Makomed's Weblog

How to Choose the Best Solvent for Nucleophilic Reactions

Posted on: May 19, 2009

This is a thread between my lab partner and I:

ME: Still having trouble with the DIELECTRIC CONSTANT and how it pertains to the EQUILIBRIUM CONSTANT. Can anybody explain? (Bonus points if you use an analogy involving unicorns, haha jk, does anybody read these things anymore?).

SAM: In general, the large dielectric c greater polarity. When your doing in reaction in which one of the reagents is a charged molecule, then then your don’t want to use a very polar solvent, because the polar solvent is going to decrease the reactivity of the reagent. Here is a simple example. if you put Na+ and Cl- in water, its not going to come together to form table salt. This is because the polar water molecules form a shell around the charged species. Same thing happens if you have a negatively charged (strong) base that is trying to attack an alkyl halide. since the base is negatively charged, a polar solvent (especially if its protic) will surround the base, or worse yet, donate a hydrogen to it. Either way, its neutralized and it can’t attack.

All in all, since a lot of the reactions that we deal with are reversible, you don’t want you reactants to be stable; then the reaction will have a higher tendency towards the reactants side rather than the product. A “stable reactant” is an oxymoron if you think about it

Stabilizing the transition state is a good thing though. If you stabilize the transition state, then your decreasing the activation energy for the reaction. if you have uncharged reagents, chances are very good that the transition state will have charged characteristics, so a polar solvent would stabilize it.

Oh, and the more favored a reaction is towards the product side, the higher the Keq. So:

charged reactants, polar solvent –> smaller Keq

uncharged reactant, polar solvent –> larger Keq

ME: Your explanation on why the sodium and chloride ions don’t automatically form table salt in water was exactly what I needed to put solvents into perspective! I had been focusing on the leaving groups and nucleophiles all this time I forgot all about the environment that the molecules had to contend with.

So you say the choice of solvents also affect the reaction and its rate? And a more protic (I’m going to assume that’s synonymous with being more polar) solvent would have a higher dielectric constant that forms an effective insulating “barrier” around charged molecules?

Then would I be correctly restating what you explained if I say it this way?:

1. PROTIC SOLVENT in an SN1 —> larger Keq —> makes the rxn go FASTER to the right
2. a. PROTIC SOLVENT in an SN2 (charged nucleophile)—> smaller Keq —> makes the rxn go SLOWER to the right
b. PROTIC SOLVENT in an SN2 (neutral nucleophile) —> larger Keq —> makes the rxn go FASTER to the right

Are these conditions similarly applicable to their E1 and E2 counterparts??

For APROTIC SOLVENTS would the opposite be true? Would it reverse what I just said about the reaction times or would it affect Keq in the same way, albeit to a lesser degree??

Wow, you just opened up a whole slew of other questions. But thanks for responding, man!

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