Problem 52 - A Complete Resource Book in Chemistry for JEE Main

Question

Tert-butyl methyl ether on heating with \(1 \mathrm{M} \mathrm{HI}\) gives (a) \(\mathrm{CH}_{3} \mathrm{I}+\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) (b) \(\mathrm{CH}_{3} \mathrm{OH}+\left(\mathrm{CH}_{3}\right)_{3} \mathrm{Cl}\) (c) \(\mathrm{CH}_{3} \mathrm{I}+\left(\mathrm{CH}_{3}\right)_{3} \mathrm{Cl}\) (d) none of these

Step-by-Step Solution

Answer

(a) \( \mathrm{CH}_{3} \mathrm{I} + \left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH} \)

Step 1: Analyze the Given Reaction

In the given question, we have tert-butyl methyl ether reacting with hydriodic acid (HI). Tert-butyl methyl ether is an ether with the formula \((CH_3)_3COCH_3\). We need to determine what this reaction will yield.

Step 2: Consider Possible Reaction Products

When ethers react with HI, they can undergo cleavage. The two possible cleavage products are methyl iodide (CH₃I) and tert-butyl alcohol \((CH_3)_3COH\). The halogen generally attaches to the less hindered, smaller alkyl group, in this case, the methyl group.

Step 3: Understand the Mechanism of Reaction

HI is a strong acid and will break the ether bond via an SN2 mechanism. The Iodide ion \(I^-\) will preferentially bond with the smaller methyl group because \(CH_3^+\) is a more stable carbocation than \((CH_3)_3C^+\) due to steric hindrance.

Step 4: Verify Products for Other Choices

Other options in the question suggest combinations involving chloride or methanol, which are not typically produced in this reaction. Only methyl iodide and tert-butyl alcohol are realistic products here.

Step 5: Compile the Correct Reaction

From the information and steps above, the correct reaction for heating tert-butyl methyl ether with \(1 \text{M} \text{HI}\) is: \( (CH_3)_3COCH_3 + HI ightarrow CH_3I + (CH_3)_3COH \).

Key Concepts

Ether Cleavage

Ether cleavage is a chemical reaction where an ether molecule (symbolized as R-O-R') is broken down by a reagent, often a strong acid. Ethers are known for their relatively stable nature, making their cleavage an interesting and significant reaction in organic chemistry. When ethers, such as tert-butyl methyl ether, are exposed to an acid like hydriodic acid (HI), they undergo cleavage, breaking the oxygen bond with the alkyl groups.
This process typically results in the formation of an alcohol and an alkyl halide. The choice of acid for cleavage often depends on the desirability of the products and the reaction conditions. In our context, heating factors in significantly, aiding the cleavage process when combined with HI.

SN2 Mechanism

The SN2 mechanism, or bimolecular nucleophilic substitution, is a fundamental concept in organic chemistry. This mechanism involves a single, concerted step where the nucleophile actively attacks the substrate, resulting in the simultaneous displacement of a leaving group. This results in a so-called 'backside attack.'
In the ether cleavage of tert-butyl methyl ether, the iodide ion from hydriodic acid functions as the nucleophile. It attacks the less hindered methyl group of the ether, displacing the oxygen and forming methyl iodide. The preference for a less hindered site makes SN2 an ideal mechanism in this scenario, since tert-butyl groups are quite bulky and sterically hinder the approach of the nucleophile.

Tert-butyl methyl ether

Tert-butyl methyl ether, with the chemical formula \((CH_3)_3COCH_3\), is an example of a simple ether. It serves as a common reagent and solvent in organic reactions due to its excellent solvent properties and relatively low toxicity.
The structure of tert-butyl methyl ether involves a larger, branched tert-butyl group attached to a simple methyl group via an oxygen atom. In reactions such as ether cleavage using HI, the properties of this structure are pivotal. Specifically, the bulkiness of the tert-butyl group means it provides steric hindrance, a key factor in determining the mechanism of attack by the nucleophile in the cleavage reaction. In SN2 reactions, the smaller, less hindered methyl group is more likely to participate, evidenced by the resulting product, methyl iodide.

Hydroiodic Acid Reaction

In the context of organic reactions, hydroiodic acid (HI) is a potent acid known for its effectiveness in cleaving ether bonds. Its strong acidic nature provides both protons for protonation and iodide ions as nucleophiles.
During the ether cleavage of tert-butyl methyl ether, hydroiodic acid performs a dual role. First, it protonates the ether oxygen, making it a better leaving group. Then, the iodide ion (I^-) participates in the nucleophilic attack on the less hindered methyl group. This results in the separation of the original compound into methyl iodide and tert-butyl alcohol.
Understanding the role of HI in this reaction highlights its importance as an agent for facilitating difficult reactions, thanks to its dual function as an acid and a source of iodide ions.