Halides weak or strong nucleophiles?

[dentalwhiz]

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Im studying for the DAT and have come across conflicting notes. Mike Christensen from DBC states that halogens* are weak nucleophiles and undergo SN1 rxns; my notes from school say that halogens* are strong nucleophiles, but weak bases and undergo SN2 rxns. I searched online and can't really find a straightforward answer, any clarification would be greatly appreciated

Edit: DBC also says RCOO- groups are weak nucleophiles due to resonance delocalization and undergo SN1 runs. My notes say that RCOO- is a strong nucleophile/weak base and do SN2...

 

[smilesandwaves]

From what I have learned, if you have something like HBr, it will participate in SN1, but if you have the salt, like NaBr (which would separate to Na+ and Br-), then you would do SN2. For halogen exchange in SN2, just make sure that your leaving group is not a better nucleophile (for example, F- can replace Br-, but not vice versa)

 

[dentalwhiz]

Im not sure if your familiar with his work, but on his quiz (8.1) the first question he reacts a secondary carbon w/ an Li-I compound and he states its SN1, and his reasoning only includes that halogens are an exception. Any insight on this?

 

[dentalwhiz]

If someone could please give an explanation for this... I thought i would breeze through this section, but these conflicting notes are holding me back big time right now

 

[smilesandwaves]

hm then I'm confused now as well lol

 

[DAT Destroyer]

Im studying for the DAT and have come across conflicting notes. Mike Christensen from DBC states that halogens* are weak nucleophiles and undergo SN1 rxns; my notes from school say that halogens* are strong nucleophiles, but weak bases and undergo SN2 rxns. I searched online and can't really find a straightforward answer, any clarification would be greatly appreciated

Edit: DBC also says RCOO- groups are weak nucleophiles due to resonance delocalization and undergo SN1 runs. My notes say that RCOO- is a strong nucleophile/weak base and do SN2...

Let me settle this......Halogens such as I- are EXCELLENT reactions when used in a polar protic solvent. The order of reactivity will be I- >Br->Cl> F-. As you can see Iodine is best. Fluorine is the worst because the polar solvent solvates the ion and renders this nucleophile useless since it is captured in a hydrogen bonded molecular solvent cage. Now....let's change the solvent to polar aprotic such as HMPT, DMSO, THF, or acetone. We now see the order is reversed ! F- > Cl- >Br->I-. Hopefully you see that F- will be a very good nucleophile. Through the many years I have been doing organic synthesis, I always was able to install an Fluorine onto a compound via an SN2 reaction using NaF in a solvent such as DMSO. For your other question, your notes are correct. Acetate ion is a fairly good nucleophile and prefers to do the SN2 reaction mechanism. I have a few challenging DESTROYER questions on nucleophilicity that I hope you will enjoy.

I hope this helps.

Dr. Romano