Before we jump in:
Remember — when ranking acid strength, we’re really asking:
“How easy is it for this molecule to give up a proton (H+)?”
The easier it is, the stronger the acid.
Step 1: Draw the conjugate base for each acid
First things first: For each acid you’re comparing, remove a proton (H⁺).
What you’re left with is the conjugate base, now carrying an extra electron pair and in most cases, a negative charge.
👉 Factors that stabilize this electron pair make it easier for the corresponding acid to give up its proton.
Key idea: The more stable the conjugate base, the stronger the corresponding acid.
We’ll analyze the stability of each conjugate base using the CARIO framework. Let’s go through each systematically.
Step 2: Consider charge (C: Charge)
Key idea: Neutral conjugate bases are typically more stable than their negatively charged counterparts.
Negatively charged conjugate bases possess high electron density, making them less stable.
Therefore, positively charged acids are generally stronger than neutral ones.
Pro tip:
- If acids differ only by charge, prioritize charge effects.
- If they differ by structure, move on to ARIO.
Step 3: Consider the atom (A: Atom)
Look at the atom carrying the electron pair (or negative charge) after deprotonation.
Two trends to keep in mind:
1️⃣ Across a row (left to right):
Stability increases with electronegativity. More electronegative atoms stabilize electron density better.
2️⃣ Down a column (top to bottom):
Stability increases with size. Larger atoms spread out electron density over a bigger volume.
Pro tip:
If conjugate bases differ by atom identity, prioritize electronegativity across a period and size down a group.
Step 4: Check for resonance stabilization (R: Resonance)
Ask yourself:
“Can the electron pair (or negative charge) be delocalized by resonance?”
Resonance spreads out the negative charge, lowering electron density and increasing stability.
Therefore, acids whose conjugate bases are resonance-stabilized are much stronger.
Pro tip:
If the conjugate bases have the same atom, look for resonance stabilization.
Step 5: Check for inductive effects (I: Induction)
Look for nearby electronegative groups (like F, Cl, NO2) that can pull electron density away from the negative charge.
Trends to consider:
1️⃣ Distance:
Inductive stabilization decreases as the withdrawing group gets farther from the negative charge.
2️⃣ Number of groups:
More electronegative groups → greater stabilization.
3️⃣ Electronegativity:
More electronegative substituents stabilize better. (For halogens: F > Cl > Br > I.)
Step 6: Analyze the orbital hybridization (O: Orbitals)
Finally, if you’re stuck between choices that all seem similar, check what orbital the negative charge occupies.
Key idea: Orbitals with higher s-character hold the negative charge closer to the nucleus, stabilizing it.
Therefore, we get the following trend: sp > sp² > sp³
Step 7: Rank the acids
Now, use all the evidence you gathered:
Charge → Atom → Resonance → Induction → Orbital
The stronger the stabilization of the conjugate base, the stronger the acid.
Quick recap
When you’re asked to rank acids without a pKa table:
1️⃣ Deprotonate → find the conjugate base
2️⃣ Analyze stability using CARIO
3️⃣ Rank acids based on conjugate base stability
Bottom line:
The more stable the conjugate base, the stronger the acid!
You did it! 🎉
With practice, these steps will become second nature—and you’ll be ranking acids by strength like a pro. Got questions or thoughts? Share them below!
