Nucleophiles &
Electrophiles
Learn to recognize electron-rich nucleophiles and electron-poor electrophiles: the two partners in every organic reaction.
Organic Chemistry 1
What Are Nucleophiles & Electrophiles?
Nucleophile
"Nucleus lover": an electron-rich species that donates a pair of electrons to form a new bond.
Electrophile
"Electron lover": an electron-poor species that accepts a pair of electrons to form a new bond.
Electron Density Is the Key
Electrons always flow from high electron density (nucleophile) to low electron density (electrophile). This is the driving force behind every organic reaction mechanism.
Why This Matters
1
Predict products: Knowing which atom attacks which tells you what bonds form.
2
Draw mechanisms: Curved arrows always start at the nucleophile and point to the electrophile.
3
Compare reaction rates: Stronger nucleophiles react faster; better electrophiles attract attack more easily.
The Core Rule of Reactivity
Identifying Nucleophiles
Look for species with available electrons. These features make something nucleophilic:
Negative Charge
\(\text{OH}^-\), \(\text{CN}^-\), \(\text{Br}^-\): anions have excess electrons ready to donate.
Lone Pairs
\(\text{H}_2\text{O}\), \(\text{NH}_3\), \(\text{ROH}\): atoms with lone pairs (N, O, S) can donate without being charged.
\(\pi\) Bonds
Alkenes (\(\text{C=C}\)) and aromatic rings: the electron cloud above/below the plane is available.
Identifying Electrophiles
Look for species that are electron-deficient. These features make something electrophilic:
Positive Charge
\(\text{H}^+\), carbocations (\(\text{R}^+\)): they desperately need electrons.
Polar Bonds (\(\delta+\))
\(\text{C}{-}\text{Cl}\), \(\text{C=O}\): the carbon carries partial positive charge due to electronegativity difference.
Incomplete Octets
\(\text{BF}_3\), \(\text{AlCl}_3\): only 6 electrons around the central atom; strong Lewis acids.
Nucleophile vs. Base: What's the Difference?
| Nucleophile | Base | |
|---|---|---|
| Attacks | An electrophilic carbon | A proton (\(\text{H}^+\)) |
| Forms | A new \(\text{C}\)–Nu bond | A new \(\text{H}\)–Base bond |
| Driven by | Polarizability and steric access | Thermodynamic stability of conjugate |
| Example | \(\text{I}^-\) (great nucleophile, weak base) | \(\text{t-BuO}^-\) (great base, poor nucleophile) |
How to Spot Nu and E in Any Reaction
1
Draw Lewis structures. Show all lone pairs and formal charges.
2
Find the electron-rich site. Look for negative charges, lone pairs, or \(\pi\) bonds: that's the nucleophile.
3
Find the electron-poor site. Look for positive charges, \(\delta+\) carbons, or incomplete octets: that's the electrophile.
4
Draw the curved arrow. Start at the nucleophile's electron pair, point to the electrophilic atom.
Example 1: \(\text{OH}^-\) Attacks \(\text{CH}_3\text{Br}\)
Example 2: \(\text{NH}_3\) as a Neutral Nucleophile
Example 3: Attack on a Carbonyl
What Makes a Stronger Nucleophile?
1
Charge: Anions are stronger than their neutral counterparts. \(\text{OH}^- \gg \text{H}_2\text{O}\).
2
Electronegativity: Less electronegative atoms hold electrons more loosely. \(\text{N} > \text{O} > \text{F}\) as nucleophiles across a row.
3
Polarizability: Larger atoms (lower in the periodic table) are more polarizable. In polar aprotic solvents: \(\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-\).
4
Steric effects: Bulky groups around the nucleophilic atom reduce reactivity. \(\text{CH}_3\text{O}^- > \text{t-BuO}^-\) as nucleophile.
What Makes a Better Electrophile?
Greater \(\delta+\)
More electronegative groups bonded to the carbon increase its partial positive charge.
Better Leaving Group
Good leaving groups (I⁻, TsO⁻, Br⁻) stabilize the departing electrons, aiding the reaction.
Less Steric Hindrance
Primary carbons are more accessible than tertiary. Less crowded = easier attack.
Common Mistakes to Avoid
✕ Arrow Backwards
Curved arrows must start at the nucleophile (electrons) and point to the electrophile, never the reverse.
✕ Confusing Nu with Base
A species can be both, but nucleophilicity describes attack on carbon; basicity describes attack on a proton.
✕ Ignoring Solvent Effects
Nucleophile strength order can reverse between protic and aprotic solvents. Always consider the medium.
Summary: Quick Reference
| Nucleophile | Electrophile | |
|---|---|---|
| Electron density | High (electron-rich) | Low (electron-poor) |
| Charge clue | Often \(\delta-\) or negative | Often \(\delta+\) or positive |
| Lewis role | Lewis base (donates e⁻) | Lewis acid (accepts e⁻) |
| Key features | Lone pairs, \(\pi\) bonds, anions | Cations, \(\delta+\) C, empty orbitals |
| Classic examples | \(\text{OH}^-\), \(\text{CN}^-\), \(\text{NH}_3\) | \(\text{H}^+\), \(\text{R}^+\), \(\text{C=O}\) carbon |
Question 1 of 5
Which of the following is the nucleophile in the reaction \(\text{CN}^- + \text{CH}_3\text{I} \rightarrow \text{CH}_3\text{CN} + \text{I}^-\)?
\(\text{CN}^-\) carries a negative charge and has a lone pair on carbon, making it electron-rich. It donates electrons to the \(\delta+\) carbon of \(\text{CH}_3\text{I}\).
Question 2 of 5
What makes the carbon in a carbonyl group (\(\text{C=O}\)) electrophilic?
Oxygen's higher electronegativity pulls the \(\pi\) electrons toward itself, leaving the carbon with a partial positive charge (\(\delta+\)). This makes it an electron-poor electrophilic center.
Question 3 of 5
In a curved-arrow mechanism, the arrow always starts at the:
Curved arrows track electron movement. Electrons flow from the nucleophile (where they are) to the electrophile (where they go). The tail starts at the electron pair; the head points to the electron acceptor.
Question 4 of 5
Which species is the strongest nucleophile in a polar aprotic solvent?
In polar aprotic solvents (like DMSO or acetone), nucleophilicity follows polarizability: larger atoms donate electrons more easily. \(\text{I}^-\) is the largest halide, so it's the most polarizable and strongest nucleophile.
Question 5 of 5
\(\text{BF}_3\) is classified as an electrophile because it:
Boron in \(\text{BF}_3\) has only 6 valence electrons (three bonds, no lone pairs). It needs two more electrons to complete its octet, making it a strong Lewis acid / electrophile that readily accepts an electron pair from a nucleophile.