The 1st involving NaSH as the nucleophile is SN2
The 2nd involving CH3OH as the nucleophile is SN1.
Both involve a secondary alkyl halide which can be used in either SN2 or SN1 so this is of no help.
The key is the nucleophiles. As Na is a metal, NaSH is ionic and therefore is really Na+ and -SH. A negatively charged nucleophile is typically strong (and -SH is indeed strong) and will lean us toward SN2.
On the other hand, CH3OH is not ionic, and is not negatively charged and is a weak nucleophile (alcohols are no more nucleophilic than H2O which is really weak itself). With a weak nucleophile, SN2 is not possible so we're left with SN1.
A general rule which works most of the time:
strong nuc (neg. charged) = SN2 (but no tertiary halides)
weak nuc (not neg. charged) = SN1
There are exceptions but, again, it works most of the time.
Anyone can explain how EWG(Electron Withdrawing Group) and weak bases are the good leaving group? Thanks.
Anyone can explain how EWG(Electron Withdrawing Group) and weak bases are the good leaving group? Thanks.
If you have a phenol (wiki it for the structure if you don't know it). If it loses an H from the OH. Then you have its carboxylate. The O- has a negative charge. It would be best if this negative charge would be spread out throughout the whole molecule. This would make it more stable (and a weaker base since the negative charge is less). So if you have a withdrawing group. It pulls the electrons towards itself making the molecule more stable and less reactive. Weaker base = better leaving group.
Hey Predent7,
A weak base is weak because it is relatively stable and therefore unreactive. That is what is needed in a good leaving group; it needs to be stable after it leaves. This is why Cl, Br and I make such good leaving groups: because Cl-, Br-, and I- are very weak bases and very stable after they leave.
