Whats a Proton anyway?

Well a hydrogen atom has one proton and one electron.  So a proton is a hydrogen atom without an electron.  Now a proton can't really exist on its own, so we usually show them bonded to some base (lone pair of electrons).  Most organic chemist hydrogen and proton interchangeably.


Arrhenius Acids/Bases

The earliest experiments to understand the nature of acids and bases were concerned with substances that would either increase hydrogen ion (proton) H+ concentration or increase hydroxide ion concentrations in aqueous (water) solutions.  These substances were termed acids and bases respectively.  For example, the addition of HCl to water increases the H+ concentration in aqueous solutions.  Likewise, NaOH is considered a base since it increases the -OH concentration when added to water.

At the time of these studies, the nature of H+ ions was unclear.  They assumed that protons could exist as "bare naked" protons.  Today we know that protons exist as hydronium ions.  A bare proton is simply an empty 1s orbital.  In a hydronium ion, the empty 1s orbital of the proton interacts with lone pairs of a water molecule forming a covalent bond.

Bronsted-Lowry Acids/Bases

The Arrhenius definition was important since it provided the concept of protons (H+) as acids and hydroxide ions (-OH) as bases.  The Bronsted-Lowry definition of acids/bases provides a more complete definition in which substance are classified as acids if they donate a proton and bases if they accept a proton.  This definition also introduced the concept of conjugate acids and conjugate bases.  The conjugate acid or base is simply the acid or base in the reverse direction.  The acid becomes a conjugate base while the base becomes the conjugate acid.


Take Note

Note that only the proton (H+) is lost from the acid leaving the bond electrons behind as a lone pair of electrons.  Likewise, the base receives the proton (H+).  One of the lone pairs of the base become the bond.  Keep track of your electrons!

Lewis Acids/Bases

While Bronstead acids/bases definition is concerned with protons, the Lewis acid view is from the perspective of the lone pairs involved.  A Lewis acid is an electron pair acceptor, while a Lewis base is an electron-pair donor.  Some molecules and ions behave like protons (acids) and they don't even possess a proton.  For example, boron and aluminum halides (BCl3 and AlCl3) are notorious Lewis acids.  Its the electronic structure of these molecules that make them good Lewis acids.  Each has an empty p orbital that can accept electron density.  Remember acids are just empty orbitals or electron sinks!  A proton is an empty s orbital, while BF3 has an empty p orbital (electron sink).

Lewis Acid

Just like Bronstead acid-base reactions, a Lewis acid will react with a Lewis base.  In the following example, the molecule on the left donates its lone pairs (Lewis base) to an empty p orbital of BH3.  This forms a Lewis Acid/Base complex as shown.


Take NoteRecall that a Lewis acid is an electron sink.  A common motif is that Lewis acids sometimes have an sp2 hybrid atom that has an empty p orbital.    CH3+  and BH3  would be considered Lewis acids.  FeCl3 is considered a Lewis acid, it has empty d orbitals.