Topic 10 · page 3 of 27
Nucleophiles, electrophiles and bond polarity
A nucleophile donates a pair of electrons, an electrophile accepts one. Bond polarity tells you which is needed: a δ+ carbon is attacked by a nucleophile, an electron-rich bond by an electrophile, and a non-polar bond by neither.
Read first: Mechanisms and curly arrows
Nucleophiles and electrophiles 10.310.4
The two words describe what a species does with a pair of electrons.
The names help you remember: a nucleophile is "nucleus-loving" and an electrophile is "electron-loving".
| Nucleophiles (lone pair to give) | Electrophiles (room for a pair) |
|---|---|
| OH⁻, CN⁻ | H⁺ |
| NH₃, H₂O (neutral, with a lone pair) | a δ+ atom in a polar molecule, such as the H in H–Br |
| Cl⁻, Br⁻, I⁻ | a carbocation, such as CH₃CH₂⁺ |
In a halogenoalkane the δ+ carbon is the centre that a nucleophile attacks.
Bond polarity decides the mechanism 10.5
Look at any bond and ask two questions. Is it polar? If so, which end is δ+? The answers tell you which kind of reagent attacks.
Electronegativity is the power of an atom to attract the bonding pair. Pauling values for the atoms in this topic:
| Atom | H | C | N | O | Cl | Br | I |
|---|---|---|---|---|---|---|---|
| Electronegativity | 2.1 | 2.5 | 3.0 | 3.5 | 3.0 | 2.8 | 2.5 |
| Bond | Polarity | Attacked by | Mechanism | Met in |
|---|---|---|---|---|
| C–H, C–C | treated as non-polar (C–C is between identical atoms; C–H differs by only 0.4) | a radical, after UV light breaks a bond evenly | radical substitution | Topic 4 |
| C=C | non-polar, but the π electrons are a region of high electron density | an electrophile | electrophilic addition | Topic 5 |
| C–Br, C–Cl, C–O | polar: carbon is δ+ | a nucleophile | nucleophilic substitution | this topic |
The C–Br bond is polar because bromine (2.8) is more electronegative than carbon (2.5). The carbon is δ+. A nucleophile has a lone pair and is attracted to that carbon, which is why hydroxide attacks bromoethane. Ethane has no δ+ atom, so hydroxide finds nothing to attack and there is no reaction.
Check yourself
- Define the term nucleophile, and give three examples that are not all negative ions.
- Define the term electrophile and give two examples.
- Explain why ammonia can act as a nucleophile although it has no charge.
- The Pauling electronegativity of chlorine is 3.0 and of carbon 2.5. State which atom is δ+ in C–Cl and what type of reagent attacks it.
- Explain why ethene is attacked by electrophiles but ethane is not.
- Chloroethane reacts with aqueous alkali but ethane does not. Use polarity to explain the difference.
Answers cover these until you have tried the questions
- A nucleophile is a species that donates a pair of electrons to form a covalent bond. Examples: OH⁻, CN⁻ (negative ions), and NH₃ or H₂O (neutral, with a lone pair).
- An electrophile is a species that accepts a pair of electrons to form a covalent bond. Examples: H⁺, and the δ+ hydrogen in H–Br (or a carbocation).
- Ammonia has a lone pair of electrons on nitrogen. The lone pair can be donated to a δ+ carbon, so it is a nucleophile. Charge is not needed.
- Chlorine is more electronegative, so the bond is polar and carbon is δ+. A nucleophile attacks it.
- The C=C bond contains a π bond, a region of high electron density that attracts electrophiles. Ethane has only non-polar σ bonds and no region of high electron density.
- In chloroethane, Cl (3.0) is more electronegative than C (2.5), so the C–Cl bond is polar and carbon is δ+, which attracts the lone pair of OH⁻. In ethane, C–H and C–C are non-polar, so there is no δ+ atom for the nucleophile to attack.