Read the passage carefully and answer the questions :

Replacement of a hydrogen atom in a hydrocarbon by an alkoxy or carboxyl group yields a class of compounds known as ethers. Ethers are classified as symmetrical or unsymmetrical on the basis of groups attached to the oxygen atoms. Diethyl ether, a symmetrical ether, has been widely used as an inhalation anesthetic. Ethers can be prepared by acid catalyzed intermolecular dehydration of alcohols and Williamson's synthesis. Acid catalyzed dehydration of alcohols is not generally preferred as it gives a mixture of elimination and substitution products.

In Williamson's synthesis, an alkyl halide is allowed to react with sodium alkoxide. Ethers containing substituted Alkyl groups may also be prepared by this method. The C-O bond in ether is weakly polar and is cleaved under drastic conditions with excess of hydrogen halides. In electrophilic substitution, the alkoxy group deactivates the aromatic ring and directs the incoming group to ortho and para positions.

Q)Which is most reactive hydrogen halide for cleavage of ethers?

The correct option is HI.

Step-by-Step Explanation:

The cleavage of the C–O bond in ethers by hydrogen halides (HX) is a nucleophilic substitution reaction. The mechanism typically involves the following steps:

1. Protonation of the ether: The oxygen atom of the ether is protonated by the hydrogen halide to form a reactive protonated ether (oxonium ion).
2. Nucleophilic attack by the halide ion: The halide ion ($X^{-}$) acts as a nucleophile and attacks the carbon atom adjacent to the protonated oxygen, cleaving the C–O bond to form an alcohol and an alkyl halide.

The rate of this reaction and the reactivity of the hydrogen halide depend on two main factors:
- Acid strength (ease of protonation): The stronger the acid, the more easily it protonates the ether oxygen. The acidic strength of hydrogen halides increases in the order:
$\mathrm{HF}<\mathrm{HCl}<\mathrm{HBr}<\mathrm{HI}$
- Nucleophilicity of the halide ion: The halide ion acts as the nucleophile that cleaves the bond. The nucleophilicity of halide ions increases down the group as size increases and polarizability increases:
$F^{-}<{\mathrm{Cl}}^{-}<{\mathrm{Br}}^{-}<I^{-}$

Because $\mathrm{HI}$ is the strongest acid among the hydrogen halides and $I^{-}$ is the strongest nucleophile, hydrogen iodide ($\mathrm{HI}$) is the most reactive hydrogen halide for the cleavage of ethers.