Protic solvents are the ones that have Hydrogen. Large atoms act as better nucleotides because they are able to spread their negative charge over a large space. This is the main reason why fluoride ion is not a good leaving group. Fluoride is smaller than Iodide and iodide is the best nucelophile out of all 4 halogens, since it is large and thus stable and an excellent leaving group.
I don't think this is quite right.
Iodide is only the best nuc in protic solvents, look at my answers below
Heres a sample of the right hand corner of the periodic table
C N O F
-------Cl
-------Br
-------I
**horizontal trend is always due to electronegativity since size is not a big enough difference, less electronegtave atoms like their e-'s less, therefore the nuc trend is (strong)<----- (weak)
1. first of all, protic solvents have hydrogen BONDS, ... huge difference than just hydrogen! anyway, H-bonds stabilize smaller ions better then larger ones, smaller the ion the more H-bonds the solution can make, therefore the trend goes DOWN for protic solvents. remember a less stabilized Nuc is a better one!
2. in aprotic solvents, size is not an issue becasue there is no H-bonding to stabilize the ions, therefore it follows the base trend (base's are electron donors, a good Nuc definetly needs this quality)
(not as positive about this one)3. if they have the same attacking atom, for example H20 and NaOH, (Oxygen is attacking in both cases) the better base is the better nucleophile, for the same reason as stated in 2.
you do NOT wanna spread out the charge as a nuclephile..
