Organic Chemistry 1 · Reaction Mechanisms

Nucleophiles, Electrophiles, and Reactive Intermediates

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Study tools

In 30 seconds

A is an electron-rich species that donates an electron pair (a Lewis base); an is an electron-poor species that accepts that pair (a Lewis acid). The reaction site is the atom carrying the lone pair, partial negative charge, or empty/partial positive center. When bonds break unevenly, reactive intermediates form: carbocations (R₃C⁺, six electrons, planar), carbanions (R₃C^-, eight electrons with a lone pair), and carbon radicals (R₃C^•, seven electrons, unpaired). Their stability trends are driven by , inductive effects, and resonance delocalization.

Why this matters

stability explains why biological alkylation events and drug metabolites funnel through resonance-stabilized benzylic/allylic positions rather than high-energy vinylic/aryl cations. Nucleophile/electrophile matching underlies how electrophilic drugs (e.g., alkylating chemotherapeutics) covalently modify nucleophilic sites on DNA and proteins, and why electron-rich antioxidants can quench carbon radicals before they propagate damaging chain reactions in cells.

The college version

1. Nucleophiles, Electrophiles, and Reaction Sites

A nucleophile ("nucleus-loving") is electron-rich — a lone pair, negative charge, or π bond — and seeks electron-poor centers. An electrophile ("electron-loving") is electron-poor — an empty orbital or a partial/full positive charge. Polar bonds create the sites: in C^δ⁺-X^{δ^-}, the electronegative X pulls density away, leaving carbon electrophilic. To identify the reaction site, ask where is the electron-rich atom? and where is the electron-poor atom? The arrow then flows from the to the .

2. Carbocations

A carbocation is a trivalent carbon with six valence electrons, a positive charge, and an empty p orbital; it is sp² hybridized and trigonal planar, so attack can occur from either face. Stability order is tertiary > secondary > primary > methyl, from two effects: hyperconjugation (donation of adjacent C–H/C–C σ-bond density into the empty p orbital) and the (electron donation by alkyl groups). further lowers the energy when the charge delocalizes: allylic and benzylic carbocations are exceptionally stable. In contrast, vinylic and aryl carbocations are very unstable — the empty p orbital sits on an sp² carbon, perpendicular to the π system, so it cannot delocalize or benefit from hyperconjugation.

3. Carbanions and Carbon Radicals

A is a trivalent carbon with a negative charge, a lone pair, and eight valence electrons; it is usually pyramidal (roughly sp³). Its stability order is the reverse of the carbocation — methyl > primary > secondary > tertiary — because donates density into an already electron-rich center; electron-withdrawing groups and resonance stabilize carbanions. A is a trivalent carbon with seven valence electrons and one unpaired electron, sp²-like (planar or shallowly pyramidal). Radical stability parallels the carbocation order (tertiary > secondary > primary > methyl) via hyperconjugation, with allylic/benzylic radicals resonance-stabilized and vinylic/aryl radicals high-energy.

How it works

Mark polar bonds and lone pairs to reveal δ⁺ and electron-rich atoms.
Label the nucleophilic center (lone pair, negative charge, π bond) and the electrophilic center (δ⁺ carbon or cation).
Predict attack: the nucleophile's electrons flow to the electrophilic center.
If a bond breaks before or during attack, decide whether a carbocation, carbanion, or radical forms.
Rank that intermediate's stability using hyperconjugation, inductive effects, and resonance.
Apply the special cases: allylic/benzylic positions are resonance-stabilized; vinylic/aryl positions are destabilized and rarely react through those intermediates.

Common confusions

Do not confuse	With	Difference
Nucleophile	Electrophile	Pair donor vs. pair acceptor
Nucleophilicity	Basicity	Kinetic (attack rate) vs. thermodynamic (proton affinity)
Carbocation stability	Carbanion stability	Alkyl groups stabilize cations, destabilize anions
Carbocation	Carbon radical	Six electrons, positive vs. seven electrons, neutral
Allylic/benzylic	Vinylic/aryl	Resonance-stabilized vs. destabilized
Inductive effect	Resonance	Through σ bonds vs. through π systems

Memory aids

"Cations and radicals love company (more alkyl groups = more stable); anions hate crowds (fewer alkyl groups = more stable)." For which resonance helps, recall "ABle to resonate" — Allylic and Benzylic are stabilized; Vinyl and Aryl (VAnt) are not.

Quick review

Topic Recap

Nucleophiles donate electron pairs and electrophiles accept them, with reaction sites at electron-rich and electron-poor atoms created by polar bonds. Carbocations (six electrons, planar) and carbon radicals follow a tertiary > secondary > primary > methyl stability order, while carbanions follow the reverse. Allylic and benzylic positions gain stability through resonance; vinylic and aryl intermediates are unusually unstable.

Knowledge Check

Is NH₃ a nucleophile, an electrophile, or potentially both? Explain.
Rank these carbocations from most to least stable: methyl, tertiary, primary, secondary, benzyl.
Why is a carbanion on a tertiary carbon less stable than one on a methyl carbon?
Why are vinylic carbocations so difficult to form?
In CH₃Cl, which atom is the electrophilic center and why?

Answers and Rationales

NH₃ is a nucleophile via its lone pair, but it also has a proton to donate, so it can act as an electrophile (Brønsted acid) toward a strong base; the role depends on the partner.
Benzyl ≈ tertiary > secondary > primary > methyl. Benzyl is resonance-stabilized, placing it among the most stable.
Alkyl groups donate density through the inductive effect and destabilize the electron-rich carbanion; the methyl carbanion is less crowded and lower in energy.
The empty p orbital sits on an sp² carbon perpendicular to the π system, so it cannot delocalize, and it lacks the hyperconjugative stabilization alkyl groups provide to sp³ cations.
The carbon is the electrophilic center: chlorine is more electronegative, making the C–Cl bond polar with carbon δ⁺.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of a reaction as a handshake between two traders. A nucleophile holds an extra item (an electron pair) to give away; an electrophile has an empty hand that needs it. The atoms actually trading are the nucleophilic center (giver) and the electrophilic center (receiver). When a molecule is left holding too much or too little, it becomes an intermediate — a temporary, charged or radical species that reacts further: a carbocation wants two electrons, a carbanion carries a surplus, a radical carries one unpaired electron.

Comparison: the relationship is a donor–acceptor exchange — one side has something to give, the other has a place to put it. Where it stops being exact: "electron-rich" and "electron-poor" are relative, not absolute. Water is a nucleophile toward a carbocation yet an electrophile (proton donor) toward a strong base; the same molecule wears either hat depending on its partner.

Simple Example

In hydroxide attacking methyl bromide, the oxygen of OH^- (lone pair) is the nucleophilic center, and the carbon bonded to bromine is δ⁺ (the C–Br bond is polar) and is the electrophilic center. Oxygen's lone pair attacks that carbon, displacing bromide.

Worked example

Locate the polarized bond. In CH₃CH₂Br, the C–Br bond is polar: C^δ⁺-Br^{δ^-}.
Identify the donor. CN^- carries a lone pair on carbon — the electron source.
Move the pair to the electrophilic center. A double-headed arrow runs from the cyanide lone pair to the δ⁺ carbon, forming the new C–C bond.
Expel the leaving group. A second arrow runs from the C–Br bond to bromine, moving that bond pair onto bromide; bromine leaves with its electrons and an octet. Charge is balanced throughout.
Classify any intermediate. Had the C–Br bond broken first without the nucleophile, an ethyl carbocation (CH₃CH₂⁺) would form; assess its stability by the rules above before the next bond forms.

Key takeaways

High yield: Nucleophile = electron-pair donor (Lewis base); electrophile = electron-pair acceptor (Lewis acid).
High yield: Carbocation stability: tertiary > secondary > primary > methyl.
High yield: Carbanion stability is reversed: methyl > primary > secondary > tertiary.
Carbon radical stability mirrors the carbocation order.
Carbocations are sp² planar with an empty p orbital; carbanions carry a lone pair; radicals carry one unpaired electron.
High yield: Allylic and benzylic carbocations and radicals are resonance-stabilized and unusually stable.
High yield: Vinylic and aryl cations/radicals are very unstable (empty orbital cannot delocalize) — do not propose them as intermediates.
Electron-withdrawing groups and resonance stabilize carbanions, opposite to carbocations.
The same species can be a nucleophile in one reaction and an electrophile in another; labels are relative.

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Practice Organic Chemistry 1

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You’ll learn to

Identify nucleophiles and electrophiles, and locate nucleophilic and electrophilic centers within a molecule.
Describe the structure, charge, and geometry of carbocations, carbanions, and carbon radicals.
Rank the stability of carbocations, carbanions, and radicals using hyperconjugation, inductive effects, and resonance.
Explain why allylic/benzylic intermediates are stabilized while vinylic/aryl intermediates are unusually unstable.

Key vocabulary

Nucleophile: Electron-rich pair donor (Lewis base)
Electrophile: Electron-poor pair acceptor (Lewis acid)
Nucleophilic center: Electron-rich atom (lone pair, π bond)
Electrophilic center: Electron-poor atom (δ⁺ or cationic)
Polar bond: Unequal electron sharing (dipole)
Carbocation: Six-electron, positive carbon with empty p orbital
Carbanion: Eight-electron, negative carbon with a lone pair
Carbon radical: Seven-electron, neutral carbon with an unpaired electron
Hyperconjugation: σ-bond donation into an adjacent empty orbital
Resonance stabilization: Delocalization of charge over a π system
Inductive effect: Electron donation/withdrawal through σ bonds
Alkyl substitution: Adding alkyl groups to a carbon center
Allylic/benzylic stabilization: Delocalization next to a π system
Vinylic/aryl intermediates: Cation/radical on an sp² carbon in a π system

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