Organic Reactions & Mechanisms (CHEM 220) · Module 2 of 9
Nucleophiles, Electrophiles & Reaction Formalism
The vocabulary and arrow-pushing notation used to describe every mechanism that follows.
Nucleophiles & Electrophiles
- A nucleophile is electron-rich and seeks a positive or partially positive center to bond with; an electrophile is electron-poor and seeks electrons.
- Nucleophilicity is affected by charge (anions are usually better nucleophiles than neutral counterparts); in protic solvents, larger, more polarizable atoms like I⁻ are often better nucleophiles than smaller ones like F⁻, due to reduced solvation.
Why is iodide (I⁻) generally a better nucleophile than fluoride (F⁻) in a protic solvent like water?
Solution. Iodide is larger and more polarizable, and it is less tightly solvated by surrounding solvent molecules than the small, highly charge-dense fluoride ion, making its electrons more available to attack an electrophile.
Curved Arrow Notation
- Curved arrows represent the movement of electron pairs, not atoms, from an electron source (like a lone pair or pi bond) to an electron sink (an electrophilic atom).
- Arrows always start at electrons and point to where they're going: a lone pair forming a new bond, or a bond breaking to form a lone pair or leaving group.
In a mechanism, what does a curved arrow starting from a bond and pointing to an atom typically represent?
Solution. It represents that bond's electron pair breaking heterolytically, moving entirely onto that atom, often the atom becoming a leaving group.
Leaving Group Ability
- A good leaving group stabilizes the negative charge, or neutral state, it takes on after leaving — which generally means it's the conjugate base of a strong acid (a weak base itself).
- Common good leaving groups include halides (I⁻ > Br⁻ > Cl⁻ ≫ F⁻) and sulfonate esters like tosylate; hydroxide (OH⁻) is a poor leaving group unless protonated first to become water.
Why is iodide a better leaving group than fluoride?
Solution. Iodide is a much weaker base — the conjugate base of the strong acid HI — and can stabilize the negative charge it takes on more effectively than fluoride, which is a stronger base and less stable once it leaves.
Acid-Base Character & Its Role in Mechanisms
- Many organic mechanism steps are fundamentally acid-base reactions (proton transfers) that activate a poor leaving group, like OH, by protonating it, or generate a nucleophile by deprotonating a weak acid.
Why might a chemist first protonate an alcohol's -OH group before it can act as a leaving group in a substitution reaction?
Solution. Protonating the -OH group converts it into -OH₂⁺, converting a poor leaving group (hydroxide) into a good one (water), since water is a much weaker base and more stable once it departs.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What best describes a nucleophile?
2. What does a curved arrow in a mechanism represent?