Nucleophiles

Common Electron Sources

Electrons must flow from somewhere, we call this an electron source.  An electron source can be a lone pair of electrons (Lewis bases) or a bond (σ or π).  The electrons are typically located in the highest occupied molecular orbital (HOMO).

Lone pairs (Lewis Bases)

Lewis bases are some of the most ubiquitous electron sources we will encounter.  Most structures with neutral or negatively charged O, N, P, S atoms are Lewis bases.  For example the acid base reaction between ammonia and acetic acid.  The first arrow begins at the N lone pairs (electron source or Nucleophile) and ends at the hydrogen atom (the sink or electrophile).

Here is another example between 2-propanol and HCl.

Electron Rich σ bonds

Organo Lithium/Sodium (LiR/NaR), Grignards (RMgX), Sodium Alkoxides (NaOR) and Lithium Amides (LiNR₂)

Organo lithium/sodium, alkoxides, lithium amides and Grignard reagents behave as very strong bases.  When C, O, or N is bound to Li, Na or MgX the bond is very highly polarized in the direction of the C,O or N atoms since Li/Na/Mg are less electronegative.

In fact most organic chemists would consider these bonds to be ionic and dissociate under typical reaction conditions in the presence of appropriate solvents.  Regardless when you see RLi or RNa or RMgX etc you should think of them as follows.

Here is an example of the methyl carbanion acting like a source (nucleophile).

π-bonds in Alkenes and Alkynes

Recall that π bonds are formed by overlap of adjacent p orbitals (Valence bond theory) and as such have region of higher electron density above and below the place of the molecule.

Both alkenes, alkynes and aromatics π systems will behave as electron sources.  In this example an alkene is reacting with an acid to form a carbocation.