Factors Affecting Acidity Overview

Why is this section important? Because not all acid-base reactions are that easy.

When to Use Factors Affecting Acidity

There are two situations in particular that making predicting equilibrium challenging:

  1. You don’t know the pKas of the acids (so how can you tell equilibrium?)
  2. The pKas for the acid and the conjugate acid are both the same (again, what do we do?)

No matter what, we know that the stronger acid will have the more stable conjugate base. Remember, reactivity and stability have that inverse relationship we talked about. 

The Element Effect

This effect describes the way different atoms donate protons. For example C-H vs. N-H. 

It consists of two trends:

  1. Electronegativity (EN) – the stronger the EN, the more stable the conjugate base will be with extra electrons.
  2. Size – The bigger (squishier) the atom, the less the conjugate base “feels” extra electrons.

NOTE: This effect can only be used when comparing the way different atoms are attached to hydrogen. If you are comparing O-H vs. another O-H, it won’t work!

Example: Without using pKa values, which of the following pairs is more acidic?

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Example: Without using pKa values, which of the following pairs is more acidic?

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Example: Without using pKa values, which of the following pairs is more acidic?

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Example: Without using pKa values, which of the following pairs is more acidic?

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The Inductive Effect

This effect describes the way that electronegative atoms that are NOT CONNECTED to the acidic proton make the conjugate base more stable. 

Any EN force that helps to pull electrons away from the conjugate is called an inductive effect.  If you can spread out that negative charge over multiple atoms, that base will be more stable. 

Example: Using electron clouds to understand the inductive effect.

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Concept: The 3 factors that determine the strength of inductive effects.

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Factors that increase inductive effects:

  1. Strength of EN forces: -F > -Cl > -Br > -I
  2. Number of EN forces: The more the better
  3. Closeness of EN forces: The closer the better

Example: Without using pKa values, which of the following pairs is more acidic?

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Hybridization Effects

The higher the %s-character of the conjugate base, the more stable it will be.

  • Recall, %s-character = (s-orbitals)/(total hybrid orbitals) x 100.
  • Aka, sp3  = ¼ = 25% s-character

Example: Which of the following hydrocarbons is the most acidic? 

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Steric Effects

This rule really only applies with alcohols for now.

Particularly with alcohols, the more easily solvated the conjugate base is, the more stable it will be.

  • The smaller the R group, the more acidic the alcohol
  • The bigger the R group, the more basic the alkoxide

Example: Which of the oxides is the most basic? 

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