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 tert-butyl alcohol.

Short answer

Question: Explain why the reaction between tert-butyl methyl ether and concentrated HI results in the formation of methanol and tert-butyl iodide instead of methyl iodide and tert-butyl alcohol. Answer: The reaction between tert-butyl methyl ether and concentrated HI undergoes acid-catalyzed ether cleavage. Due to steric hindrance, the iodide ion preferentially attacks the less crowded methyl carbon, resulting in a more stable carbon-iodine bond. This selectivity leads to the formation of methanol and tert-butyl iodide as the observed products instead of methyl iodide and tert-butyl alcohol.

Step-by-step solution

Write down the reaction

First, let's write down the reaction for the treatment of tert-butyl methyl ether with concentrated HI: tert-butyl methyl ether + HI → methanol + tert-butyl iodide Notice that we expected to form methyl iodide and tert-butyl alcohol, but instead, methanol and tert-butyl iodide are formed.

Identify the mechanism

This reaction is an example of acid-catalyzed cleavage of ethers. Ethers are susceptible to cleavage in the presence of a strong acid. The mechanism involves protonation of the ether oxygen, followed by a nucleophilic attack by the iodide ion and finally a deprotonation step.

Examine the reaction selectivity

In this case, the ether oxygen in tert-butyl methyl ether is protonated by the strong acid, HI: (OCH_3)C(CH_3)_3 + HI → (OCH_3)C(CH_3)_3^+I^- Once protonated, it becomes more electrophilic, susceptible to a nucleophilic attack by the iodide ion. The iodide ion attacks the most sterically accessible carbon to form the most stable carbon-iodine bond. Due to the tert-butyl group's size, the iodide's nucleophilic attack is preferentially directed toward the less crowded methyl carbon: I^- + (OCH_3)C(CH_3)_3^+ → I-C(CH_3)_3 + HOCH_3 Now we have an intermediate, tert-butyl iodide, and methanol. A final deprotonation of methanol yields the observed products: HOCH_3 + HI → H_2O + CH_3I Since the reaction is performed with a limited amount of HI, the excess methanol remains unreacted, thus giving methanol and tert-butyl iodide as the observed products.

Summarize the reaction process and result

In summary, tert-butyl methyl ether reacts with HI under acid-catalyzed ether cleavage conditions. Due to steric hindrance, the iodide ion preferentially attacks the less crowded methyl carbon with a more stable carbon-iodine bond. As a result, methanol and tert-butyl iodide are observed as products instead of the expected methyl iodide and tert-butyl alcohol.

Key concepts

Nucleophilic Attack

In organic chemistry, a nucleophilic attack occurs when a nucleophile, an electron-rich species, donates a pair of electrons to an electrophile, an electron-deficient species, forming a new covalent bond. During the acid-catalyzed cleavage of ethers like tert-butyl methyl ether, the protonated ether becomes highly electrophilic, making it an ideal target for attack by a nucleophile, such as an iodide ion (I-).

The iodide ion is attracted to the positive charge on the protonated oxygen of the ether. It has a lone pair of electrons which it can donate to the carbon atom bonded to the oxygen. This nucleophilic attack is key to breaking the ether bond and driving the reaction forward, leading to the products of the reaction, methanol and tert-butyl iodide. The ease and outcome of the attack are influenced by factors such as steric hindrance and the stability of the forming bond.

Understanding the Role of Protonation

The first step in making the ether susceptible to nucleophilic attack is the protonation of the oxygen atom. This step increases the electrophilic character of the carbon atoms, as the positive charge on oxygen makes it a better leaving group. This prepares the stage for the incoming nucleophile, the iodide ion, to form a bond with the carbon.

Outcome of the Nucleophilic Attack

The result of the nucleophilic attack in the given reaction is the formation of the more stable tert-butyl iodide rather than methyl iodide. This preference is explained by the difference in steric environments around the carbon atoms in the starting ether, a concept deeply intertwined with steric hindrance, which is explored further in the next section.

Steric Hindrance

Steric hindrance refers to the resistance that atoms or groups of atoms encounter when their volumes prevent them from getting close enough to participate in reactions. In the case of the acid-catalyzed cleavage of tert-butyl methyl ether, the bulky tert-butyl group offers significant resistance to nucleophilic attack, inhibiting reactions at the tertiary carbon atom.

The iodide ion, acting as the nucleophile, favors the path of least resistance, hence, it attacks the less hindered methyl carbon, which is more accessible due to its small size and lesser steric bulk. This preferential attack is directly related to the ease with which the nucleophile can approach the reactive site and form a new bond with the carbon atom.

Impact of Steric Hindrance on Reactivity

Steric hindrance can profoundly affect the rate and outcome of chemical reactions. Large groups can slow down or even prevent certain reactions from occurring at particular sites due to the spatial difficulty in accommodating the reacting species. This plays a crucial role in determining which products are formed, especially in reactions having more than one potential site for nucleophilic attack, such as in our case with tert-butyl methyl ether.

Adjusting Predictions Based on Steric Effects

Understanding steric hindrance allows chemists to predict the products of reactions more accurately. By considering the size and arrangement of groups around a reactive center, it's possible to anticipate which bonds will be broken or formed, improving our ability to design and carry out chemical syntheses effectively.

Organic Reaction Mechanisms

Organic reaction mechanisms are step-by-step descriptions of the changes in the structure of molecules during a chemical reaction. These provide a detailed account of the breaking and forming of bonds, along with the intermediate states and transition states that species pass through during the course of a reaction.

Delving into the reaction mechanism helps provide insight as to why certain products are formed in a reaction over others. In the cleavage of tert-butyl methyl ether by concentrated HI, the sequence of steps - protonation, nucleophilic attack, and deprotonation - illustrates how methanol and tert-butyl iodide are produced rather than methyl iodide and tert-butyl alcohol.

Analyzing Each Step in the Reaction

Detailed knowledge of the steps allows chemists to predict the speed of different stages, identify which step is likely to be the rate-determining step (the slowest step that controls the overall rate of the reaction), and understand how the reaction can be influenced by changing the reaction conditions such as temperature, concentration, and solvents.

Significance of Reaction Intermediates

Organic reaction mechanisms often involve unstable intermediates that do not appear in the final reaction products but are essential for the reaction to progress. In our example, the protonated ether and the tert-butyl iodide are intermediates that help us understand how the reactants are transformed into the final products.
By mapping out these steps, organic chemists can not only explain the outcomes of known reactions but can also predict and design new synthetic pathways for producing desired chemical compounds, thus organically reaction mechanisms are foundational tools in the field of organic chemistry.