Nucleophile and leaving groups in more detail
Nucleophiles are electron rich species and may be anionic or molecular species with no charge.
Nucleophiles that react by substitution at saturated carbon have nonbonding electrons at one atom.
In general the nucleophilicity of a species depends on
whether the nucleophile is an anion (negatively charged) or a molecule (no charge)
When comparing the same atom, anions are stronger than the corresponding molecule.
> 
>
the size of the nucleophilic atom
the electronegativity (Electronegativity is the tendency of a bonded atom to attract the bonding electrons to itself. In the bond H-X where X has a higher electronegativity than H, there is unequal sharing of the bonding electrons, and the H is slightly positive and the X is slightly negative. The range of electronegativity values for main group elements is given in a table in the Glossary (see Options).) of the nucleophilic atom.
The leaving group is displaced from the organic substrate when it reacts with a nucleophile.
Better leaving groups are weaker bases.
Nucleophiles that react by substitution at saturated carbon have nonbonding electrons at one atom.
In general the nucleophilicity of a species depends on
whether the nucleophile is an anion (negatively charged) or a molecule (no charge)
When comparing the same atom, anions are stronger than the corresponding molecule.
> >Larger atoms are better nucleophiles.
>
> 
> 
> 
>> > > the electronegativity (Electronegativity is the tendency of a bonded atom to attract the bonding electrons to itself. In the bond H-X where X has a higher electronegativity than H, there is unequal sharing of the bonding electrons, and the H is slightly positive and the X is slightly negative. The range of electronegativity values for main group elements is given in a table in the Glossary (see Options).) of the nucleophilic atom.
Less electronegative atoms hold their nonbonding electrons less strongly and are better nucleophiles.
>
>The leaving group is displaced from the organic substrate when it reacts with a nucleophile.
Better leaving groups are weaker bases.
H2O > NH3 > HO–
Better leaving groups have weaker C-X bonds. I– > Br– > Cl–> F–