The Structure of Alkenes

When we first discussed the concepts of nucleophiles and electrophiles, we saw that nucleophiles are electron-rich species eager to donate electrons, while electrophiles are electron-deficient and seek to accept them. Alkenes, with their characteristic carbon-carbon double bond, are examples of nucleophiles.  Let's look at the simplest alkene, ethene shown below. 

From a valence bond argument, we would say that the sigma bond is a result of the overlap of two sp2 hybrid orbitals one on each carbon.  The π bond results from sideways overlap of two p-orbitals, resulting in significant electron density above and below the plane of the molecule. This concentration of electrons makes the pi bond highly nucleophilic and reactive toward electrophiles.  This unsymmetrical overlap is also prevent rotation about the two carbon atoms.

Molecular orbital theory show a similar picture.  Below is the quantum mechanically derived HOMO (i.e Highest Occupied Molecular Orbital) shows that the electrons in the π bond are high in energy and readily available for interactions with electron-deficient species. This explains the alkene's role as a nucleophile in addition reactions.

Action

Rotate the QM derived HOMO shown below.  The red and blue indicate the "Phase" of this molecular orbital.  You can think of the blue and red as where the electrons are located.  Notice how these regions are above and below the plane of the molecule.

 

The electrostatic potential map shows where the electron density is located.  The yellow areas are regions of higher electron density.  These areas indicate where alkenes are most likely to react with electrophiles, reinforcing their nucleophilic character.

By connecting structure with reactivity, we see that alkenes are primed for interactions with electrophiles, driven by the accessible electron density of the π bond. This nucleophilic behavior is a fundamental aspect of alkene chemistry, making them key players in various reactions.