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

Answer: Acetals have higher chemical reactivity towards dilute aqueous acids compared to ethers due to the increased electrophilicity of the central carbon atom in the C-O bond and the stabilization of the intermediate species during the reaction. In acetals, the presence of two ether groups makes the central carbon more electrophilic, and the adjacent oxygen atom provides better leaving group stabilization, allowing the reaction to proceed more readily than in ethers.

Step-by-step solution

Analyzing Ether Structure and Reactivity

Ether has the general structure R-O-R', where R and R' can be alkyl or aryl groups. Ethers are relatively unreactive towards dilute aqueous acids because the oxygen atom in an ether is less electrophilic compared to an acetal. This is due to the lone pair electrons on the oxygen atom that is involved in resonance stabilization with the neighboring carbons (if they contain double bonds) in case of aryl groups or is hindered by the electron-donating effect of alkyl groups (inductive effect) in case of alkyl groups.

Analyzing Acetal Structure and Reactivity

Acetals have the general structure R-CH(OR')2, where R and R' can be alkyl or aryl groups. Here, the central carbon is bonded to two ether groups, making it more electrophilic. The presence of two oxygen atoms with lone pair electrons not only makes the central carbon more electrophilic but also creates a better leaving group for the reaction to proceed.

Reaction Mechanism with Dilute Acids

In the presence of dilute aqueous acid, the mechanism for acetal hydrolysis proceeds in a step-by-step fashion. The acid donates a proton to one of the oxygen atoms of the acetal, forming an oxonium ion intermediate. This intermediate is stabilized by the adjacent oxygen atom, which donates its lone pair electrons to form a resonance-stabilized structure. This stabilization makes it easier for the leaving group to depart, which breaks the C-O bond and forms the respective alcohol and carbocation species. This carbocation can react with water to form the final alcohol product.

Acetal Hydrolysis vs Ether Cleavage

In contrast, the cleavage of ethers using strong acids (HI or HBr) involves breaking a C-O bond through a nucleophilic substitution mechanism, which is less favorable with weak acids, as the electrophilic nature of the central carbon is hindered by the electron-donating alkyl groups. Acetals, however, undergo hydrolysis reactions readily in the presence of dilute aqueous acids, due to the increased electrophilicity of the central carbon, and the stabilizing effect of the adjacent ether group on the intermediate. In conclusion, the marked difference in chemical reactivity toward dilute aqueous acids between ethers and acetals is due to the difference in electrophilicity of the central carbon atom (C) in the C-O bond, and the stabilization of the intermediate species during the reaction. Acetals are more reactive because of their increased electrophilicity and the presence of a better leaving group compared to ethers.