What Determines the Strength of an Acid?
The question “what determines the strength of an acid?” is complex as there are many factors. In this section, we will provide a step-by-step method to ensure you know what determines the strength of an acid.
To determine the acidity of a molecule, one must first identify the most acidic proton (that is, the proton that can be most easily dissociated). This is a proton that’s normally attached to a highly electronegative atom, like O, N, S, F, Cl, Br, or I. Then, one must look at the conjugate base after the most acidic proton has been dissociated. We examine the conjugate base with a hierarchy of 4 acid/base rules to find what determines the strength of an acid:
The atom that holds the charge
After forming the conjugate base, first look at the atom on which the negative charge is located. The better the atom is at stabilizing the negative charge, the more stable the conjugate base is. The more stable the conjugate base, the stronger its corresponding acid.
To stabilize a negative charge properly, the atom must either be very electronegative (because electronegative atoms strongly seek electrons) or considerably large (because the larger the atom, or the higher its atom number, the less electronegative). If the atoms in question are in the same row of the periodic table, one should rely on the degree of electronegativity as a reliable indicator of acidity; if the atoms are in the same column, we should consider the atom’s size as well.
Delocalization of charge through resonance
One crucial principle that we must recognize: In general, charged species are not favored in nature. They are of course formed, as we have seen above in various acid/base reactions, but these happen because one atom has a very strong desire to not share electrons (acids) or has a strong tendency to share its electrons (bases).
When a charged species does form, the molecules will do what they can to shift or spread the charge; this way, more than one particular atom gets to be charged or several atoms are partially charged. Therefore, if the negative charge on the conjugate base can be delocalized, then that adds to the stability of the conjugate base (and the acid is then stronger). When you can draw a resonance structure for a molecule, you are establishing that the charge can be spread through the molecule.
In the resonance structures above, the negative charge is not just focused on one oxygen, but rather, it is spread out on 2 oxygen atoms, which results in increased stability because the charge is not localized, rather distributed within the entire molecule. In summary, the more delocalized the negative charge of the conjugate base, the more resonance structures we can draw, so the protonated precursor is more acidic.
Stabilization of charge through inductive forces
This is yet another way to stabilize negative charge on a conjugate base, making it more stable. If a particularly electronegative atom (such as O, Cl, or F) is near the negative charge, then it can lower the cost of electron accumulation (negative charge) by pulling the electron density towards itself through the covalent bonds that attach it to the site. An electronegative atom thus stabilizes the electron density by what we call an inductive effect, leading to higher acidity of the protonated form.
Hybridization state of the charged atom
Electrons that reside in an s orbital within the same atomic shell are lower in energy than those that are in the corresponding p orbitals; therefore, the more “s character” of an atom, the lower the energy of its electrons. This means an atom that is sp-hybridized (such as a carbon that is part of a triple bond) is better able to stabilize a negative charge on the conjugate base than one that is sp2-hybridized (since it has comparatively more p character, such as the carbons of an alkene), which, in turn, is better able to stabilize a negative charge than an atom that is sp3-hybridized (such as carbons within an alkane).
Our goal at the beginning of this section was to find out what determines the strength of an acid. From this section, you can see that what determines the strength of an acid is multi-factorial, but it can easily be found out by following our method.