[G O D I V A]

What would be the stereostructure of the product obtained when (R)-2-bromo-propionitrile () undergoes SN2 reaction with
ethanethiolate (Na^{+ -}SCH₂CH₃); in ethanol.

I dont know which would be the nucleophile and which would be the leaving group.  The question goes on to ask what the configuration (R/S) is on the product so it must be either the cyanide or the bromine leaving and the ^-SCH₂CH₃ adding?  Is that right?

[Dan]

it must be either the cyanide or the bromine leaving and the ^-SCH₂CH₃ adding?  Is that right?

Yes. So which is the better leaving group - bromide or cyanide?

[pantsboy]

good Leaving Groups are typically weak bases which are conjugates of strong acids.  HBr is a strong acid with a pKa of -9 and HCN is weak with a pKa of 9.21.  In general, if the pKa of a conjugate acid is higher than 2 it will not function as a leaving group.

[Scatter]

See ya later, Br!

Seriously, Br just likes to get up and walk out all the time.  Another molecule starts getting frisky with its carbon's nucleus and it just backs off.  A "weak" base for sure.