Question

Treatment of (S)-2-bromobutane with sodium hydroxide results in the production of a compound with an (R) configuration. This reaction has most likely taken place through: A an \(\mathrm{S}_{\mathrm{N}} 1\) mechanism. B an \(S_{N} 2\) mechanism. C an \(\mathrm{S}_{\mathrm{N}} 1\) and \(\mathrm{S}_{\mathrm{N}} 2\) mechanism in sequence. D an \(\mathrm{S}_{\mathrm{N}} 1\) and \(\mathrm{S}_{\mathrm{N}} 2\) mechanism simultaneously.

Short answer

B. an \(\text{S}_\text{N}2\) mechanism.

Step-by-step solution

Understand the problem

Identify the initial and final configurations of the compound and the reaction mechanism to understand the question.

Initial Structure

(S)-2-bromobutane is given as the starting compound, where (S) refers to its stereochemistry configuration.

Reaction with Sodium Hydroxide

Treatment with sodium hydroxide indicates a substitution reaction as hydroxide ion will replace the bromine atom.

Final Configuration

The final compound has (R) configuration, which indicates an inversion of configuration.

Identify Mechanism Types

Recall the two main substitution mechanisms. 1. \(\text{S}_\text{N}1\): leads to racemization due to the planar intermediate.2. \(\text{S}_\text{N}2\): leads to inversion of configuration due to the backside attack.

Determine the Mechanism

Since there is an inversion of configuration, the reaction must have taken place through an \(\text{S}_\text{N}2\) mechanism.

Key concepts

stereochemistry

Stereochemistry deals with the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In organic chemistry, understanding stereochemistry is crucial since it affects how molecules interact with each other. For instance, in a substitution reaction involving (S)-2-bromobutane, the ‘(S)’ denotes its chiral configuration. Chiral molecules have non-superimposable mirror images, much like left and right hands.
When we say a molecule is (S) or (R), we are referring to its absolute configuration, which is determined by the Cahn-Ingold-Prelog priority rules. Knowing the stereochemistry of reactants and products can provide insights into the reaction mechanism involved.

inversion of configuration

Inversion of configuration is a key concept in substitution reactions, particularly in the \(\text{S}_\text{N}2\) mechanism. This phenomenon occurs when a nucleophile attacks a carbon atom from the opposite side of the leaving group, resulting in a 'flipping' of the molecular configuration. For example, if you start with (S)-2-bromobutane and it undergoes an \(\text{S}_\text{N}2\) reaction with sodium hydroxide, the product will have an (R) configuration.
This can be imagined like an umbrella turning inside out in a gust of wind! This inversion is often referred to as Walden inversion and is a hallmark of the \(\text{S}_\text{N}2\) mechanism due to its backside attack.

substitution reactions

Substitution reactions are one of the fundamental reaction types in organic chemistry where one atom or group of atoms in a molecule is replaced by another. There are two main types: \(\text{S}_\text{N}1\) and \(\text{S}_\text{N}2\).
\(\text{S}_\text{N}1\) stands for unimolecular nucleophilic substitution and involves a two-step mechanism with a carbocation intermediate. This tends to occur with tertiary carbons.
\(\text{S}_\text{N}2\) stands for bimolecular nucleophilic substitution and involves a single, concerted step where the nucleophile attacks the substrate from the opposite side of the leaving group, leading to inversion of configuration. This is more common with primary carbons.
Knowing whether a reaction follows an \(\text{S}_\text{N}1\) or \(\text{S}_\text{N}2\) mechanism helps predict the stereochemistry and the reaction rate.

SN1 vs SN2 mechanisms

Comparing \(\text{S}_\text{N}1\) and \(\text{S}_\text{N}2\) mechanisms is essential to understand their unique characteristics and when each is likely to occur. Here are key differences:

• \(\text{S}_\text{N}1\) Mechanism: Occurs in two steps. First, the leaving group departs, forming a carbocation intermediate. Then, the nucleophile attacks the planar carbocation. This mechanism often leads to a mixture of retention and inversion of configuration (racemization).


• \(\text{S}_\text{N}2\) Mechanism: Occurs in a single, concerted step. The nucleophile attacks the carbon at the same time as the leaving group exits, from the opposite side. This always leads to inversion of configuration due to the backside attack.

Whether a reaction follows \(\text{S}_\text{N}1\) or \(\text{S}_\text{N}2\) depends on factors like the substrate’s structure, the strength of the nucleophile, the leaving group, and the solvent used. For example, the reaction of (S)-2-bromobutane with sodium hydroxide, which results in an (R) configuration product, clearly follows the \(\text{S}_\text{N}2\) mechanism, showcasing the importance of these distinctions.