Racemic mixture of diasteromers impossible??

[chaser0]

I was wondering why this was the case as the Berkley Review states this verbatim...

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[PiBond]

I was wondering why this was the case as the Berkley Review states this verbatim...

racemic mixture corresponds to a 50/50 (equimolar) mixture of enantiomers. i've never heard of a racemic mixture of diastereomers

could it be that they were referring to resolution and how you separate enantiomers by turning them into diastereomers?

 

[venacontracta]

It's just how things are defined.

Enantiomers are nonsuperimposable mirror images of each other (absolute definition)

A racemic mixture contains a chiral compound and its enantiomer in equal amounts

Diastereomers are stereoisomers that are not enantiomers

There's nothing really to understand about any of the above, it's just the way things are named, labeled, and defined.

That being said, let's say you have a two carbon compound with both carbons chiral. All six of the substituents coming off of these two carbons are different (I'm adding this condition to prevent getting into meso compounds).

The two carbons can be RR, SS, RS, or SR.

You can say RR and SS are enantiomers (enantiomers have an opposite configuration at each chiral carbon)
You can say RS and SR are enantiomers
RS and (RR or SS) are diastereomers
SR and (RR or SS) are diastereomers

Now what your book is saying is:

You can have a racemic mixture of RR and SS because they are enantiomers.
You can have a racemic mixture of RS and SR because they are enantiomers

You cannot have a racemic mixture of RS and RR, RS and SS, SR and RR, or SR and SS. The reason is that are these all pairs of diastereomers and not enantiomers.

 

[chaser0]

This is just the case I was thinking of:

-You have a molecule with two optical stereocenters (one is R and the other is S)
-The S stereocenter has an attatched OH, and is easily susceptible to an SN1 reaction. The R stereocenter has four alkyl groups and cannot participate in any reaction.
-An SN1 reaction then occurs on the S stereocenter

-----> you then have a racemic mixture of RR and RS
a racemic mixture of diastereomers was created from an SN1 reaction of one of the stereocenters of a two stereocenter molecule~~



I don't see how this is incorrect. I seemed to have just demonstrated a creation of a racemic diastereomer product mixture...

 

[chaser0]

Or is it perhaps because the definition of racemic ONLY refers to enantiomers due to their opposite optical rotation??

Hence, even though a 50:50 mix of diastereomers can be made, it is not racemic as the optical rotation is not Zero...




Does a racemic mixture have to have an optical rotation of Zero?

 

[venacontracta]

This is just the case I was thinking of:

-You have a molecule with two optical stereocenters (one is R and the other is S)
-The S stereocenter has an attatched OH, and is easily susceptible to an SN1 reaction. The R stereocenter has four alkyl groups and cannot participate in any reaction.
-An SN1 reaction then occurs on the S stereocenter

-----> you then have a racemic mixture of RR and RS
a racemic mixture of diastereomers was created from an SN1 reaction of one of the stereocenters of a two stereocenter molecule~~



I don't see how this is incorrect. I seemed to have just demonstrated a creation of a racemic diastereomer product mixture...

Your reactant is R,S-someunnamedorganicmolecule

Your products are R,R-asecondorganicmolecule (50%) and R,S-asecondorganicmolecule (50%)

1) When you only have one chiral carbon, SN1 produces a racemic mixture. You cannot however say that SN1 always produces a racemic mixture without that "one chiral carbon" qualification, because in cases where your starting material has multiple chiral carbons, that statement is no longer true.

The general statement you can make about SN1 that is always true is that 50% of the product will have the R configuration at the carbon where SN1 occurred and 50% will have S at that carbon.

(there is a slight simplification here that is beyond the scope of the MCAT, look up SN1 ion pairing if you're interested … otherwise, ignore this)

2) The statement "a racemic mixture of diastereomers was created" is an abuse of terminology - specifically, of the term racemic. It is an incorrect statement, and the correct statement would be "a mixture of equal amounts of two diastereomers was created." Racemic always means 50/50 of two enantiomers. Always. "Racemic mixture of RR and RS" should throw up a big red flag because it should be immediately obvious that RR and RS are diastereomers.

 

[venacontracta]

Or is it perhaps because the definition of racemic ONLY refers to enantiomers due to their opposite optical rotation??

Hence, even though a 50:50 mix of diastereomers can be made, it is not racemic as the optical rotation is not Zero...




Does a racemic mixture have to have an optical rotation of Zero?

Yes. That is why racemic has such a precise definition, because the significance of a racemic mixture is that it doesn't rotate plane polarized light.

 

[UCB05]

Also keep in mind that while enantiomers will have the same physical properties, diastereomers do not. While equal amounts of two enantiomers creates a racemic mixture, a mixture of equal amounts of two diastereomers like that discussed in this thread will likely not be optically inactive because they will have different optical rotations.

 

[Rucap09]

Wanted to bump this, wondering if these responses are right. The BR book clearly says that a racemic mixture can't be formed because a 50/50 mixture of diastereomers won't result, NOT because of a terminology/enantiomer-definition technicality.

I feel like maybe BR is generalizing this to smaller compounds as I could see how that would be the case, but in a large enough compound couldn't you say that a second chiral center is not always going to influence substitution at another depending on where they are relative to each other?

 

[DrknoSDN]

I just discovered the IUPAC GoldBook website recently and it seems to be helpful for "definitions" because IUPAC makes the rules.

Definition: "An equimolar mixture of a pair of enantiomers. It does not exhibit optical activity. The chemical name or formula of a racemate is distinguished from those of the enantiomers by the prefix (±)- or rac- (or racem-) or by the symbols RS and SR."
Source: http://goldbook.iupac.org/R05025.html

By definition they have to be enantiomers to be racemic.
Lots of SN1 reactions yield 50/50 products but if they are not enantiomers, they are not racemic. You could post the reaction that BR is referencing because it may be a mechanism (steric etc).

 

[Teleologist]

I just discovered the IUPAC GoldBook website recently and it seems to be helpful for "definitions" because IUPAC makes the rules.

IUPAC makes the rules but also is slow to make major changes to definitions. Some definitions from the Gold Book are next to useless. Such as its definition of "oxidation state."