Question

You are given a supply of ethyl iodide, tertbutyl iodide, sodium ethoxide, and sodium tert-butoxide. Your task is to use the \(\mathrm{S}_{\mathrm{N}} 2\) reaction to make as many different ethers as you can. In principle, how many are possible? In practice, how many can you make?

Short answer

Two ethers can be made in practice.

Step-by-step solution

Identify the Starting Materials

List the given starting materials: ethyl iodide, tert-butyl iodide, sodium ethoxide, and sodium tert-butoxide.

Understand the \( \mathrm{S}_{\mathrm{N}} 2 \) Reaction

Recall that the \( \mathrm{S}_{\mathrm{N}} 2 \) reaction involves a single step in which the nucleophile attacks the electrophile, leading to the inversion of stereochemistry. This type of reaction is favored by strong nucleophiles and less hindered electrophiles.

Determine Possible Combinations

Combine each electrophile (ethyl iodide, tert-butyl iodide) with each nucleophile (sodium ethoxide, sodium tert-butoxide). Ideally, there are four combinations: ethyl iodide with sodium ethoxide, ethyl iodide with sodium tert-butoxide, tert-butyl iodide with sodium ethoxide, and tert-butyl iodide with sodium tert-butoxide.

Evaluate Feasibility for Each Combination

Consider the feasibility of each \( \mathrm{S}_{\mathrm{N}} 2 \) reaction based on steric hindrance: 1. Ethyl iodide with sodium ethoxide: feasible2. Ethyl iodide with sodium tert-butoxide: feasible3. Tert-butyl iodide with sodium ethoxide: not feasible due to steric hindrance4. Tert-butyl iodide with sodium tert-butoxide: not feasible due to steric hindrance

Final Count of Ethers

Conclude that in principle, four different ethers are possible, but in practice, only two ethers can be synthesized through the \( \mathrm{S}_{\mathrm{N}} 2 \) mechanism.

Key concepts

nucleophile

A nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction. Imagine it as a particle with extra electrons looking for a partner with fewer electrons. Nucleophiles are typically either negatively charged ions or molecules with a lone pair of electrons that can be shared. In the \( \mathrm{S}_{\mathrm{N}} 2 \) reaction, strong nucleophiles are essential for attacking the electrophile (usually a carbon center with a leaving group like iodide).
Examples of nucleophiles include sodium ethoxide \( \mathrm{NaOC_2H_5} \) and sodium tert-butoxide \( \mathrm{NaO(tBu)} \). These nucleophiles have lone pairs of electrons on oxygen that drive them to attack carbon atoms in electrophiles.

electrophile

An electrophile is a species deficient in electrons, making it eager to accept an electron pair. Think of electrophiles as electron-seeking species. They are often characterized by a positive charge or by having a partial positive charge due to an electron-withdrawing group. In the \( \mathrm{S}_{\mathrm{N}} 2 \) reaction context, the electrophile typically bears a leaving group like iodide, making it susceptible to nucleophilic attack.
For our exercise, the electrophiles are ethyl iodide \( \mathrm{C_2H_5I} \) and tert-butyl iodide \( \mathrm{(CH_3)_3CI} \). Ethyl iodide has less steric hindrance, making it a good candidate for an \( \mathrm{S}_{\mathrm{N}} 2 \) reaction, while tert-butyl iodide is hindered due to the bulky tert-butyl group, making \( \mathrm{S}_{\mathrm{N}} 2 \) reaction less favorable.

steric hindrance

Steric hindrance refers to the prevention of reactions at a particular location within a molecule due to the size of substituent groups around it. In simple terms, bulkier groups around the reaction center can block access to nucleophiles, making the reaction more difficult or even impossible.
For instance, in the exercise, tert-butyl iodide has a bulky tert-butyl group which hinders the entry of nucleophiles to the carbon center. This steric hindrance makes it challenging for any \( \mathrm{S}_{\mathrm{N}} 2 \) reaction to occur with tert-butyl iodide, whereas ethyl iodide, which is less bulky, does not experience significant steric hindrance and is more reactive in \( \mathrm{S}_{\mathrm{N}} 2 \) conditions.

stereochemistry inversion

Stereochemistry inversion is a key feature of the \( \mathrm{S}_{\mathrm{N}} 2 \) reaction. During the reaction, the nucleophile attacks the electrophile from the side opposite to the leaving group. This backside attack leads to an inversion of configuration around the carbon atom, similar to flipping an umbrella inside out.
For instance, if the carbon in ethyl iodide had substituents arranged in a particular spatial configuration, once sodium ethoxide nucleophile attacks, the spatial arrangement of substituents around the central carbon would be inverted. This inversion is crucial in understanding why stereochemistry can dramatically change in products formed through \( \mathrm{S}_{\mathrm{N}} 2 \) reactions.