🖐️ Before we start,
Make sure you’re familiar with a few key ideas:
- Brønsted-Lowry acids and bases
- How to draw the conjugate base for an acid
- Factors affecting the stability of conjugate bases
Got it? Great! Let’s get started.
Challenge 1: Ranking neutral acids by strength
Question:
What is the order of acidity from strongest to weakest acid for these compounds?
Here’s how we’ll do it:
Step 1: Draw the conjugate base of each acid
You’ll often deal with Brønsted-Lowry acids and bases in these types of questions. Remember, that a Brønsted-Lowry acid donates a proton.
All conjugate bases include:
Because the acids are neutral, their conjugate bases will be negatively charged.
H-A → A- + H+
Step 2: Evaluate and rank the stability of the conjugate bases
Here’s the big idea: a stronger acid has a weaker, more stable conjugate base. So, by ranking the stability of the conjugate bases, we can determine the relative strength of the acids.
To evaluate stability, we’ll go step-by-step through Charge, Atom, Resonance, Induction, Orbitals, taking into account exceptions to the order of priority.
Charge
What is the charge on each conjugate base?
All conjugate bases here carry the same -1 charge, so this factor doesn’t help us rank them. Let’s move on.
Atom
Which atom is holding the negative charge?
In this case, the negative charge is on an oxygen atom in all conjugate bases. This factor doesn’t help either.
Resonance
Is the negative charge delocalized over multiple atoms?
Conjugate base A
The negative charge is localized on the oxygen atom (no resonance).
Conjugate base B
The negative charge is delocalized over two oxygen atoms, making it more stable.
Conjugate base C
The negative charge is delocalized over one oxygen atom and three carbon atoms, which provides some stability, but not as much as B (since oxygen stabilizes negative charges better than carbon).
Conjugate base D
The negative charge is localized on the oxygen atom (no resonance).
Ranking so far: B > C > ? > ?
Induction
Are there electronegative atoms nearby that stabilize the charge?
Conjugate base A
Stabilized by nearby fluorine atoms (highly electronegative), which pull electron density away, stabilizing the negative charge.
Conjugate base D
There are no electronegative atoms nearby, so it’s less stable than A.
Updated ranking: B > C > A > D
Orbitals
Which orbital is holding the negative charge, and how close is it to the nucleus?
This factor isn’t relevant here since we’ve already ranked the bases using the previous factors.
Step 3: Determine the relative acidity of the corresponding acids
Now that we’ve ranked the conjugate bases by stability (B > C > A > D), we can rank the acids. The more stable the conjugate base, the stronger the acid.
So, the acids are ranked: B > C > A > D
