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: Compare the reactivity of ethers and acetals towards dilute aqueous acid. Explain the differences in their reactivity based on their structure and other factors. Answer: Acetals react more readily with dilute aqueous acid as opposed to ethers. This difference in reactivity can be attributed to the weaker bond between carbon and oxygen in acetals, the increased nucleophilicity of the oxygen atoms due to inductive effects, and the presence of two ether groups bound to the same carbon atom. Ethers, on the other hand, have a stable bond between carbon and oxygen and low nucleophilicity, which makes them less reactive towards dilute aqueous acids.

Step-by-step solution

Understand the structure of ethers and acetals

Ethers have a structure R-O-R' where R and R' represent alkyl or aryl groups linked by an oxygen atom. Acetals, on the other hand, have the following structure: R-CH(OR')(OR'') where R, R', and R'' represent alkyl or aryl groups, and two ether groups are linked to the same carbon.

Factors affecting reactivity of ethers and acetals

Let's consider the factors affecting the reactivity of ethers and acetals: 1. Stability of the resulting product after the reaction 2. Nucleophilicity of the lone pair on the oxygen atom 3. Steric hindrance in the molecules

Stability of products after reaction with dilute aqueous acid

When ethers react with dilute aqueous acid, they form alcohol and alkyl halide. For this reaction to happen, the bond between carbon and oxygen must be broken, which is relatively strong. Acetals, when reacted with dilute aqueous acid, undergo hydrolysis and form an aldehyde or ketone and two alcohol molecules. In this case, the bond that needs to be broken is between the carbon and the oxygen linked to an alkyl or aryl group, which generally has a weaker strength compared to the carbon-oxygen bond in ethers.

Nucleophilicity of the lone pair on the oxygen atom

The lone pair of electrons on the oxygen atom in ethers is quite stable and isn't highly nucleophilic, which makes it difficult to react with dilute aqueous acid. In acetals, since there are two oxygen atoms linked to the same carbon atom, the lone pair electrons on both oxygen atoms become more nucleophilic as they are influenced by the inductive effect of another alkyl or aryl group linked to it. Hence, acetals are more reactive.

Steric hindrance in the molecules

In ethers, the steric hindrance around the oxygen atom is comparatively low and doesn't affect its reactivity significantly. However, in acetals, due to the presence of two ether groups linked to the same carbon atom, the steric hindrance may affect its reactivity.

Conclusion

In conclusion, the marked difference in chemical reactivity towards dilute aqueous acids between ethers and acetals can be attributed to the differences in the stability of the resulting products, the nucleophilicity of the lone pair electrons on the oxygen atoms and the steric hindrance in the molecules. Acetals are more reactive due to the weaker bond between carbon and oxygen, increased nucleophilicity of the oxygen atoms, and presence of two ether groups bound to the same carbon atom.

Key concepts

Hydrolysis of Acetals

Hydrolysis of acetals is an important reaction in organic chemistry that converts acetals back to aldehydes or ketones and alcohols in the presence of an acidic aqueous solution. Acetals consist of a carbon atom connected to two alkoxy groups (R-O-R'), where R can represent any alkyl or aryl group. This structure is sensitive to hydrolysis because the carbon is electrophilic and susceptible to attack by water molecules.

During the hydrolysis, the acid acts as a catalyst, activating the water molecule to become a stronger nucleophile. This nucleophilic water can then attack the electrophilic carbon, eventually leading to the cleavage of the C-O bonds and restoring the original carbonyl compound along with two alcohol molecules. The mechanism involves protonation of the acetal oxygen, nucleophilic attack by water, and departure of the alkoxy groups as alcohols.

Examining the Mechanism

When delving into the hydrolysis mechanism, we see multiple steps, including the creation of a protonated hemiacetal intermediate, which is more unstable and therefore more readily reacts further to produce the final products. This multi-step process serves as an example of the importance of understanding the interactions between functional groups and the influence of the acidic environment on the reactivity of organic compounds.

Reactivity with Dilute Aqueous Acid

The reactivity of organic molecules with dilute aqueous acid varies significantly between different classes of compounds. Ethers and acetals, for instance, display quite distinct reactivity profiles. The key to understanding this difference lies in the varying stabilities of the functional groups within these molecules under acidic conditions.

Dilute aqueous acids can act as proton donors, which leads to increased electrophilicity of certain atoms within the molecule. In acetals, the central carbon atom becomes more electrophilic upon protonation, thereby facilitating nucleophilic attack. Ethers, on the other hand, don’t undergo this process as readily because their more stable C-O bonds resist protonation and subsequent nucleophilic attack.

Protonation and Its Effects

In the context of dilute aqueous acids, protonation is a pivotal step that enhances the electrophilic character of a molecule. Acetals are particularly prone to protonation at the oxygen, leading to a cascade of steps resulting in the cleavage of their C-O bonds. This increased susceptibility to protonation underlies their greater reactivity compared to ethers in such environments.

Structure and Stability of Organic Compounds

The structure of an organic compound deeply influences its stability and reactivity. In the case of ethers and acetals, the presence of oxygen atoms connected to carbon forms a key part of their structural identity, imparting specific chemical properties to these compounds.

For ethers, the single ether group attached to the carbon creates a relatively stable structure that is resistant to attack, especially from dilute acids. Acetals contain two ether groups connected to the same carbon, which introduces elements of both steric hindrance and inductive effect that influence their stability. While steric hindrance can decrease reactivity by preventing the approach of nucleophiles or acids, it is the inductive effect from the dual alkoxy groups that primarily drives the increased nucleophilicity and subsequent reactivity in acetals.

Inductive Effect on Reactivity

Steric factors may often play a lesser role compared to electronic effects such as the inductive effect, which can stabilize or destabilize charges within the molecule. This helps to explain why acetals, despite potential steric crowding, are still more reactive in the presence of dilute aqueous acid as their structure promotes electronic effects that enhance their susceptibility to hydrolysis.