Organic question

[Erhatstil]

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can someone explain how an Sn2 reaction would proceed with a protic solvent? i've seen a few examples like #127 in destroyer (i have 2008 version) where the answer explains the substitution reaction as being Sn2 but our solvent is methanol.

even if we have a primary alkyl halide and a strong nucleophile, which are recipes for an Sn2, why would a protic solvent be present? doesn't make sense...

thanks for the help

 

[Cool Beans]

Hey Erhatstil,
SN1 rxns must be carried out in protic solvents, whereas for SN2 it is just a very strong preference that they be carried out in aprotic solvents. Some of the larger nucleophiles like SH- do SN2 just fine in protic solvents.
So don't look at the solvent first to determine whether the substitution rxn will be SN1 or SN2; look at the nucleophile 1st to see if it is strong (SN2) or weak (SN1) as this will be the most telling indicator.

 

[Erhatstil]

i figured that was prob the case since so many E2 reactions work in protic solvents.

Thanks for the help. Much appreciated!

 

[Maygyver]

I think a lot of times you might see E2 reactions happening in protic solvents that are also the conjugate acids of the strong base. So like ethoxides with ethanol, etc. I think the acetoacetic acid synthesis or whatever it's called is like that too. For the most part, pay attention to the substitution of the carbons for determining SN1 SN2, etc.