Question

Heating methyl phenyl ether with HI gives (a) methanol + iodobenzene (b) methyl alcohol + benzyl alcohol (c) methyl iodide \(+\) phenol (d) methyl iodide + iodobenzene

Short answer

(c) methyl iodide + phenol.

Step-by-step solution

Understanding the Reaction

When methyl phenyl ether ( ext{C}_6 ext{H}_5 ext{OCH}_3) is treated with HI, the iodide ion ( ext{I}^-), which is a strong nucleophile, is able to break the ether bond. The cleavage typically occurs at the carbon-oxygen bond, leaving one part with an iodine substituent.

Reaction Mechanism Overview

The reaction occurs via an S ext{N}2 mechanism. The incoming iodide ion attacks the less hindered methyl group because it forms a more stable and faster reacting intermediate than attacking the phenyl group.

Identifying the Reaction Products

In the presence of HI, methyl phenyl ether will undergo cleavage at the methyl-oxygen bond. This leads to the formation of methyl iodide ( ext{CH}_3 ext{I}) because the methyl group is less sterically hindered and can better accommodate the iodide ion attack. The phenyl group will then be attached to the hydroxyl group left behind, forming phenol ( ext{C}_6 ext{H}_5 ext{OH}).

Choosing the Correct Option

From the analysis, we know the products of the reaction are methyl iodide and phenol. Therefore, the correct answer corresponds to option (c) methyl iodide \(+\) phenol.

Key concepts

SN2 Reaction Mechanism

The SN2 reaction mechanism is a fascinating type of nucleophilic substitution reaction where the nucleophile attacks a substrate from the back. This mechanism stands for "Substitution Nucleophilic Bimolecular." It is called bimolecular because the rate of the reaction depends on the concentration of two reactants: the nucleophile and the substrate. In the context of an SN2 reaction, the nucleophile is typically a species rich in electrons and seeking an electron-poor site.

During the SN2 reaction, these nucleophiles attack the less hindered side of the substrate molecule. As a result, the leaving group is displaced, allowing for a new bond formation with the incoming nucleophile at the same site. SN2 reactions are characterized by their inversion of configuration. This means that the spatial arrangement of groups around the carbon atom changes, akin to flipping an umbrella inside out. The SN2 mechanism is particularly common in reactions involving primary alkyl halides, as seen in the reaction of methyl phenyl ether.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental concept in organic chemistry that describes the replacement of a leaving group by a nucleophile. Nucleophiles are species that contain a pair of electrons that they can share with electron-deficient atoms like carbon within a molecule.

Understanding nucleophilic substitution is vital for studying a wide range of chemical reactions. It involves two main types of mechanisms: SN1 and SN2. In the case of SN2, nucleophiles attack the backside of the carbon atom to prevent electronic crowding. This specific mode of attack ensures that efficiencies in the reaction pathway are retained. Crucially, these reactions can occur under neutral or slightly acidic conditions, as seen when methyl phenyl ether reacts with HI.

Methyl Phenyl Ether

Methyl phenyl ether, also known as anisole, is a common organic compound with the formula \(\text{C}_6\text{H}_5\text{OCH}_3\). It consists of a methoxy group \((\text{OCH}_3)\) attached to a phenyl ring \((\text{C}_6\text{H}_5)\). This ether has a sweet smell reminiscent of anise seeds, hence its name.

Ethers like methyl phenyl ether are generally quite stable because of their ether bonds \((\text{C-O-C})\). However, under certain conditions, such as when reacting with strong acids like HI, these compounds can undergo cleavage. The breaking of the ether bond leads to new compounds through nucleophilic substitution reactions. The stability and characteristics of the substituent groups in the ether, such as the methoxy and phenyl groups in methyl phenyl ether, largely influence the chemical behavior during reactions.

Reaction with HI

When methyl phenyl ether is exposed to hydrogen iodide (HI), a notable reaction occurs, showcasing a classic example of nucleophilic substitution. The strong nucleophile, iodide ion \((\text{I}^-\)), acts to cleave the stable ether bond in methyl phenyl ether through an SN2 mechanism.

The iodide ion attacks the less hindered methyl group, breaking the oxygen–carbon bond. As a result, the products formed are methyl iodide \((\text{CH}_3\text{I})\) and phenol \((\text{C}_6\text{H}_5\text{OH})\). Methyl iodide is formed when the iodide ion replaces the methoxy group's oxygen atom. Meanwhile, the phenyl group gains a hydroxyl group, resulting in phenol's creation. This transformation illustrates how ethers can be broken down into valuable, smaller compounds when adequately treated with a suitable reagant like HI.