Questions: Organic Reaction Mechanisms and Arrow Pushing
3 questions to test your understanding
Score: 0 / 3
Question 1 Multiple Choice
In curved arrow notation, a double-headed curved arrow represents the movement of:
AA single electron
BAn electron pair from a nucleophile to an electrophile
CA proton from one atom to another
DAn entire atom between two molecules
Double-headed (full) curved arrows always represent the movement of two electrons — an electron pair. They originate at an electron-rich source (a lone pair, a pi bond, or a sigma bond) and point toward an electron-poor sink (an electrophile or antibonding orbital). Single-electron movement uses fishhook (half-headed) arrows in radical mechanisms.
Question 2 True / False
A fishhook (half-headed) curved arrow in a reaction mechanism means the same thing as a regular double-headed curved arrow, just drawn differently for stylistic reasons.
TTrue
FFalse
Answer: False
The distinction is mechanistically significant, not stylistic. A double-headed arrow represents a two-electron movement (ionic mechanism), while a fishhook arrow represents a one-electron movement (radical mechanism). Confusing them changes the entire mechanistic interpretation — radical and ionic pathways involve different intermediates, conditions, and reactivity patterns.
Question 3 Short Answer
In the reaction step where H₂O donates a lone pair to H⁺ forming H₃O⁺, which species is the nucleophile and which is the electrophile? Explain why.
Think about your answer, then reveal below.
Model answer: H₂O is the nucleophile because its oxygen has lone pairs (electron-rich) and donates them to H⁺. H⁺ is the electrophile because it has an empty orbital and no electrons to contribute — it accepts the electron pair. The curved arrow is drawn from the lone pair on oxygen pointing to H⁺.
This proton-transfer is the simplest example of nucleophile/electrophile thinking. Nucleophiles are electron-pair donors (Lewis bases); electrophiles are electron-pair acceptors (Lewis acids). Practicing on familiar acid-base reactions builds the arrow-pushing intuition needed for more complex substitution and addition mechanisms.