Question

Account for the fact that treatment of tert-butyl methyl ether with a limited amount of concentrated HI gives methanol and tert-butyl iodide rather than methyl iodide and tertbutyl alcohol.

Short answer

Answer: Treating tert-butyl methyl ether with a limited amount of concentrated HI gives methanol and tert-butyl iodide because the limited amount of HI causes methanol to react quickly with the remaining HI, forming methyl iodide and water. This shifts the reaction equilibrium towards methanol consumption, leading to the preferential formation of methanol and tert-butyl iodide over the expected products, methyl iodide and tert-butyl alcohol. The reaction follows an acid-catalyzed nucleophilic substitution mechanism.

Step-by-step solution

Understand the reactants and products

We are given tert-butyl methyl ether as the starting material and concentrated HI as the reagent. The products obtained are methanol and tert-butyl iodide. The starting material, tert-butyl methyl ether, has the following structure: (CH3)3COCH3. When reacting with concentrated HI, we expect cleavage of the ether bond, forming either methanol (CH3OH) and tert-butyl iodide ((CH3)3CI) or methyl iodide (CH3I) and tert-butyl alcohol ((CH3)3COH).

Describe the reaction mechanism

The reaction of ethers with HI is an acid-catalyzed nucleophilic substitution (SN1 or SN2) reaction. In the case of tert-butyl methyl ether, the reaction proceeds as follows: 1. The oxygen atom in the ether molecule attacks the proton in HI, forming a protonated ether. 2. The iodide ion (I-) acts as a nucleophile and attacks the less substituted carbon (the carbon in the methyl group), displacing the oxygen atom and forming methanol. 3. At the same time, the less substituted alkyl iodide - in this case, methyl iodide - is also formed. However, since we have a limited amount of concentrated HI, the methanol produced would further react with the remaining HI to form methyl iodide and water. The final products would be tert-butyl iodide and methyl iodide.

Explain the product selectivity

The selectivity of the products depends on the reaction conditions. In this case, we are given a limited amount of concentrated HI. Due to the limited amount of HI, once methanol is formed from the initial ether cleavage, it quickly reacts with the remaining HI to form methyl iodide (CH3I) and water (H2O), shifting the equilibrium of the reaction towards methanol consumption. This leads to the preferential formation of methanol and tert-butyl iodide, instead of the expected products, methyl iodide and tert-butyl alcohol.

Key concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution is a fundamental class of reactions whereby a nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group. This type of reaction is essential for understanding many processes involving organic compounds, like the cleavage of ethers.

There are two main types of nucleophilic substitution reactions: SN1 and SN2.

• SN1 reactions: This is a two-step process where the leaving group departs before the nucleophile attacks. It's typical of tertiary carbons, where the carbocation intermediate formed is relatively stable.
• SN2 reactions: This involves a single, concerted step where the nucleophile attacks the electrophilic carbon at the same time as the leaving group departs. This type of mechanism is common with primary and secondary carbons, where backside attack can easily occur.

When tert-butyl methyl ether reacts with hydrogen iodide (HI), the iodide ion can be seen as the nucleophile that attacks the electrophilic carbon atom in the molecule, leading to the formation of methanol and tert-butyl iodide.

Acid-Catalyzed Reaction

An acid-catalyzed reaction involves the addition of an acid to initiate or speed up a chemical process. In the context of ether cleavage, the acid provides a proton which can be added to the ether oxygen. This protonation step increases the susceptibility of the ether to nucleophilic attack because it makes the adjacent carbon atoms more electrophilic.

In our specific case concerning tert-butyl methyl ether treatment with HI, the concentrated acid (H+ from HI) activates the ether oxygen making it more electrophilic. As a result, the oxygen atom takes on a positive charge, facilitating the subsequent attack by the iodide ion (I-), a good nucleophile. This results in the cleavage of the C-O bond and forms the desired products, methanol and tert-butyl iodide.

Organic Reaction Mechanisms

Organic reaction mechanisms outline the step-by-step sequence of elementary reactions by which overall chemical change occurs. These mechanisms help scientists and students understand how atoms rearrange during reactions, the intermediates formed, and the types of bonds broken and formed.

In the case of tert-butyl methyl ether and HI reaction, understanding the mechanism allows us to rationalize product formation. The ether oxygen gets protonated first, enhancing its leaving ability. Then, the iodide ion performs a backside attack on the more accessible methyl carbon (due to steric hindrance by the tert-butyl group) leading to the cleavage and the formation of methanol and tert-butyl iodide. This reaction showcases the interplay between steric effects and the nucleophilicity of reactants, which are crucial concepts in deciphering organic mechanisms.

Tert-Butyl Methyl Ether

Tert-butyl methyl ether is an organic compound commonly used as a solvent or as an etherifying agent. Its structure consists of a methyl group (CH3) and a tert-butyl group ((CH3)3C) attached to an oxygen atom. This molecule is important to study as it's a model for steric effects in organic reactions, particularly in nucleophilic substitutions.

Due to the bulky tert-butyl group, certain reactions are hindered, which affects the reactivity of the compound. For example, with concentrated HI, the iodide is more likely to attack the less hindered methyl group rather than the bulky tert-butyl side, leading to the preferential formation of methanol instead of tert-butyl alcohol. This demonstrates how the structure of a compound can influence its chemical behavior and the outcome of reactions.