Question

In Section 11.5, we saw that ethers, such as diethyl ether and tetrahydrofuran, are quite resistant to the action of dilute acids and require hot concentrated HI or HBr for cleavage. However, acetals in which two ether groups are linked to the same carbon undergo hydrolysis readily, even in dilute aqueous acid. How do you account for this marked difference in chemical reactivity toward dilute aqueous acid between ethers and acetals?

Short answer

Question: Explain the difference in reactivity between ethers and acetals towards dilute aqueous acid. Answer: The difference in reactivity between ethers and acetals towards dilute aqueous acid lies in their molecular structures. Ethers have a strong and stable C-O bond that cannot be easily cleaved by dilute acids due to resonance stabilization. Acetals, however, have two ether groups connected to a single carbon, which upon protonation forms an unstable intermediate that can readily undergo hydrolysis. This is why acetals are more reactive than ethers towards dilute aqueous acid.

Step-by-step solution

Understand the molecular structure of ethers and acetals.

Ethers have the general formula R-O-R', where R and R' are alkyl or aryl groups. Acetals, on the other hand, have the general formula R-C(OR')2, where R and R' can be alkyl or aryl groups. The key difference is that in acetals, two ether groups are connected to the same carbon atom.

Understand the reactivity of ethers towards acids.

Ethers are relatively unreactive with dilute aqueous acid. This is mainly because the oxygen atom in ethers cannot be protonated easily by the acid due to resonance stabilization of the lone pair electrons. Moreover, the C-O bond in ethers is generally strong and not susceptible to cleavage.

Understand the reactivity of acetals towards acids.

Acetals, on the other hand, are much more susceptible to hydrolysis by dilute aqueous acid. This is due to the presence of two ether groups linked to a single carbon atom. When protonated by the acid, a positively charged intermediate species is formed that eventually undergoes cleavage of one of the C-O bonds. The resulting carbocation can then be attacked by water, leading to the eventual hydrolysis of the acetal.

Summary of the difference in reactivity.

The key difference in reactivity between ethers and acetals towards dilute aqueous acid lies in their molecular structures. While ethers have a strong and stable C-O bond that cannot be easily cleaved by dilute acids, acetals have two ether groups connected to a single carbon, which, upon protonation, leads to the formation of an unstable intermediate that can readily undergo hydrolysis. This explains the marked difference in chemical reactivity between ethers and acetals.