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Electrophiles and Nucleophiles Explained with Examples

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An electrophile is an electron-deficient species that accepts an electron pair to form a new bond. Examples include H+, NO2+, carbocations like CH3+, neutral molecules with incomplete octets such as BF3 and AlCl3, and the carbonyl carbon of aldehydes and ketones.

A nucleophile is an electron-rich species that donates an electron pair. Examples include OH-, CN-, NH3, halide ions, and the pi electrons of alkenes. Almost every organic reaction is a meeting between an electrophile and a nucleophile.

Key Concepts

  • Electrophile (E+): has a positive charge or a partial positive charge, or an atom with fewer than eight valence electrons. It seeks an electron pair.
  • Nucleophile (Nu-): has a negative charge or a lone pair of electrons it can share. It seeks a positive or partially positive centre.
  • Strength trends: among halides in polar protic solvents, nucleophilicity increases down the group, I- > Br- > Cl- > F-, because larger ions hold their electrons less tightly.
  • Electrophilic and nucleophilic centres in one molecule: in a carbonyl compound, the carbon is electrophilic and the oxygen is nucleophilic, which is why both acids and bases can attack it.

Formulas

  • Formal charge of NO2+: nitrogen has 5 valence electrons, 0 nonbonding electrons, and 8 bonding electrons (two double bonds). Formal charge = 5 − 0 − (8 ÷ 2) = +1. The positive charge is shared by resonance, but nitrogen formally carries +1, which is why NO2+ is such a strong electrophile. Charge is measured in units of the elementary charge e.
  • General substitution: R−X + Nu- → R−Nu + X-. The nucleophile donates its pair to carbon while the leaving group X- takes the bonding pair away.
  • Electrophilic attack on alkene: CH2=CH2 + E+ → CH3−CH2+ (via +E shift), then Nu- adds to the carbocation. The pi bond acts as the nucleophile.

Worked Example

Verify numerically that the nitronium ion NO2+, the electrophile in nitration, really carries a positive charge on nitrogen.

  1. Draw the Lewis structure: O=N+=O, linear, with nitrogen forming two double bonds, one to each oxygen. Each oxygen has two lone pairs.
  2. Count for nitrogen: valence electrons of N = 5; nonbonding electrons on N = 0; bonding electrons around N = 8.
  3. Formal charge = 5 − 0 − (8 ÷ 2) = 5 − 4 = +1.
  4. Check oxygen: each O has 6 valence, 4 nonbonding, 4 bonding electrons → 6 − 4 − 2 = 0. Total charge = +1 + 0 + 0 = +1, matching the ion's charge. The +1 on nitrogen explains why NO2+ hungrily accepts an electron pair from benzene.

Common Misconceptions

  • All electrophiles are positively charged. Wrong. BF3, AlCl3, and SO3 are neutral yet strongly electrophilic because of incomplete octets or highly polarized bonds.
  • All nucleophiles are negatively charged. Wrong. Ammonia, water, and alcohols are neutral nucleophiles; their lone pairs do the donating.
  • A good base is always a good nucleophile. Not necessarily. Basicity is about affinity for H+, nucleophilicity about attacking carbon. Iodide is a weak base but an excellent nucleophile, while the bulky tert-butoxide ion is a strong base but a poor nucleophile.

Real-World Applications

  • Nitration of benzene: the electrophile NO2+ attacks benzene to make nitrobenzene, the starting point for dyes, explosives like TNT, and the drug paracetamol.
  • Soap making: the nucleophile OH- attacks the electrophilic carbonyl carbon of fats in saponification.

Electrophiles accept electron pairs and nucleophiles donate them. Spot the electron-poor and electron-rich centres — confirmed by a quick formal-charge calculation — and you can predict where bonds will form.

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