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:

 

What determines the strength of an acid? A hierarchy of 4 factors

 

 

 

 

 

  1. 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.

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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.

Example

Which molecule is more acidic?

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Answer

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We first identify that the most acidic proton on each molecule is the –OH and -NH2 respectively.  We then write down the corresponding conjugate bases:

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The question now is: which of these two atoms (the oxygen or the nitrogen) is better able to stabilize the negative charge?  Remember: the more stable the conjugate base, the stronger the acid.

Oxygen and nitrogen are in the same row of the periodic table, so we assess each atom’s electronegativity.  Because oxygen is more electronegative than nitrogen (essentially meaning it wants electrons more), it is more stable to have an oxygen atom that carries a negative charge.  Because the conjugate base is more stable, it must arise from the stronger acid.

 

  1. 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.

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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.

 

Example

Which of the following molecules is more acidic?

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Answer

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We first find the most acidic proton.  In each case, this can be found on a N atom.  We then draw out the conjugate bases:

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According to our hierarchy, we first compare the atom on which the negative charge resides.  Since both are on nitrogen atoms, we then look to see if loss of a proton in either case can lead to resonance stability.  The first structure drawn can be stabilized by a resonance structure (see if you can draw it), while the second can’t.  The resonance structures add stability to the conjugate base, making the molecule more acidic.

 

  1. 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.

Example

Which of the following molecules is more acidic?

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Answer

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We first identify the most acidic proton on each molecule, and we see this is the proton attached to the oxygen atom (an OH group) on each.  We then draw out the conjugate bases:

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After seeing that the negative charge is on the same kind of atom (both oxygen) and that there are no resonance structures, we then look at inductive effects.  Both atoms have an electronegative atom near the negative charge; however, Cl is more electronegative that Br, so it is able to pull more electron density away from the negatively charged oxygen, more effectively stabilizing the conjugate base and making the precursor molecule more acidic.

 

  1. 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).

Acids & Bases Graphics 1

 

 

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Example

In which direction does the equilibrium lie in the following equation?  Assume that the hydrogen explicitly shown is the most acidic proton.

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Answer

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This is simply an acid/base question in disguise.  Remember, a strong acid wants to dissociate from one of its protons, so we must only find which acid is the stronger of the two.  When we establish which acid is stronger, the equilibrium will lie towards the opposite side because the stronger acid will dissociate more.  We first compare the conjugate bases, both of which are already given to us:

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After seeing that both negative charges are on a carbon atom, no resonance structures are possible, and no inductive forces are present, we next turn to examining the hybridization state of the atom that carries the negative charge.  The left structure’s negative charge is on a double bond, so the electrons are in an sp2-hybridized carbon.  The second structure has the negative charge on a triple bond in an sp-hybridized carbon.  Because sp-hybridized atoms can better stabilize negative charge (due to its greater “s character”), this conjugate base is more stable, making it’s precursor a stronger acid.

We know the triple bond acid is the stronger acid.  Therefore, this molecule will dissociate (or break apart) from its most acidic proton more easily so that the equilibrium will lie more strongly towards the right.

 

 

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.