This lesson introduces the unimolecular nucleophilic substitution reaction, its stepwise mechanism, and the relationship between its mechanism and its rate law. Understanding these ideas helps explain why some substrates react by SN1 and why changing the nucleophile may not change the reaction rate.
The Hughes–Ingold symbol SN1 describes a nucleophilic substitution reaction in which the rate-determining step is unimolecular. In a common SN1 pathway, the substrate first loses its leaving group to form a carbocation. The nucleophile attacks that carbocation afterward.
All flashcards
What is an SN1 reaction?
It is a nucleophilic substitution reaction whose rate-determining step involves ionization of one substrate molecule, usually forming a carbocation.
What are the three stages of tert-butyl bromide hydrolysis by an SN1 pathway?
Slow ionization forms a tert-butyl carbocation; water attacks to form an oxonium ion; deprotonation produces tert-butanol.
Why is the first step usually rate-determining?
Breaking the carbon–leaving-group bond to form a high-energy carbocation requires the greatest activation energy in the usual pathway.
When does the simple first-order SN1 rate law apply?
It applies when capture of the carbocation by the nucleophile dominates over recombination with the leaving group, so the rate is approximately proportional to substrate concentration alone.
What is the common ion effect in the example reaction?
Adding bromide can favor recombination of the carbocation with bromide, reducing the amount captured by water and slowing product formation.
Quiz questions
Why does attack by neutral water on a carbocation initially produce an oxonium ion?Answer: Water forms a bond using a lone pair, leaving oxygen positively charged.
What is the characteristic stereochemical outcome of an SN2 reaction at a stereogenic carbon?Answer: Inversion of configuration at the reacting center.
How does an E2 reaction form an alkene?Answer: A base removes a proton as the leaving group departs in one concerted step.
How can bulky groups near the reacting carbon favor SN1 over SN2?Answer: They hinder the close nucleophile approach required for SN2.
Which solvent properties commonly favor SN1 ionization?Answer: Polar solvents stabilize ions, and protic solvents can solvate leaving-group anions.
Based on the Wikipedia article SN1 reaction, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.