A negatively charged species can function as nucleophile as well as like base but its nucleophil icity and basicity are dillerent. Nucleophilicity or 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^(+)` . Its function as a nucleophile is shown as

(IMAGE)

and its role as base is indicated as

`B^(⊖) + H - A ⇋ B - H +A^(⊖)`

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 retlected by its `K_b`.

The order or nucleophilicity of di!lerent 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 `(CCl_4)` , polar protic `(H_2O),` polar aprotic `(CH_3SOCH_3)` and weakly polar aprotic `(CH_3COCH_3)`.

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 u- l, thus the reactivity of `X^-` decreases with decreasing size. Thus, the nucleophilicity order of `X^-` in such solvents would be

`I^(-) > Br^(-) > F^(-)`

In polar protic solve nts, hyd rogen 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^(-)`

On this basis, certain nucleophilicity orders are

(i) In polar protic solvents, `HS^(-) > HO^(-)`

(ii) In weakly polar aprotic solvents, `CsF > RbF > KF > Naf > LiF`

(iii) Oases are better nucleophiles than their conjugate acids. For example,

`OH^(-) > H_2O` and `NH_2 > NH_3`

(iv) In non- polar solvents, `text( )^(-) CH_3 > text( )^(-) NH_2 > text( )^(-) OH > text( )^(-) F`

(v) When nucleophilic and basic sites are same, nucleophilicity parall els basicity. For example,

`RO^(-) > HO^(-) > R- CO- O^(-)`

(vi) When the atom bonded to nucleophilic site also has an tmshared pair of electrons, nucleophilicity of species increases. ror example,

`HOO^(-) > HO^(-)` and `H_2overset(. .)N - overset(. .)NH_2 >overset(. .)NH_3`

Edwards and Pearson gave following order of nucleophilicity for `S_N2` reactions in protic solvents.

`RS^(-) > ArS^(-) > I^(-) > CN^(-) > OH^(-) > N_3^(-) > Br^(-) > ArO^(-) > Cl^(-) > text(pyridine) > AcO^(-) > H_2O.`

Some nucleophiles have lone pair of electrons on more than one atom and can attack through more than one site. Such nucleophiles are called wnbident nucleopltiles. In such cases, different products are possible due to attack through clifferent sites. Attack by a specific site can be promoted under special conditions. Two well - known examples are discussed in detail.

`text(Attack By)` `CN^(-)` `text(Nucleophile)` `( text( )^(-) : CN : )`

`R-X + overset(CN^(-))-> undersettext(nitriles)(R-CN)+ undersettext( isonitriles)(R -NC+X^(-))`



In `CN^(-)` , carbon (negatively charged) will be a soft base as compared to nitrogen. So, if the reaction proceeds via `S_N1` mechanism, which produces a free carbocation (a hard acid), then attack through nitrogen (hard base) will take place. But if the reaction proceeds via `S_N2` 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 nitrites, we can use NaCN or KCN etc in a less polar solvent, which facilitates `S_N2` substihJtion. Similarly, if we want to increase the yield of isonitriles, we can use `AgCN . Ag^(+)` has very strong affinity tor `X^(-)` , so it favours the formation of `R^(+)` and the reaction proceeds via `S_N1` mechanism. This will result in attack by hard base giving N- NC. Further if we compare primary, secondary and tertiary alkyl halides, formation of R- NC should be favoured due to more favourable `S_N1` substimtion 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.


`text(Attack By)` `NO_2^(-)` `text(Nucleophile)` `(text( )^(-) O - N=O)`

`R-X overset(NO_2^(-))-> undersettext(alkane nitrite)(R-O-N=O ) +undersettext( nitro alkane)(R-NO_2) + X^(-)`



In `NO_2^(-)` oxygen (negatively charged) will be a hard base as compared to nitrogen. So, if the reaction proceeds via `S_N1` mechanism, then attack through oxygen (hard base) will take place to produce alkane nitr ite. l.lut if the reaction takes place via `S_N2` mechanism then attack through nitrogen (soft base) takes place to give nitro alkane .

If we want to increase the yield of nitro alkane, the reaction should proceed via `S_N2` mechanism. i.e. we can use `NaNO_2, KNO_2` etc. Moreover, the yield will be best if we use primary alkyl halide and less polar solvent. Formation of nitrite will dominate, if we use tertiary alkyl halide, more polar solvent and `AgNO_2` because `Ag^(+)` has strong affinity for `X^(-)` and can form a carbocation to force the reaction to proceed via `S_N1` mechanism. Primary alkyl halide with `AgNO_2` chiefly gives nitro alkane but if secondary and tertiary alkyl halides are used then `AgNO_2` will yield nitrite as the major product.