🖐️ Before we begin,
Make sure you’re familiar with the factors that affect the stability of conjugate bases: Charge, Atom, Resonance, Induction, and Orbitals
Challenge 1: Ranking negatively charged bases by strength
Question:
Rank the following bases in order of increasing basicity
We’ll figure out which base is the least basic and which is the most basic by focusing on the stability of their negative charges. Here’s the key idea:
💡 Less stable = more basic.
Sound straightforward? Good! Let’s break it down step by step.
Step 1: Evaluate and rank the stability of the bases
We’ll analyze the stability of each base using five factors in this order (Charge, Atom, Resonance, Induction, Orbitals), taking into account exceptions to the order of priority.
Charge
What is the charge on each base?
Each base carries a negative one (-1) charge
Conclusion: Factor 1 does not indicate which base is more stable.
Atom
Which atom is holding the negative charge?
In bases A, B, and D, the negative charge is located on oxygen atoms, while in base C, it is on a nitrogen atom.
Both nitrogen and oxygen are in the same row of the periodic table, so electronegativity is a determining factor here.
Oxygen is more electronegative than nitrogen, therefore it can stabilize a negative charge better.
💡 Key takeaway: Base C, with its less stable charge on nitrogen, is the strongest base so far.
Ranking so far: ? < ? < ? < C
Resonance
Is the negative charge delocalized over multiple atoms?
Base A
The negative charge is delocalized over one oxygen atom and three carbon atoms.
Base B
The negative charge is ‘stuck’ on the oxygen atom (localized).
Base D
The negative charge is delocalized over one oxygen atom and three carbon atoms.
💡 Key takeaway: Bases A and D are stabilized by resonance, while Base B is not. This makes Base B less stable than A and D, and therefore, B is a stronger base.
Ranking so far: ? < ? < B < C
Since A and D have similar resonance stabilization, we’ll use the next factor (Induction) to break the tie.
Induction
Are there electronegative atoms nearby that stabilize the charge?
Base D
The negative charge in base D is significantly stabilized by the inductive effects of the nearby fluorine atom.
Base A
Base A lacks the presence of electronegative atoms in proximity to the negatively charged oxygen. As a result, there is less stabilization from inductive effects compared to base D.
We can conclude that base A is less stable than base D, making it a stronger base.
Final ranking: D < A < B < C
Orbitals
How close is the orbital that houses the charge to the nucleus?
This factor is not relevant in this case as we have successfully ranked each base using the previous factors.
Step 2: Summarize findings
After evaluating the factors, our final ranking from least basic to most basic is D, A, B, C.
