[wiseguy]

                 1. Do all SN2 reactions result in inversions to enantiomers at the site of attack? If not which ones do not? Also, are all SN2 reactions backside attacks and if so, why does that happen?
             
                 2. S and R are called absolute configurations right? S and R describe the "handedness" of a molecule, so S and R are just another name given to enantiomers right, one that we consider clockwise and counterclockwise?
                 
                 3. How is it that one attack can automatically change its left or right configuration when we don't know what the new priorities are? i.e. when the nucleophilic substitution occurs, can the inverse still be the same absolute configuration, or is that not possible, ever?
         
                 Thanks in advance, I'm teaching myself and I don't want to learn something and have to unlearn it later.

[fawad0418]

                 1. Do all SN2 reactions result in inversions to enantiomers at the site of attack? If not which ones do not? Also, are all SN2 reactions backside attacks and if so, why does that happen?
             
                 2. S and R are called absolute configurations right? S and R describe the "handedness" of a molecule, so S and R are just another name given to enantiomers right, one that we consider clockwise and counterclockwise?
                 
                 3. How is it that one attack can automatically change its left or right configuration when we don't know what the new priorities are? i.e. when the nucleophilic substitution occurs, can the inverse still be the same absolute configuration, or is that not possible, ever?
         
                 Thanks in advance, I'm teaching myself and I don't want to learn something and have to unlearn it later.

1-Yes all SN2 rxns follow inversion of configuration. i.e., backside attack.
2- yes probably but ''S'' is for sinister(anticlockwsie) and R is rectus ( for clockwise rotation). We use Cahn Ingold prelog rule for giving notations.
3- If the reaction follows SN1 path, then u can have both enantiomers, if it is SN2 u get inversion of configuration.
If u r teacher then please learn also.

[Dan]

                 1. Do all SN2 reactions result in inversions to enantiomers at the site of attack? If not which ones do not? Also, are all SN2 reactions backside attacks and if so, why does that happen?

All S_N2 reactions at asymmetric centres result in inversion of the configuration of that centre because backside attack is a mechanistic requirement of S_N2. The reason for this is that the nucleophile lone pair overlaps with the sigma* antibonding orbital of the C-X (X = leaving group) bond - which is pointing backwards 180^o from the C-X sigma bonding orbital.

Substitution reactions in general do not necessarily proceed with inversion - there are other mechanisms such as S_N1.

An S_N2 reaction only produces the enantiomer if the leaving group is the same as the nucleophile [in which case racemisation will probably occur from repeated S_N2 at that centre] and the asymmetric centre that is inverted is the only asymmetric centre in the molecule. Make sure you understand the definition of enantiomer.

             
                 2. S and R are called absolute configurations right? S and R describe the "handedness" of a molecule, so S and R are just another name given to enantiomers right, one that we consider clockwise and counterclockwise?

Not necessarily. R and S are absolute stereochemical descriptors that refer to a single centre. As such the R/S descriptor only describes the whole molecule if there is only one asymmetric centre. For example, 3-methyl-pentan-2-ol could be 2R,3R; 2R,3S; 2S,3R or 2S,3S since it has two asymmetric centres.

Furthermore, R/S does not imply the direction [clockwise/anticlockwise] the molecule rotates a plane of polarised light - the optical activity is experimentally determined.

       
                 3. How is it that one attack can automatically change its left or right configuration when we don't know what the new priorities are? i.e. when the nucleophilic substitution occurs, can the inverse still be the same absolute configuration, or is that not possible, ever?

I think I see what you're getting at here. Generally speaking, the leaving group has highest priority in the starting material, and nucleophile will end up having highest priority in the product - so that an inversion results in R-->S or vice versa.

This isn't always the case though. If there is another substituent with higher priority, the inversion reaction may result in the same stereochemical descriptor. For example:

S_N2 reaction of cyanide with MeCClHCH(SEt)₂; in this case S alkyl chloride-->S nitrile (or R-->R) because the -CH(SEt)₂ substituent is higher priority than -CN. The -Cl was 1st priority in the starting material.

[wiseguy]

Thanks for that brilliant explanation, I was worried people wouldn't know how to answer that 3rd one.