Challenge 1
Question:
What is the order of rates, from slowest to fastest, for the reactions of the four nucleophiles with ethyl iodide?
Ready? Let’s dive in! 🚀
Let’s talk about the nucleophile—the molecule that’s doing the attacking in a substitution reaction.
Key rule: In substitution reactions, a stronger nucleophile leads to a faster reaction.
Remember: A nucleophile is an electron donor. Atoms that are willing to share their electrons make strong nucleophiles.
But what makes a nucleophile strong? We’ll break that down step by step as we rank the nucleophiles.
Sound good? Let’s get started!
Step 1: Identify the atoms that can donate electrons
The first thing to figure out is which atoms in each molecule have lone pairs that can be donated. These are the nucleophilic atoms.
Note:
Lone pairs on heteroatoms like nitrogen, oxygen, and sulfur are often omitted in chemical structures. Be sure to recognize that these atoms almost always have lone pairs, and it’s important to know how many each one has in a given structure.
Here’s the breakdown:
- Choice A: Oxygen
- Choice B: Oxygen
- Choice C: Nitrogen
- Choice D: Sulfur
So, we’ve identified the key players. Let’s move on to how their charge affects their nucleophilic strength.
Step 2: Consider charge
Key rule: Negatively charged nucleophiles are stronger than neutral ones. Why? Because they have more electron density, which makes them more eager to share electrons and attack an electrophile.
Quick Check!
Which choices are negatively charged?
Answer:
- Choice A: Neutral (Oxygen)
- Choice B: Negatively charged (Oxygen)
- Choice C: Neutral (Nitrogen)
- Choice D: Negatively charged (Sulfur)
Since neutral nucleophiles are weaker than negatively charged ones, we know: A and C (neutral) will react slower than B and D (negatively charged).
But we still need to figure out:
- Which is slower between A and C?
- Which is faster between B and D?
Let’s tackle these one at a time.
Step 3: Compare different atoms
Now, let’s figure out which of the neutral nucleophiles (A and C) is the slowest, and which of the charged nucleophiles (B and D) is the fastest.
A vs. C: Oxygen vs. Nitrogen
Now we’re comparing A (oxygen) and C (nitrogen). Both are neutral nucleophiles, but they’re in the same row of the periodic table.
When comparing atoms in the same row:
Key rule: Nucleophilic strength decreases as electronegativity increases. More electronegative atoms hold onto their electrons tightly and are less willing to share them.
- Oxygen is more electronegative than nitrogen, so it’s less willing to donate its lone pairs.
- Therefore, A is a weaker nucleophile than C.
Ranking so far:
A < C < ? < ?
B vs. D: Oxygen vs. Sulfur
Next, let’s compare B (oxygen) and D (sulfur). Both are negatively charged, but they’re in the same column of the periodic table.
When comparing atoms in the same column:
Key rule: Nucleophilic strength increases as atomic size increases. Larger atoms have valence electrons that are farther from the nucleus, making them more reactive.
- Sulfur is larger than oxygen, so it’s a stronger nucleophile.
- Therefore, D reacts faster than B.
Final Ranking:
A < C < B < D
Looking ahead 👀
You might wonder: What happens when the nucleophilic atoms are the same? In that case, we consider resonance stabilization, induction, and orbitals which we’ll explore in the next challenge. 😊
Challenge 2
Question:
Rank the anions according to nucleophilic strength from weakest to strongest
Here’s how we’ll do it!
Step 1: Identify the atoms that can donate electrons
The first thing to figure out is which atoms in each molecule have lone pairs that can be donated. These are the nucleophilic atoms.
Here’s what we’ve got:
- Choice A: Oxygen
- Choice B: Oxygen
- Choice C: Nitrogen
- Choice D: Oxygen
Now that we’ve identified the key players, let’s see how their charge affects their nucleophilic strength.
Step 2: Consider charge
Recall that negatively charged species are stronger nucleophiles because they have extra electron density to share.
In this challenge, all the nucleophilic atoms are negatively charged, so we’ll need to move on to the next step to differentiate them.
Step 3: Compare different atoms
We’re comparing nitrogen (Choice C) and oxygen (Choices A, B, and D).
Since these atoms are in the same row of the periodic table, electronegativity plays a key role.
Quick check!
What’s the key rule for comparing atoms in the same row of the periodic table?
When comparing atoms in the same row of the periodic table, nucleophilic strength decreases as electronegativity increases. This is because more electronegative atoms hold onto their electrons tightly and are less willing to share them.
Nitrogen is less electronegative than oxygen, making it more willing to share its lone pairs and the strongest nucleophile.
Ranking So Far:
Weakest to strongest: ? < ? < ? < C
Now, let’s figure out how to rank the oxygen-containing anions (Choices A, B, and D).
Step 4: Compare the same atoms
To differentiate between the oxygen anions, we consider Resonance, Induction, and Orbitals in that order. These factors help determine how stable the negative charge is on the oxygen atom. The more stable the charge, the weaker the nucleophile.
Resonance
Is the negative charge delocalized over multiple atoms?
- Choices A and B: The negative charge is localized on the oxygen atoms
- Choice D: The negative charge is delocalized over two oxygen atoms, which makes it more stable and less nucleophilic.
Result: Choice D is the most stable anion and, therefore, the weakest nucleophile.
Ranking So Far:
Weakest to strongest: DÂ < ? < ? < C
Now between A and B which is the weaker nucleophile? Let’s consider the next factor, induction, to find out.
Induction
Are there electronegative atoms nearby that stabilize the charge?
- Choice A: No nearby electronegative atoms to stabilize the charge.
- Choice B: Three fluorine atoms nearby stabilize the negative charge via induction, making it more stable (and weaker) than Choice A.
Result: Choice B is weaker than Choice A.
Final Ranking:
Weakest to strongest: D < B < A < C
