Different between regiospecific and stereospecific?

[JP2740]

Also, what can we conclude from D and L about R or S or enantiomers, etc.

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[PiBond]

Also, what can we conclude from D and L about R or S or enantiomers, etc.

Regiospecific pertains to our markovnikov vs. anti-mark. thus different connectivity is involved in regioisomers. if you were to add HBr to a double bond and noticed that the H had a preference for one site over the other then this would illustrate a regiospecific reaction. H goes to where more H's reside in mark, and the opposite is true of anti-mark

stereoisomers pertains to difference in spatial (3D) arrangement. thus, stereoisomers have the same connectivity.

D and L and R and S don't have any special relationship necessarily. We can't say a D sugar is R because D refers to the overall molecule where R denotes a specific chiral center (glyceraldehyde is the exception here). the 2nd to last carbon (which denotes D/L in sugars) may have an R configuration but we can't make the blanket statement that all D sugars are R and L sugars are S, if that makes any sense. D sugars have the OH on the penultimate carbon on the right, and vice versa for L sugars.

 

[lesset01]

I agree with PiBond, but, just to clarify, regiospecificity does not refer exclusively to markovnikov vs anti-markovnikov rxns. A rxn is regiospecific if it prefers one structural isomer in the products vs other isomers that are possible. Markovnikov rxns are regiospecific rxns.

 

[orgohacks]

Also, what can we conclude from D and L about R or S or enantiomers, etc.

Regiospecific - given a reaction that could potentially form multiple structural isomers (structural isomers = regioisomers) a regiospecific reaction only forms one regioisomer. Example, as above - hydroboration is regiospecific for adding boron to the least substituted carbon.***

Stereospecific - Starting with one steroisomer, the reaction produces one (and only one) stereoisomer. The classic example is the SN2. Given a chiral alkyl halide, SN2 will lead to inversion of configuration at the carbon, 100% of the time.

Regioselective and stereoselective - refer to above, except produce mixtures where one regioisomer/stereoisomer is dominant.

D/L and R/S. Caution: it's coincidental that D tends to be R and L tends to be S. All 20 natural amino acids are L, but cysteine is R because of the SH. If you remember that exception, you should be OK.

Hope this helps - James


PS - not on the exam: D/L refers to the absolute configuration assigned to sugars/amino acids by Fischer back in the early 1900s. R/S was developed in the 1950s, much more broadly applicable than D/L.] ***this is a white lie - hydroboration is actually highly regioselective (~99:1 in some cases) not regiospecific, but pretend I didn't tell you this.]

 

[Dieumerci]

Thanksss alottt. This was more than helpful to me. I've been struggling with this concept for a long time