Talk:Stereospecificity

The sentence "Stereospecificity always yields 100% of a particular stereoisomer, while stereoselectivity does not necessarily give 100% of the favored isomer (although 100% stereoselectivity is possible)" is misleading on a couple of counts, and I have tried to fix it in the article. First the "always yields 100% of a particular stereoisomer" ignores the many situations where less than perfect transmission of stereochemistry occurs because of competing mechanisms (e.g. SN1 and SN2). Second, the statement "(although 100% stereoselectivity is possible)" is not quite true - during the creation of a new stereocenter there will ALWAYS be both products formed, although the selectivity may be so high that the minor isomer could not be detected by the analytic method used. Chemists then talk about a "100/0 ratio of isomers," or "only" one isomer being formed or "complete stereselectivity" when what they mean is "I couldn't detect the other isomer," the detection limits rarely being better than 99/1, and often no better than 95/5, since proton or carbon NMR spectroscopy is by far the predominant technique for this purpose."ChemistHans (talk) 23:17, 24 November 2008 (UTC)[reply]

I disagree vehemently. The first sentence makes clear that stereospecificity requires the pre-existence of stereochemistry. The later sentence on chiral synthesis reinforces this.

I attribute this comment to an all-too-common misunderstanding of the concept -- a confusion of specificity and selectivity. The misunderstanding can be alleviated if one shifts the focus away from reaction outcomes or product distributions and onto mechanisms. The product distribution in a particular reaction arises from competing mechanisms or from competing channels (pathways) of the same mechanism, whereas a mechanism is or is not (chemo-, regio-, stereo- or diastereo-) specific and either gives one or more than one product, through a single or multiple channels. An example is the S_N2 mechanism, as opposed to the S_N1 mechanism: it is stereospecific in that it ALWAYS results in inversion of stereochemistry (whether or not the starting product was stereochemically pure to start with), whereas the S_N1 mechanism does not and may in fact, depending on conditions, show some or no selectivity. There is no actual selectivity in the S_N2 mechanism, because there is only one channel from which to select, whereas the S_N1 mechanism has two channels (inversion or retention). If a nucleophilic substitution gives less than 100% inversion, it is because S_N1 is competing (such as at secondary carbons) or because of double inversion (as with iodide acting as both nucleophile and leaving group).

To address this, I would do the following:

shift the focus from reaction to mechanism:

"In chemistry, stereospecificity is the property of a ~~chemical~~ reaction mechanism that yields different stereoisomeric reaction products from two stereoisomeric reactants ~~depending on the reaction conditions~~ or operates on only one of the stereoisomers. Stereospecificity differs from stereoselectivity in that stereospecificity describes a reaction that necessarily yields a given stereoisomer because of the mechanism of the reaction (i.e. the mechanism specifies the product stereochemistry), assuming that only one mechanism is operating, while stereoselectivity describes a reaction where ~~the~~ a non-stereospecific mechanism allows for the formation of ~~both~~ multiple products, but the formation of one of the products is favored (i.e. selected) by factors, such as steric access, that are independent of the mechanism. Given a stereoisomerically pure starting material, a stereospecific reaction ~~can~~ will give 100% of a particular ~~diastereomer~~ stereoisomer, although loss of ~~stereochemistry~~ stereochemical integrity can easily occur through competing mechanisms with a different stereochemical outcomes. A stereoselective process will normally give ~~two~~ multiple products even if only one mechanism is operating. "

The above suggested paragraph also makes minor corrections.
add a sentence to the introduction such as "The term 'stereospecificity' is commonly misused to mean 'very high stereoselectivity'. A mechanism or reaction is or is not stereospecific, and the specificity cannot be high or low -- it does not occur in degrees, as does selectivity. "

This has been a source of much frustration for me. I have taught this subject for decades. Students understand it, then read the literature where the word is usually misused more often than not, then end up thinking that I had it wrong. Pgpotvin (talk) 20:15, 20 December 2008 (UTC)[reply]

I could not quite detect what Pgpotvin "disagrees with vehemently". Could you clarify? ChemistHans (talk) 20:17, 21 December 2008 (UTC)[reply]

I certainly hope I misread your objections.

I interpreted your sentence:

'First the "[stereospecificity] always yields 100% of a particular stereoisomer" ignores the many situations where less than perfect transmission of stereochemistry occurs because of competing mechanisms (e.g. SN1 and SN2).'

as a misinterpretation itself of the original sentence, owing to a confusion of stereospecificity and stereoselectivity. The original statement

"Stereospecificity always yields 100% of a particular stereoisomer"

is not wrong. One can only talk of competing mechanisms when talking of reactions (as the results of reactant combinations), whereas the stereospecificity of the original sentence is used in the absolute sense, without reference to particular reactant combinations, and would apply to the pure S_N2 reaction, for example. Whereas a particular reactant combination may adopt a mix of stereospecific and non-stereospecific mechanisms (e.g. S_N1 and S_N2), the stereospecific component remains stereospecific. Hence, my suggestion was to move away from talking about reactions which, when meant as a combination of reactants instead of refering to a single-mechanism transformation, can proceed by competing mechanisms, to talking of mechanisms.

Secondly, I did not like the objection to "100% stereoselectivity is possible" because (a) it is theoretically possible and (b) denying this statement on the basis of a practical limitation (analytical detection limits) is no better than affirming that there is no such thing as a stereospecific reaction on the same practical grounds, yet the word stereospecific is routinely bandied about to mean stereoselective, based on practical grounds. Further, the reference to the creation of a new chiral centre takes away from the main point (on specificity), and the main point is not about the creation of new chiral centres, and thus leads to a confusion of selectivity and specificity.

Pgpotvin (talk) 07:00, 23 December 2008 (UTC)[reply]

Since there appears to be no further discussion, I have gone ahead and made appropriate changes. Pgpotvin (talk) 03:02, 9 January 2009 (UTC)[reply]

In English?

If there is anything worth knowing about this subject, there is probably 85%-94.5% of the population of potenial readers who would appreciate some clarity here. -Peace-

In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers.[1][2][3] In contrast, stereoselectivity[1][2] is the property of a reactant mixture where a non-stereospecific mechanism allows for the formation of multiple products, but where one (or a subset) of the products is favored by factors, such as steric access, that are independent of the mechanism

In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers.[1][2][3]

In contrast, stereoselectivity[1][2] is the property of a reactant mixture where a non-stereospecific mechanism allows for the formation of multiple products, but where one (or a subset) of the products is favored by factors, such as steric access, that are independent of the mechanism 105.108.42.242 (talk) 14:41, 15 May 2022 (UTC)[reply]