[ariaxxx]

What would the problem be? A strong nucleophile can just as well attach to the bonding site after a leaving group leaves right??? Wouldn't a strong Nu be better?

[izchief360]

Consider charge. In an SN1 reaction, we have a very good leaving group that just leaves, creating a carbocation. Now, we have two charges. We have a positive charge on the substrate and a negative charge on the leaving group. If we had a strong nucleophile (generally, a strong nucleophile is negatively charged) then it will be able to react with the substrate and neutralize the positive charge, but the problem arises when considering the negative charge on the leaving group. How can it neutralize its charge?

A weak nucleophile is neutral, so when it reacts with the substrate, the positive charge doesn't get neutralized. Instead, the positive charge jumps onto the newly attached nucleophile and the leaving group is able to neutralize itself (usually by pulling off a proton). This gets rid of both the negative charge on the leaving group and the positive charge on the newly attached nuc.

[Dan]

izchief360 - I do not follow your argument at all, can you clarify?

What would the problem be? A strong nucleophile can just as well attach to the bonding site after a leaving group leaves right??? Wouldn't a strong Nu be better?

The rate of S_N1 reactions is nucleophile independent. The use of a strong nuclephile vs a weak nucleophile has no effect whatsoever on the rate of S_N1. Strong nucleophilies do not favour S_N1, but they don't inhibit it either - they can participate in S_N1 reactions depending on the other factors at play.

Hint: What competing pathways can strong nucleophiles promote?

[azmanam]

In my mind, it all has to do with electron density and energy. See here for how I think about it:

http://www.masterorganicchemistry.com/2013/11/12/guest-post-on-sn1sn2e1e2-3-step-1-the-nucleophile/

To be a ‘1’ reaction, the nucleophile has to wait around long enough for the leaving group to spontaneously leave on its own. If the nucleophile is strong enough to invoke one of the ‘2’ mechanisms without having to wait around for the carbocation, that mechanism will dominate – it won’t give the electrophile enough time to form the carbocation. It doesn’t have to. It has plenty of excess energy – more than enough to go straight to the ‘2’ mechanism.

[orgopete]

What would the problem be? A strong nucleophile can just as well attach to the bonding site after a leaving group leaves right??? Wouldn't a strong Nu be better?

While this seems a reasonable premise, molecules may not follow this logic. If the concentration of a reactant is changed, it will alter the number of collisions that occur. In an SN1 reaction, formation of a carbocation must precede the nucleophile reaction with the substrate. Using a higher concentration, stronger base, or stronger nucleophile will not increase carbocation formation. It will do the opposite. It will catalyze an E2/SN2 reaction. In order to increase the rate of carbocation formation, you need to suppress E2/SN2 reactions. The opposite is weak bases and poor nucleophiles.

I think there is more to these reactions that that. A common kinetic experiment done in some labs is to treat dilute solutions of t-butyl halides with varying concentrations of water and solvents. NaOH is the base, but because the concentrations are low, a solvent dependent carbocation formation is a kinetically faster reaction. Water increases the rate of carbocation formation.

[Babcock_Hall]

Ingold Lecture. How does a reaction choose its mechanism?
W. P. Jencks  Chem. Soc. Rev., 1981,10, 345-375
DOI: 10.1039/CS9811000345

I used to have a paper copy of this article...