Uni.

DL is an equal mixture, but both types of molecule are present in the same crystal.

It is possible to answer. Think molecularly and think structurally of the crystal. Just in general terms, don't need to know detail of structures beyond what you're told.


And if you really can't get it that way there is a trick way of knowing. If the correct answer were untrue it would have consequences you would probably know about.

 

[Borek]

In your context L would be unpure.

No, it would be pure.

Now that I think about it, my wording - calling them pure/unpure, in the context of melting point, could be confusing.

Think about it this way - apparently DL (which is a mixture) crystallizes in an entirely different way than the D does. How would you expect L to crystallize?

 

[disneychannel]

Well they are enantiomers. So they would have the same MP? Therefore it will also be 125? But why is the mixture(I believe this would refer to racemix be higher)?

 

[Borek]

Same MP would be my guess.

As for DL - my guess is that in some way they "fit" each other, so when the amount is exactly equimolar, they crystallize as D-L pairs, or something like that.

 

[Mike H]

Just to add -

For properties like melting point and solubility, enantiomers are identical. It's when you impose a "chiral environment" that you can resolve the two, e.g., reactions with a chiral molecule that prefers one enantiomer to another.

You might find the Wiki article on racemic mixtures to be worthwhile additional reading.

 

[epenguin]

Well they are enantiomers. So they would have the same MP? Therefore it will also be 125?

Justify that a bit.

But why is the mixture(I believe this would refer to racemix be higher)?

Is this coursework? Then you need to show some effort, try and think of an explanation - we have given some hints. I would expect as a rule DL to have a higher mp than L or D but there might be some exceptions. It's just broad general principles. (Though if you could find out the actual structure of crystals of this or any other substance that would exemplify them that would be brilliant.)