Organic Chemistry · An Overview of Organic Reactions

Polar Reactions

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Polar reactions break and form bonds heterolytically: one partner keeps the electron pair, and electrons move as a unit. Every pairs two complementary species: a ("nucleus-loving"), an electron-rich donor of an electron pair, and an ("electron-loving"), an electron-poor acceptor. The nucleophile is a Lewis base; the electrophile is a Lewis acid. When they meet, the nucleophile's lone pair (or π electron pair) forms a new bond to the electrophile, and a or π bond departs with its electron pair.

This partnership explains much of organic chemistry: the addition of HBr to ethylene, substitutions of alkyl halides, and additions of nucleophiles to carbonyls are all polar reactions. Here you learn to identify the nucleophile and electrophile in any polar reaction, predict electron flow, and describe steps with curved arrows — the notation showing where electrons start and end.

Why this matters

Identifying reactive partners: Spotting the nucleophile and electrophile is the single most useful skill for predicting organic products.
Mechanism literacy: Polar mechanisms are drawn with curved arrows; every arrow is a nucleophile–electrophile event.
Biochemistry: Nearly all enzyme reactions are polar: a nucleophilic amino acid side chain attacks an electrophilic carbonyl carbon, a phosphate, or a protonated heteroatom.
Synthesis design: Choosing a nucleophile or electrophile with the right properties (strength, steric bulk, leaving group) is how chemists steer reactions toward the desired product.
Exams: "Label the nucleophile and electrophile" and "draw the curved arrows" are staple questions from this chapter forward.

The college version

Core Concepts

Nucleophiles: electron-rich donors

A nucleophile has a lone pair or a π electron pair that it can donate to form a new bond. Nucleophiles include anions (OH⁻, CN⁻, Br⁻, HS⁻), neutral molecules with lone pairs (H₂O, NH₃, ROH, R₂S), and π systems (alkenes, aromatic rings). Key properties:

is the kinetic tendency to donate electrons to carbon; basicity is the thermodynamic tendency to donate to a proton. They usually track each other but sterics and solvent can separate them.
Negative charge increases nucleophilicity: OH⁻ is a stronger nucleophile than H₂O.
Among halides in polar aprotic solvents, nucleophilicity increases down the group: I⁻ > Br⁻ > Cl⁻ > F⁻ (large, polarizable anions donate more readily).
Electron-rich atoms are nucleophilic: oxygen, nitrogen, sulfur, and carbanions (R₃C⁻) are all common nucleophiles.

Electrophiles: electron-poor acceptors

An electrophile has an empty orbital, a positive charge, or a polar bond that makes an atom electron-poor. Electrophiles include cations (H⁺, carbocations R₃C⁺, NO₂⁺), Lewis acids (BF₃, AlCl₃), and molecules with polar bonds such that one atom carries a partial positive charge (the carbonyl carbon of aldehydes/ketones, the carbon of alkyl halides C–X, the carbon of esters and amides). Key properties:

The electrophilic atom is the one that accepts the electron pair — usually the carbon attached to an electronegative atom or the carbon of a C=O group.
A full positive charge makes a strong electrophile (carbocations react instantly); a partial positive charge (δ⁺) makes a weaker but still reactive one — the carbonyl carbon is the classic case.

The nucleophile–electrophile partnership

A polar reaction step always pairs an electron donor with an electron acceptor:

$Nu - + E + \to Nu-E$

In curved-arrow notation, the tail starts at the nucleophile's electron pair (lone pair or π bond) and the head points at the electrophilic atom, forming the new Nu–E bond; if the electrophile carries a leaving group, that group departs with the old bond's electron pair (another arrow). Two rules govern every polar step:

Electrons move from the electron-rich partner to the electron-poor partner.
The total electron count is conserved: every arrow that takes electrons out of a bond or lone pair is balanced by an arrow that forms a new bond or lone pair.

Carbocations and carbanions as partners

The simplest polar intermediates are the charged carbon species:

Carbocations (R₃C⁺) are electrophiles: they have an empty p orbital and accept an electron pair. They are sp²-hybridized, planar, and stabilized by alkyl substitution (tertiary > secondary > primary > methyl).
Carbanions (R₃C⁻) are nucleophiles: they carry a lone pair in an sp³ orbital. They are pyramidal, and stability is the reverse of carbocations (methyl > primary > secondary > tertiary), with electron-withdrawing groups providing extra stabilization.

A formed in a polar reaction is attacked immediately by the nearest nucleophile — which is why polar additions to alkenes are fast, and why rearranging carbocations do so before the nucleophile arrives.

Polar vs radical reactions

Polar and radical reactions are the two mechanistic families from the previous topic. Polar: electrons move in pairs (double-headed curved arrows), intermediates are ions. Radical: electrons move one at a time (fishhook arrows), intermediates carry unpaired electrons. Polar bonds and polar solvents favor polar mechanisms; nonpolar bonds and energetic conditions (heat, light) favor radical ones. The same reaction class can occur by either family.

Curved arrows: the language of electron movement

A always shows electron movement — never atom movement — and points from source to sink. In polar mechanisms:

A lone pair attacking an electrophile: arrow from the lone pair to the electrophilic atom.
A π bond attacking an electrophile: arrow from the middle of the π bond to the electrophilic atom (the π electrons become a new σ bond).
A leaving group departing: arrow from the bond being broken to the leaving group atom (the bond electrons become a lone pair on the leaving group).

Reading and drawing these arrows correctly is the mechanical skill of the chapter — the next two topics (the HBr/ethylene example and curved arrows in polar mechanisms) drill exactly this.

Common Confusions

Do Not Confuse	With	Difference
Nucleophile	Electrophile	Nucleophile donates electrons (Lewis base); electrophile accepts them (Lewis acid). "Electron-rich" vs "electron-poor"
Basicity	Nucleophilicity	Basicity is affinity for a proton (thermodynamic); nucleophilicity is reactivity toward carbon (kinetic). They usually correlate but can diverge (sterics, solvent)
Carbocation	Carbanion	Carbocation: empty orbital, electrophile, planar, stabilized by alkyl groups; carbanion: lone pair, nucleophile, pyramidal, destabilized by alkyl groups
Curved arrow showing electron movement	Arrow showing atom movement	Curved arrows move electrons only; equilibria arrows and reaction arrows show overall change
"Nucleophile attacks the positive atom"	"Nucleophile attacks any electron-poor atom"	The electrophilic atom need not carry a full charge; δ⁺ atoms (carbonyl C, alkyl-halide C) are the usual targets
Polar reaction	Ionic reaction	All polar reactions involve electron-pair shifts, but the bond need not be fully ionic — polar bonds (δ⁺/δ⁻) suffice
Lewis acid	Brønsted acid	Lewis acids accept electron pairs (BF₃, carbocations); Brønsted acids donate protons (a subset of Lewis behavior)
The leaving group's electrons	The nucleophile's electrons	The nucleophile's pair forms the new bond; the leaving group's pair departs as a lone pair — two separate curved arrows

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a game of catch where one player (the nucleophile) always has extra balls and loves to pass them, and the other player (the electrophile) always wants more balls. The pass is a new bond: the nucleophile throws its extra pair of electrons to the electrophile, and sometimes the electrophile has to drop an old ball (a leaving group) to catch the new one. The curved arrow in chemistry is just a drawing of the throw — it shows where the ball starts and where it lands.

Worked example

Example 1: Identifying the nucleophile and electrophile

Question: For each polar reaction, label the nucleophile, the electrophile, and the atom where the new bond forms.

Reaction A — CH₃Br + OH⁻ → CH₃OH + Br⁻: OH⁻ has a lone pair and donates it → nucleophile. CH₃Br has a polar C–Br bond; the carbon carries δ⁺ → electrophile (the new C–O bond forms at this carbon). Br⁻ departs as the leaving group.

Reaction B — CH₂=CH₂ + HBr → CH₃CH₂Br: the alkene π bond is electron-rich → nucleophile. HBr's proton is electron-poor (the H carries δ⁺) → electrophile. The π electrons form a new C–H bond; the resulting carbocation is then attacked by Br⁻.

Reaction C — CH₃CHO + CN⁻ → CH₃CH(CN)O⁻: CN⁻ is the nucleophile; the carbonyl carbon of acetaldehyde (δ⁺ on C) is the electrophile. The new bond forms at the carbonyl carbon, and the π electrons of C=O move onto oxygen, creating an alkoxide.

Method: find the species with the lone pair or π electrons (nucleophile) and the atom with a positive charge or δ⁺ (electrophile); the arrow connects them.

Example 2: Predicting the product of a polar addition

Question: Predict the product of HBr addition to propene, CH₃CH=CH₂, and justify the regiochemistry.

Step 1 — Identify the partners: the alkene π bond is the nucleophile; HBr's proton is the electrophile.

Step 2 — Protonate the alkene: the π electrons form a C–H bond. Two possible carbocations form: protonation at C1 gives a secondary carbocation (CH₃–CH⁺–CH₃); protonation at C2 gives a primary carbocation (CH₃–CH₂–CH₂⁺).

Step 3 — Choose the more stable intermediate: the secondary carbocation is more stable, so protonation occurs at C1 (the less-substituted carbon) — this is Markovnikov's rule restated in mechanism language.

Step 4 — Capture with the nucleophile: Br⁻ attacks the carbocation, forming the new C–Br bond at the substituted carbon.

Product: 2-bromopropane, CH₃CHBrCH₃. The mechanism explains the regiochemistry — no memorization of "Markovnikov" needed, just carbocation stability.

Example 3: Using pKa to judge a polar proton-transfer step

Question: In the polar addition H₂O + HBr → H₃O⁺ + Br⁻, why does the proton transfer favor products? Use the pKa values: H₃O⁺ has pKa ≈ −1.7; HBr has pKa ≈ −9.

Step 1 — Write the acid–base logic: the equilibrium favors the side with the weaker acid (the larger pKa). H₃O⁺ (pKa −1.7) is a weaker acid than HBr (pKa −9).

Step 2 — Compare: HBr has pKa ≈ −9 and H₃O⁺ has pKa ≈ −1.7. Since −1.7 > −9, the product acid is weaker and the equilibrium lies far to the right.

Step 3 — Connect to the mechanism: this is why HBr is essentially fully dissociated in water and why water can protonate alkenes only in strong acid — the proton donor must be a stronger acid than the conjugate acid of the nucleophile that forms. pKa comparisons are the quantitative backbone of many polar mechanism steps.

Key takeaways

Polar reactions = heterolytic electron movement: nucleophile (electron-rich, Lewis base) attacks electrophile (electron-poor, Lewis acid).
Curved arrow: tail at the electron source (lone pair or π bond), head at the electron sink (electrophilic atom); electrons move, atoms don't.
Typical nucleophiles: OH⁻, CN⁻, halides, H₂O, NH₃, carbanions, alkenes. Typical electrophiles: H⁺, carbocations, carbonyl carbons, alkyl-halide carbons.
Carbocations are electrophiles (empty p orbital, planar, tertiary > primary stability); carbanions are nucleophiles (lone pair, pyramidal, primary > tertiary stability).
Polar vs radical: paired electrons vs unpaired electrons; double-headed vs fishhook arrows; ions vs radicals.
Every polar step conserves electron count: an arrow removing electrons is balanced by an arrow forming a bond or lone pair.
In polar additions to alkenes the carbocation (if formed) is attacked immediately — watch for rearrangements first.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Define nucleophile and electrophile, and give two examples of each.
Show answer
Nucleophile: electron-rich donor (OH⁻, CN⁻, H₂O, NH₃, carbanions). Electrophile: electron-poor acceptor (H⁺, carbocations, carbonyl carbons, Lewis acids).
In CH₃I + HS⁻ → CH₃SH + I⁻, which species is the nucleophile and where does the new bond form?
Show answer
HS⁻ is the nucleophile; it donates its lone pair to the carbon of CH₃I (the electrophilic carbon bearing I), forming the new C–S bond, while I⁻ leaves.
Why are carbocations electrophiles and carbanions nucleophiles?
Show answer
A carbocation has an empty p orbital and accepts an electron pair; a carbanion has a lone pair it can donate.
What does a curved arrow tail and head mark in a polar mechanism?
Show answer
Tail marks the electron source (lone pair or π bond); head marks the electron sink (the electrophilic atom where the new bond forms).
Using carbocation stability, explain why HBr adds to propene to give 2-bromopropane.
Show answer
Protonation gives the more stable secondary carbocation (CH₃–CH⁺–CH₃) rather than a primary one; Br⁻ then attacks that cation, placing Br at the more substituted carbon (2-bromopropane).
Is NH₃ (pKa of NH₄⁺ ≈ 9.3) strong enough to deprotonate acetic acid (pKa ≈ 4.8)? Justify with the weaker-acid rule.
Show answer
Yes — acetic acid (pKa 4.8) is the stronger acid, so the equilibrium favors the weaker acid, NH₄⁺ (pKa 9.3): CH₃COOH + NH₃ → CH₃COO⁻ + NH₄⁺ lies to the right.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Polar reaction: A reaction proceeding by heterolytic bond changes with electron-pair movement
Nucleophile: An electron-rich species that donates an electron pair
Electrophile: An electron-poor species that accepts an electron pair
Lewis base / Lewis acid: Electron-pair donor / acceptor (general definition)
Curved arrow: Notation showing electron movement from source to sink
Leaving group: A group that departs with the electron pair of its bond
Carbocation: Positively charged carbon with an empty p orbital
Carbanion: Negatively charged carbon with a lone pair
Nucleophilicity: Kinetic ability to donate electrons to carbon

Sources & references

openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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