Nucleophiles and electrophiles are central concepts in organic chemistry, especially in reaction mechanisms. Every bond-forming step in organic chemistry involves one species donating electrons (nucleophile) and one species accepting electrons (electrophile).
A nucleophile is a species that *donates* a pair of electrons to form a new covalent bond.
Anions (charged nucleophiles):
Neutral molecules with lone pairs:
Pi bonds (π electrons act as nucleophiles):
| Factor | Effect on Nucleophilicity |
|---|---|
| Negative charge | Increases nucleophilicity (OH⁻ > H₂O) |
| Atom size (going down a group) | Increases (I⁻ > Br⁻ > Cl⁻ > F⁻) in polar aprotic |
| Electronegativity | Decreases nucleophilicity (N > O > F) |
| Lone pair availability | More lone pairs = better nucleophile |
| Solvent (protic) | Solvation reduces nucleophilicity of smaller ions |
Key rule: In polar aprotic solvents, nucleophilicity follows basicity order. In protic solvents, larger/softer nucleophiles are stronger due to lower solvation.
These are related but different properties:
An electrophile is a species that *accepts* a pair of electrons to form a new covalent bond.
Cations (charged electrophiles):
Polarized neutral molecules:
Lewis acids:
| Factor | Effect |
|---|---|
| Positive charge | Strongly electrophilic |
| Partial positive (δ⁺) | Moderately electrophilic |
| Electron-withdrawing groups nearby | Increases electrophilicity |
| Empty orbitals | Make atom strongly electrophilic (e.g., BF₃) |
| Steric hindrance | Decreases reactivity toward nucleophiles |
Some nucleophiles have two possible sites from which they can attack. These are called ambident nucleophiles.
| Ambident Nucleophile | Attacks via... | Product |
|---|---|---|
| CN⁻ | C → forms nitrile (R–CN) | Nitrile |
| CN⁻ | N → forms isonitrile (R–NC) | Isonitrile |
| SCN⁻ | S → forms thiocyanate | R–SCN |
| SCN⁻ | N → forms isothiocyanate | R–NCS |
| Enolate | C → C-alkylation | Alkylated at carbon |
| Enolate | O → O-alkylation | Alkylated at oxygen |
The actual site of attack depends on solvent, hard-soft considerations (HSAB), and reaction conditions.
The HSAB principle (Pearson's theory) helps predict which nucleophile will attack which electrophile:
| Type | Description | Examples |
|---|---|---|
| Hard nucleophiles | Small, electronegative, non-polarizable | OH⁻, F⁻, RO⁻, NH₃ |
| Soft nucleophiles | Large, polarizable, electron-rich | I⁻, RS⁻, R₃P, CN⁻ |
| Hard electrophiles | Small, high charge density | H⁺, Li⁺, Mg²⁺, carbonyl C |
| Soft electrophiles | Large, polarizable, low charge density | Alkyl halides (C–X), π systems |
Rule: *Hard prefers Hard, Soft prefers Soft*
- Hard nucleophile + Hard electrophile → thermodynamically controlled
- Soft nucleophile + Soft electrophile → kinetically preferred
In every organic reaction:
Example 1: SN2 Reaction
CH₃–Br + OH⁻ → CH₃–OH + Br⁻
Example 2: Nucleophilic Addition to Carbonyl
R–CHO + CN⁻ → R–CH(OH)(CN)
Example 3: Electrophilic Addition to Alkene
CH₂=CH₂ + HBr → CH₃–CH₂–Br
Example 4: Electrophilic Aromatic Substitution (EAS)
C₆H₆ + NO₂⁺ → C₆H₅–NO₂ + H⁺
| Feature | Nucleophile | Electrophile |
|---|---|---|
| Electron role | Electron donor | Electron acceptor |
| Charge tendency | Often negative or neutral | Often positive or δ⁺ |
| Lewis acid/base | Lewis base | Lewis acid |
| Lone pairs | Has lone pairs | Lacks/needs electrons |
| Examples | OH⁻, CN⁻, NH₃, I⁻ | H⁺, BF₃, R–C⁺, C=O |
| Role in reaction | Attacks electrophilic center | Attacked by nucleophile |
Understanding nucleophiles and electrophiles helps you:
The easiest way is by looking at their names:
The nitronium ion (NO₂⁺) is a very strong electrophile. It is commonly used in electrophilic aromatic substitution reactions, such as the nitration of benzene.
Ammonia (NH₃) is a nucleophile. It has a lone pair of electrons on the nitrogen atom that it can readily donate to form a bond.
An electrophile is a chemical species that accepts a pair of electrons to form a new covalent bond. They are electron-deficient and act as Lewis acids.
Common examples of nucleophiles include the hydroxide ion (OH⁻), water (H₂O), ammonia (NH₃), and halide ions like chloride (Cl⁻) or bromide (Br⁻).
A nitrile (R-C≡N) exhibits both characteristics depending on the reaction. The carbon atom is partially positive and acts as an electrophile, while the nitrogen atom has a lone pair and can act as a weak nucleophile. The cyanide ion (CN⁻), however, is a strong nucleophile.
They can be either:
Nucleophiles and Electrophiles is a fundamental concept in organic chemistry. Understanding the mechanisms, reaction conditions, and stereo-chemical outcomes is crucial for mastering organic chemistry. Our curated resources provide step-by-step visualizations to help you excel.
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What defines a nucleophile?
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The easiest way is by looking at their names: Nucleophile means 'nucleus lover'. Since nuclei are positively charged, a nucleophile must be negative or electron-rich to be attracted to it. Electrophile means 'electron lover'. Since electrons are negatively charged, an electrophile must be positive or electron-deficient to be attracted to them.
The nitronium ion (NO2+) is a very strong electrophile. It is commonly used in electrophilic aromatic substitution reactions, such as the nitration of benzene.
Look for lone pairs or negative charges—these indicate a nucleophile (e.g., OH-, NH3, Cl-). Look for positive charges, partial positive charges (δ+), or empty orbitals—these indicate an electrophile (e.g., H+, BF3, carbocations).
Common examples include water (H2O), ammonia (NH3), hydroxide ion (OH-), and cyanide ion (CN-). They all have lone pairs of electrons to donate.
In polar aprotic solvents, strong bases with minimal steric hindrance, such as the methyl anion (CH3-) or amide ion (NH2-), are the strongest nucleophiles. Among halogens, fluoride (F-) is strongest in aprotic solvents, while iodide (I-) is strongest in protic solvents.
A nucleophile is also called a Lewis base because it donates an electron pair. It can also be referred to as an 'electron donor' or 'nucleus lover'.
Common examples of nucleophiles are species with lone pairs or negative charges, such as hydroxide (OH-), water (H2O), ammonia (NH3), and cyanide (CN-).
A good nucleophile usually has a full negative charge, is less electronegative, and has less steric hindrance. Poor nucleophiles are usually neutral molecules (like H2O or alcohols) or bulky bases that struggle to reach the electrophilic center.
If a species is electron-rich (has lone pairs, pi bonds, or a negative charge), it's a nucleophile. If it's electron-deficient (has an empty p-orbital, a positive charge, or a partial positive charge), it's an electrophile.
Hydrogen gas (H2) is not a nucleophile because it does not have any lone pairs of electrons or pi bonds to donate. Its two electrons are tightly held in a stable, non-polar sigma bond.
Yes, H2O is generally considered a weak (or poor) nucleophile. Because it is a neutral molecule, it is less reactive and slower to attack an electrophile compared to its negatively charged counterpart, the hydroxide ion (OH-).
The hydroxide ion (OH-) is a much better nucleophile than water (H2O). A negatively charged species is always a stronger nucleophile than its neutral conjugate acid because it has a higher electron density and is more eager to donate electrons.