How are Acids and Bases Different?

How are acids and bases different? What are conjugate acids and bases? How do you draw a curved arrow in organic chemistry. All this is covered in this section.


A Brønsted acid (we’ll often just call this an acid) is a molecule that is an H+ (proton) donor.  This is a slight simplification, but for the sake of Introductory Organic Chemistry, this definition will suffice.  In reality, the trait of acids that makes them proton donors derives from their desire to increase their collection of electrons by releasing a proton (an H+). Some examples of acids are HCl, H2SO4, and H3O+.  For example, H3O+ is an acid because, by breaking an O–H bond, the oxygen atom no longer has to share the two electrons (it “owns” them exclusively).  Again, note that this can be simplified by just calling the H3O+ molecule a proton donor.  Acids always have hydrogens that they’re willing to give up (the stronger the acid, the more willing), and they sometimes have a positive charge.


A Brønsted base (we’ll often just refer to this as a base) is a proton acceptor.  Yet again, this is a simplification of the true chemistry here, but this remains sufficient for Introductory Organic ChemistryIn more rigorous terms, the trait of a Brønsted base that truly makes it a base is that the molecule is relatively willing to give away its electrons, and it does this by sharing a pair of electrons with a proton.  Thus, the fact that bases are proton acceptors is a consequence of their willingness to give away electrons. Some common bases are NH3 and OH.  Bases always have a lone pair of electrons that are reasonably high in energy and they like to share with a proton (H+).  Bases often have a negative charge.


Quick summary of how are acids and bases different: Acids give away protons, while bases want to take in protons.


Acid and base reactions have the general form that looks like this:

How are acids and bases different? Acids want to give away protons, while bases want to take them in.





What are conjugate acids and bases?

The deprotonated form of the acid is called the conjugate base.  The protonated form of the base is called the conjugate acid.  It’s important to understand “what are conjugate acids and bases” for the remaining sections of this chapter.

Let’s look at how you would draw the arrows for acid/base reactions.  Remember:


How to draw a curved arrow in organic chemistry







From this figure, we can see that the tail of our arrow will begin at the base (because there is at least 1 lone pair of electrons on a base), and the head will be at the proton.  Here is an example that shows the curved arrows with HCl as the acid.


What are conjugate acids and bases? They are the products of an acid base reaction.




The longer equilibrium arrow that points to the right shows that the equilibrium tends to lie preferentially in this direction.  This is because strong acids tend to dissociate (come apart), and HCl is a strong acid.

The term pKa is often used as the quantitative measure for the strength of an acid.  The lower the pKa, the more acidic the molecule is.  The higher the pKa, the weaker the acid (and its conjugate form is therefore a stronger base).