Difficulty: Intermediate | Prerequisites: CHEM 2301 Ch. 5 Part 1 (reaction types, bond cleavage), Lewis structures, electronegativity, formal charge.
Polar reactions are the dominant class of organic reactions and the ones you will spend most of your time on in CHEM 2301. They arise from heterolytic bond cleavage in polar compounds, where electron density is unevenly distributed. The central idea is simple: electron-rich species (nucleophiles) donate electrons to electron-poor species (electrophiles), forming new bonds. Once you can identify the nucleophile and electrophile in any reaction, you can predict the direction of electron flow and draw the mechanism. This section maps the four patterns of electron flow in polar reactions (addition, elimination, substitution, rearrangement) and gives you the tools to classify and draw arrows for each.
Polar reactions involve heterolytic bond cleavage. Nucleophiles (electron-rich) attack electrophiles (electron-poor), and double-headed curved arrows track where the electron pairs go. The four electron-flow patterns, nucleophilic attack, loss of a leaving group, substitution (both combined), and rearrangement, account for every polar mechanism you will see this semester.
Nucleophile (Nu⁻)
An electron-rich species that donates a pair of electrons to form a new bond. Nucleophiles are the site of high electron density. Think of it as: the electron donor, the species that "loves nuclei" (nucleus-seeking).
Electrophile (E⁺)
An electron-poor species that accepts a pair of electrons to form a new bond. Electrophiles are the site of low electron density. In simple terms, the electron acceptor, the species that "loves electrons."
Leaving group (LG)
A group that departs from a molecule during an elimination or substitution reaction, taking the bonding electrons with it. Good leaving groups are stable as anions (they can stabilise the negative charge). Think of it as: the group that gets kicked out, leaving with the electrons from the broken bond.
Polar reaction
A reaction involving heterolytic bond cleavage, where both electrons in a breaking bond move together to one atom. Tracked with double-headed curved arrows. In simple terms, reactions driven by the attraction between opposite partial (or full) charges.
Charge balance
The principle that the total formal charge of the starting materials must equal the total formal charge of the products. No charge is created or destroyed in a balanced reaction. Think of it as: a bookkeeping check on your mechanism. If charges do not balance, something is wrong.
The first step in any polar mechanism is finding the nucleophile and electrophile.
Nucleophile features: atoms with lone pairs (N, O, S, halide anions), pi bonds (C=C), negatively charged species, Grignard reagents (RMgBr). The curved arrow always starts from the nucleophile.
Electrophile features: carbocations (6 electrons, empty p orbital), atoms bonded to more electronegative atoms (partial positive charge, e.g. the carbon in C-Br where carbon is delta-positive), protons on strong acids. The curved arrow always points toward the electrophile.
The nucleophile donates a pair of electrons to the electrophile, forming a new bond. Two reactants become one product (A + B → C).
Example: Br⁻ attacks a carbocation to form an alkyl bromide.
The curved arrow goes from the lone pair on Br⁻ to the positive carbon.
Charge balance: (-1) + (+1) = 0. The product is neutral.
A bond within the molecule breaks heterolytically. The leaving group departs with both bonding electrons. One reactant becomes two products (A → B + C).
Example: an alkyl bromide loses Br⁻ to form a carbocation.
The curved arrow goes from the C-Br bond to bromine.
The leaving group is anionic: it leaves with the electron pair and stabilises the negative charge.
A leaving group is defined as a group lost from carbon in an elimination or substitution reaction.
A nucleophile attacks while a leaving group departs. Two reactants, two products (A + B → C + D). Bond formation and bond breaking happen in the same step or in sequence.
Example 1: HO⁻ + H-Cl → H-OH + Cl⁻. The nucleophile (HO⁻) attacks the electrophilic hydrogen, and Cl⁻ leaves.
Example 2: a phenoxide anion attacks CH₂Cl (with N(CH₃)₂ as the nucleophile in a separate example), displacing Cl⁻.
Two curved arrows: one from nucleophile to electrophilic centre (forming bond), one from breaking bond to leaving group.
A single reactant reorganises bonds internally. In polar reactions, rearrangement typically involves carbocation intermediates shifting a hydrogen or alkyl group to an adjacent carbon to form a more stable carbocation.
The nucleophile/electrophile framework is how medicinal chemists design drug molecules: they identify electrophilic sites on biological targets and synthesise nucleophilic drugs that bind to them (or vice versa). Substitution reactions are used industrially to attach functional groups to organic scaffolds in everything from dye manufacturing to agrochemical production.
Students often draw curved arrows going from the electrophile to the nucleophile. Arrows always go from electron-rich to electron-poor: from the nucleophile toward the electrophile.
A common error is forgetting charge balance. If your starting materials have a net charge of -1 and your products have a net charge of 0, you have made an error somewhere. Charge is conserved.
Students sometimes think a leaving group must be a halide. Many groups can serve as leaving groups (water, tosylate, etc.), provided they are stable once they depart with the bonding electrons.
Pi bonds can act as nucleophiles, which surprises students who think only lone pairs donate electrons. The electron density in a C=C double bond is accessible and can attack electrophiles.
⚠️ You will be asked to identify the nucleophile and electrophile in a reaction, then draw curved arrows showing electron flow. Practise this until it is automatic (LOs 1k).
⚠️ Be able to classify a polar reaction as addition, elimination, or substitution by examining the number of reactants and products.
⚠️ Always check charge balance after drawing your mechanism. This is a fast way to catch errors.
⚠️ Know examples of common nucleophiles (HO⁻, RO⁻, halide ions, amines, pi bonds, Grignard reagents) and common electrophiles (carbocations, polarised C-X bonds, protons on acids).
True or false: A nucleophile is the site of low electron density. False. A nucleophile is the site of high electron density. The electrophile is the electron-poor site.
Fill in the blank: In a substitution reaction, the nucleophile forms a new bond while the ______ departs. Leaving group.
True or false: Curved arrows in polar mechanisms point from the electrophile to the nucleophile. False. Arrows point from the nucleophile (electron source) toward the electrophile (electron sink).
Fill in the blank: A good leaving group must be able to ______ the negative charge once it departs. Stabilise.
True or false: A pi bond can act as a nucleophile. True. The electron density above and below the plane of a C=C bond can donate into an electrophile.
Q: In the reaction HO⁻ + H-Cl → H₂O + Cl⁻, identify the nucleophile, electrophile, and leaving group.
A: HO⁻ is the nucleophile (electron-rich, has lone pairs). The hydrogen of H-Cl is the electrophilic site (bonded to the more electronegative Cl, so H carries a partial positive charge). Cl⁻ is the leaving group.
Q: Classify this reaction: a tertiary alkyl bromide loses Br⁻ to form a carbocation. What type of polar reaction pattern is this?
A: Loss of a leaving group (elimination pattern). One reactant becomes two products: the carbocation and Br⁻.
Q: Draw the curved arrows for the reaction: Br⁻ attacks a tertiary carbocation to form a C-Br bond. What pattern is this?
A: Nucleophilic attack (addition pattern). One curved arrow from a lone pair on Br⁻ to the positive carbon. Two reactants combine into one product.
Q: A reaction has two reactants and two products. The nucleophile forms a bond to carbon while a leaving group departs. Classify this reaction.
A: Substitution. A + B → C + D with simultaneous (or sequential) bond formation and bond breaking.
Q: Why is Cl⁻ a reasonable leaving group but CH₃⁻ is not?
A: Cl⁻ is stable because chlorine is electronegative and can hold the negative charge comfortably. CH₃⁻ (a carbanion) is a very strong base and highly unstable, making it a poor leaving group.
The nucleophile/electrophile concept builds directly on electronegativity and polarity from general chemistry and Ch. 2. Substitution patterns here are the conceptual preview of SN1 and SN2 mechanisms you will study in detail. The leaving group concept returns in elimination reactions (E1, E2). Understanding electron flow arrows is the single most transferable skill in organic chemistry: every mechanism you draw for the rest of the course uses these same patterns.
Polar reactions, nucleophile, electrophile, electron flow, curved arrows, nucleophilic attack, leaving group, LG, substitution, addition, elimination, charge balance, heterolytic cleavage, electron-rich, electron-poor, lone pair, pi bond nucleophile, Grignard reagent, carbocation, partial charge, delta plus, delta minus, CHEM 2301, organic chemistry I, Chapter 5, LOs 1k