A negatively charged species can function as nucleophile as well as like base but its nucleophilicity and basicity are different. Nucleophilicity of the species is the ability of the species to attack an electrophilic carbon while basicity is the ability of the species to remove H+ from an acid. Let us have a species, B-.
RS- > ArS- > I- > CN- > OH- > N3 -> Br- > ArO- > Cl- > pyridine > AcO- > H2O
The nucleophilicity is determined by the kinetics of the reaction, which is reflected by its rate constant (k) while basicity is determined by the equilibrium constant, which is reflected by its Kb.The order of nucleophilicity of different species depends on the nature of solvent used.
For instance, let us take F-, Cl-, Br- and I- with their counter cation as Na+ and see their nucleophilicity order in different solvents. There are four categories of solvents, namely non-polar (CCl4), polar protic (H2O), polar aprotic (CH3SOCH3) and weakly polar aprotic(CH3COCH3)
Polar solvents are able to dissociate the salts i.e. ion-pairs can be separated. On the other hand, non-polar and weakly polar solvents are unable to dissociate salts, so they exist as ion-pairs. The ion-pairing is strong when ions are small and have high charge density In non-polar and weakly polar aprotic solvents, all the salts will exist as ion-pairs.
The ion-pairing will be strongest with the smallest anion (F-) and weakest with the largest anion (I-), thus the reactivity of X- decreases with decreasing size. Thus, the nucleophilicity order of X- in such solvents would be
I- > Br- > Cl- > F-
In polar protic solvents, hydrogen bonding or ion-dipole interaction diminishes the reactivity of the anion. Stronger the interaction, lesser is the reactivity of anion. F- ion will form strong H-bond with polar protic solvent while weakest ion-dipole interaction will be with I- ion. Thus, the nucleophilicity order of X- in polar protic solvent would be I- > Br- > Cl- > F-.Polar aprotic solvents have the ability to solvate only cations, thus anions are left free.
The reactivity of anions is then governed by their negative charge density (i.e. their basic character). Thus, the order of nucleophilicity of X- in polar aprotic solvents would be
F- > Cl- > Br- > I-
(i) In polar protic solvents, HS- > HO-
(ii) In weakly polar aprotic solvents, CsF > RbF > KF > NaF > LiF
(iii) Bases are better nucleophiles than their conjugate acids. For example, OH- > H2O and > NH3
(iv) In non-polar solvents, -CH3 > -NH2 > -OH > -F
(v) When nucleophilic and basic sites are same, nucleophilicity parallels basicity. For example, RO- > HO- > R - CO - O-
RS- > ArS- > I- > CN- > OH- > > Br- > ArO- > Cl- > pyridine > AcO- > H2O
Some nucleophiles have lone pair of electrons on more than one atom and can attack through more than one site. Such nucleophiles are called ambident nucleophiles. In such cases, different products are possible due to attack through different sites. Attack by a specific site can be promoted under special conditions. Two well-known examples are discussed in detail
Attack By CN- Nucleophile (-:CN:)
R-X R-CN → R-NC + X-
In CN-, carbon (negatively charged) will be a soft base as compared to nitrogen. So, if the reaction proceeds via SN1 mechanism, which produces a free carbocation (a hard acid), then attack through nitrogen (hard base) will take place. But if the reaction proceeds via SN2 mechanism (small positively charged carbon is soft acid) then attack through carbon (soft base) will take place. So, if we want to increase relative yield of nitriles, we can use NaCN or KCN etc in a less polar solvent, which facilitates SN2 substitution. Similarly, if we want to increase the yield of isonitriles, we can use AgCN. Ag+ has very strong affinity for X-, so it favours the formation of R+ and the reaction proceeds via SN1 mechanism. This will result in attack by hard base giving R-NC. Further if we compare primary, secondary and tertiary alkyl halides, formation of R-NC should be favoured due to more favourable SN1 substitution in tertiary alkyl halide. But the exception is that tertiary alkyl halides undergo elimination and the yield decreases. This is because CN- is a strong base, which can also cause elimination reaction.