Factors that contribute to nucleophilicity
There are four key factors that contribute to a species' nucleophilicity:
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Charge A nucleophile reacts by donating electrons. This means, that the higher the electron density on a species, the more nucleophilic it is, all other things being equal. In general, a negatively charged species will be more nucleophilic than its neutral counterpart.
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Size A nucleophile donates electrons to an electrophile, but to do so it needs to be in close proximity to the electrophile. This can be hard if the nucleophile is a large and bulky molecule. In general, a smaller nucleophile is a stronger nucleophile!
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Electronegativity Highly electronegative atoms have a high electron density, but they also have a high electron affinity, meaning that they attract the electrons strongly. A nucleophile reacts by donating electrons, which a highly electronegative atom is less willing to do. Therefore, a less electronegative atom is more nucleophilic, all other things being equal.
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Solvent Solvents can be either protic or aprotic. A protic solvent can participate in hydrogen bonding with the nucleophile, which causes the nucleophile to be cushioned by the solvent. This makes the nucleophile less reactive, than if it was dissolved in an aprotic solvent.
Role of Acid in Elimination (E1 Mechanism)
In an E1 (unimolecular elimination) reaction, an acid plays a key catalytic role:
- Protonates the hydroxyl group (–OH) of an alcohol to form a better leaving group — water (H₂O).
- This sets up the formation of a carbocation intermediate, which can then undergo loss of a β-hydrogen to form a double bond.
Without acid, the –OH group is a poor leaving group on its own.
Phosphoric Acid (H₃PO₄)
Phosphoric acid is commonly used in dehydration (elimination) of alcohols because:
- It is a moderately strong acid (pKa ≈ 2.1).
- It is non-nucleophilic, so it doesn’t add to or interfere with the reaction intermediates.
- It creates a controlled acidic environment, minimizing side reactions.
Sulfuric Acid (H₂SO₄)
Sulfuric acid is an effective substitute because it shares similar properties:
| Property | Phosphoric Acid | Sulfuric Acid |
|---|---|---|
| Strength | Moderate acid | Stronger acid (pKa ≈ –3) |
| Protonates –OH? | Yes | Yes |
| Nucleophilic? | No | No |
| Common in E1? | Yes | Yes |
| Volatility/Corrosiveness | Low | Higher |
Why it works: Sulfuric acid can efficiently protonate –OH groups, turning them into water, just like phosphoric acid. Since it’s non-nucleophilic, it won’t interfere with the formation or stability of the carbocation intermediate.
Why Other Acids Don’t Work as Well
| Acid | Why It’s Less Suitable |
|---|---|
| HBr | Strong acid and a strong nucleophile → favors substitution (SN1) instead of elimination. |
| HF | Weak acid; strong hydrogen bonding makes it ineffective at protonating –OH groups. |
| Acetic acid | Weak acid (pKa ≈ 4.76); not strong enough to protonate –OH groups effectively. |
You need an acid that’s strong enough to protonate an alcohol, but not so reactive that it participates in unwanted side reactions.
OR
Why use sulfuric acid in elimination reactions?
Sulfuric acid is strong enough to protonate the –OH group on an alcohol, turning it into water — a much better leaving group. This makes elimination (E1) possible.
Like phosphoric acid, sulfuric acid is:
- A strong acid
- Non-nucleophilic, so it won’t interfere with the reaction
- Commonly used in alcohol dehydration reactions
Other acids like HBr or acetic acid are either too reactive or too weak to do the job properly.