Question

Tert-butyl methyl ether on heating with \(1 \mathrm{M}\) 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

Short answer

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

Step-by-step solution

Understanding the Reactants

Tert-butyl methyl ether, or TBME, is an ether compound with the formula \( (CH_3)_3C-O-CH_3 \). The exercise states that it reacts with \( 1 \text{M} \) HI, which is a source of \( I^- \) and \( H^+ \), under heating.

Mechanism of Reaction - Ethers and HX Acids

Ethers react with acids such as HI by an SN2 mechanism because the C-O bonds can be cleaved to form alkyl iodides. The acidic environment favors this reaction. In this mechanism, the proton (H+) from HI protonates the ether oxygen, which helps in cleaving the C-O bond.

SN2 Reaction Pathway

In the given ether \( (CH_3)_3C-O-CH_3 \), the methyl group \( CH_3 \) is less sterically hindered than the tert-butyl group \( (CH_3)_3C \), making it the more likely site for nucleophilic attack by \( I^- \). The result is the cleavage of the O-CH3 bond, forming \( CH_3I \) and \( (CH_3)_3COH \).

Compare with Given Options

Comparing the reaction outcome with the given options, we see that the result \( CH_3I + (CH_3)_3COH \) matches option (a). None of the other options reflect the expected products of the reaction based on the mechanism.

Key concepts

SN2 Reaction

The SN2 reaction, or bimolecular nucleophilic substitution, is a fundamental concept in organic chemistry. It involves a nucleophile approaching a substrate from the opposite side of a leaving group, leading to a single, concerted step reaction. This mechanism is highly stereospecific; if the carbon atom undergoing substitution is chiral, the reaction results in an inversion of configuration, often compared to an umbrella turning inside out.
This reaction is favored when the carbon center is less hindered by bulky groups, allowing the nucleophile easy access. In the reaction with tert-butyl methyl ether, the methyl group is smaller and less hindered compared to the tert-butyl group, making it an ideal site for SN2 reactions. The iodide ion (\(I^-\)) acts as the nucleophile, leading to the formation of \(CH_3I\) as one of the products.

Ethers

Ethers are a class of organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups. Their general formula is \(R-O-R'\). In organic reactions, ethers can be relatively stable, but they are susceptible to cleavage under acidic conditions.
In the context of acid-catalyzed reactions, the presence of a strong acid like HI can protonate the ether's oxygen, transforming the ether into a more reactive intermediate. This protonation increases the oxygen's positive charge, favoring the subsequent bond cleavage that defines the reaction mechanism.

• Ethers generally require strong acids to undergo cleavage.
• Acid acts to protonate the oxygen, enhancing its leaving ability.
• Ethers are cleaved at the less hindered carbon, here the methyl group, yielding \(CH_3I\).

Nucleophilic Substitution

Nucleophilic substitution is a type of reaction where a nucleophile replaces a leaving group in a molecule. In our example, the iodide ion (\(I^-\)) acts as the nucleophile. This reaction type is central to many transformations in organic chemistry.
Substrates with good leaving groups, often anions such as halides, are more reactive. The presence of a stronger nucleophile can speed up the reaction. In the reaction of tert-butyl methyl ether with HI, \(CH_3I\) is formed because \(I^-\) preferentially attacks the less hindered methyl group, and \(OH\) becomes the leaving group forming \((CH_3)_3COH\).

• The SN2 mechanism involves a single transition state.
• The steric hindrance around the reacting carbon affects the rate and outcome of the reaction.

Acid-Catalyzed Reaction

Acid-catalyzed reactions are a common means of facilitating reactions that require the protonation of a functional group. In ether cleavage by HI, the acid's role is to protonate the ether's oxygen, making a better leaving group and facilitating the subsequent nucleophilic attack.
The strength of the acid influences the reaction rate and outcome. A strong acid like HI not only protonates but also provides a good nucleophile (\(I^-\)) that participates in the substitution. This reaction typically results in the cleavage of the ether into two separate products:

• The ether's oxygen is protonated to enhance the leaving group potential.
• The nucleophile causes the breakage of bonds, releasing an alcohol and an iodide product.
• The stronger the acid, the more effective and faster the reaction, often requiring heat to proceed smoothly.