In a substitution reaction, if the nucleophile is more basic than the solvent, will it always depro

[manohman]

In a substitution reaction, if the nucleophile is more basic than the solvent, will it always deprotonate the solvent rather than attacking the electrophile/substrate?

Normally, you want a good nucleophile (less substituted, less beta and alpah substitution), and a not too crowded electrophile for subsitution to occur.

if your nucleophile is too branched, or the electrophile is too crowded, the nucleophile will instead act as a base (assuming alpha hydrogens are available) in an elimation reactoin.

But if you have a strong nucleophile like NH2- and a weakly acidic solvent like CH3OH, and an electrophile CH3I, rather than attacking CH3I, the nucleophile deprotonates CH3OH.

NH2- + CH3I ----CH3OH---> ?

= NH3 + CH3O-


Is this a rule?

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[paingains]

where did you learn these rules? a nucleophile doesnt just start to become a base as a result of the substitutions on the to be attacked substrate. certain reagents can act as both strong nuc's and bases, weak nuc and bases, or one or the other depending on exactly what they are when deprotonated. Ex. NaOEt is a strong base and strong nuc. The branching of the substrate/electrophile along with the nuc/base will determine what type of products you get and how many. NH2 is a strong base because it is the conjugate of NH3 which has a pka of 38ish. NH2 deprotonates methanol favorably because the resultant ammonia is much less acidic than methanol. If you calculate the Keq of the reaction you will see that it is positive and thus favors the deprotonation of methanol.

that glorious OH and the end of the methyl is all you need to see where you instantly, almost instinctively know what to do. Memorize PKA's. its not that hard and will make acid/base chem a breeze. You can always employ ARIO if you are a rules based person, but that would not be fruitful on a time sensitive exam...

 

[amy_k]

I wouldn't be applying the rule "if the nucleophile is more basic than the solvent, will it always deprotonate the solvent rather than attacking the electrophile/substrate"

You should just remember that in the case where we have a strong base and an acidic solvent and a favorable acid/base equilibrium as the above post states, the acid/base reaction occurs rather than substitution because it is MUCH quicker.