By the end of this guide,
You’ll learn how to identify the most acidic proton in a molecule, a fundamental skill in organic chemistry.
Let’s dive in!
Challenge 1
Question:
In the following molecule, which of the labeled protons is the most acidic?
Step 1: Draw the corresponding conjugate base for each proton
Let’s draw the conjugate base for each labeled proton.
Step 2: Evaluate and rank the stability of the conjugate bases
Next, we need to figure out how stable each conjugate base is. Remember, the stronger the acid, the more stable its conjugate base. By figuring out which conjugate base is more stable, we can identify the most acidic proton.
To do this, we’ll look at five factors: Charge, Atom, Resonance, Induction, and Orbitals.
Charge
What is the charge on each conjugate base?
Each base carries a negative one (-1) charge.
Conclusion: The charge factor doesn’t help us here since all the bases have the same charge.
Atom
Which atom is holding the negative charge?
In conjugate bases a, b, and c, the negative charge is on carbon.
Conclusion: The atom factor doesn’t help us here either since the negative charge is on the same atom in all bases.
Resonance
Is the negative charge delocalized over multiple atoms?
Conjugate base a
The negative charge is ‘stuck’ on the carbon atom (localized).
Conjugate base b
The negative charge is spread out over two carbon atoms and one oxygen atom.
Conjugate base c
The negative charge is spread out over one carbon atom and one oxygen atom.
Conjugate bases b and c are stabilized by resonance, while a is not. This makes a the least stable conjugate base and proton a the least acidic of the three protons.
🤔 But, between b and c, which is more stable?
Conjugate base b
The negative charge is spread out over three atoms (two carbon atoms and one oxygen atom).
Conjugate base c
The negative charge is spread out over two atoms (one carbon atom and one oxygen atom).
This makes conjugate base b more stable and the most stable of all three conjugate bases.
Step 3: The more stable conjugate base corresponds with the more acidic proton
Since conjugate base b is the most stable conjugate base, proton B is the most acidic of the three highlighted protons.
By following these steps, we see that proton b is the most acidic because its conjugate base is the most stable. Keep practicing with different molecules to get the hang of identifying the most acidic proton!
