Chad says halogens are strong nucleophiles, thus, reactions with halogens will proceed SN2.
However, DAT Bootcamp says the electron on halogens are delocalized (therefore, is weakened) so Cl-, Br-, and I- are weak nucleophiles, which will make reactions go SN1.
I asked Dr. Mike at DAT Bootcamp about this and he gave me very thorough response:
Some sources claim that halides are strong SN2 nucleophiles, such as the example webpage https://tigerweb.towson.edu/jdiscord/www/331_problem_sets/chapter10_11/nucleophiles_overview.pdf.
This particular webpage gives the specific examples of NaI, LiBr, and KI as being strong SN2 nucleophiles. You’ll notice that it does not give NaBr, KBr, or LiI. Why not? Because these are all weak nucleophiles. Thus, the identity of the metal (K, Li, or Na) can alter the nucleophilicity of the halide. So can solvent, but I’ll ignore that for now. You’ll also notice that there are no chloride or fluoride examples given on this webpage.
The question is: what is the driving force behind a halide being a strong or a weak nucleophile? Is it the halide itself? The answer is actually no. In reality, it’s driven by the strength of the bond that forms between the leaving group (LG) and the metal. For example, if I react an alkyl-bromide with NaI, iodide will do an SN2 reaction to displace the bromide.
Why is that? Is Br- a better LG than I-? Surely not, because I- is much more stable than Br-, so an I- shouldn’t be able to take the place of a Br-. So what’s going on?
As it turns out, an Na-Br bond is much shorter and stronger than an Na-I bond. Thus, when I- displaces a Br, the Br- forms NaBr, which precipitates out of solution. This thereby prevents the Br- from coming back and re-displacing the I in the original electrophile. This whole issue is called the Finkelstein reaction (Finkelstein reaction - Wikipedia) and is driven by the LG-metal bond being stronger than the nucleophile-metal bond. Does that make sense?
So to be absolutely technically perfect, I would have to say that halides are strong nucleophiles under certain conditions and weak nucleophiles under other conditions. However, to avoid making you learn tons of complicated nuances, I instead just have you learn that halides are weaks (SN1) unless the question specifically indicates otherwise. This is summarized pretty clearly in the flowchart on Page 5 of the attached document.
Now, as further evidence to support my case: as you probably know, the binary haloacids HCl, HBr, and HI are very strong acids. Furthermore, you probably also know that “the stronger the acid, the weaker its conjugate base.” Because HCl, HBr, and HI are all strong acids, their conjugate bases (Cl-, Br-, and I-) must be weak bases. Does that make sense?
Now, strictly speaking, basicity is a subcategory of nucleophilicity. Nucleophilicity, in general, is a substance’s ability to bond with another atom. Basicity is the same thing, except SPECIFICALLY with hydrogen. (For a great resource on this, check out Nucleophilicity vs. Basicity — Master Organic Chemistry.)
It follows, then, that if Cl-, Br-, and I- are weak bases (which they are, because HCl, HBr, and HI are strong acids), then they must also be weak nucleophiles. Now, that does NOT mean that they can never do SN2 reactions (as I explained above). However, in my effort to keep things simple (once again), I teach it that way.
As you might imagine, a halide CAN displace a leaving group SN2 style under certain conditions, IF the leaving group is a better one than the halide itself. Additionally, under Finkelstein conditions, halides can do SN2 reactions. However, this occurs not because the halide is a strong nuc, but because the metal-LG bond (such as Na-Br) that gets formed is very strong and makes a precipitate, which removes the LG from re-displacing the newly-added halide.
which makes sense.
But I just do not know who to follow when I'm actually taking DAT.
Let me settle this. Flummoxing indeed at the confusion. I have worked with halogens in SN2 and SN1 reactions for over 30 years. A halogen can be a great nucleophile or a horrible nucleophile depending on the solvent. In polar aprotic solvents such as DMSO, HMPT, acetone, DMF,,,,the order is F- > Cl- > Br- > I-. Thus if you carried out an SN2 reaction using NaF, the solvent needs to be polar aprotic for best results. Exceptions do exist, but not much where the halogens are concerned. I have been most successful using what is called a crown ether. If you change the solvent to polar protic,,,,F- is HORRIBLE, since it becomes solvated and captured in what I call a " molecular solvent cage ". Thus in solvents such as water....I- is the BEST nucleophile. Thus the order is I- > Br->Cl- >>F-. Thus as you can see, this question depends mainly on the solvents. Solvents are a complicated business. I have read books that span hundreds of pages on solvent discussion. Get a text written by a PhD organic chemist such as David Klein, Francis Carey, Skip Wade, or Peter Vohlhardt if you need further clarity.
As far as " strength " goes.......SH- is usually much stronger than any of the halogens.....PH3 is also very powerful. The reason for this lies on the border of Quantum-Physical Chemistry and Advanced Organic Chemistry. Simply put....a super " strong " nucleophile has a high energy HOMO......From the data I have seen presented by Ian Fleming PhD of Cambridge,,,,,none of the halogens, nor matter the solvent are as good as CN-, SH-, PH3, or HOO_. For the DAT,,,,,,as long as you understand the order I have presented,,,,,which is also seen in any standard organic text.....you will be fine for the DAT exam.
I hope this helps.....
Dr. Romano
