Question

tert-Butyl ethers can be prepared by the reaction of an alcohol with 2 -methylpropene in the presence of an acid catalyst. Propose a mechanism.

Short answer

Use acid to protonate alkene, followed by alcohol nucleophilic attack, then deprotonate to form the ether.

Step-by-step solution

Protonation of Alkene

The mechanism begins with the protonation of the alkene, 2-methylpropene, by the acid catalyst. The double bond in 2-methylpropene attacks a proton (H⁺) from the acid, resulting in the formation of a more stable carbocation intermediate on the more substituted carbon.

Nucleophilic Attack by Alcohol

The alcohol molecule acts as a nucleophile and attacks the carbocation intermediate. The oxygen atom of the alcohol, which has lone pairs, donates a pair of electrons to the positively charged carbocation, forming an oxonium ion.

Deprotonation to Form Ether

The oxonium ion formed in the previous step is not very stable, so it undergoes deprotonation. A base (often the conjugate base of the acid catalyst) abstracts a proton from the oxygen, leading to the formation of the tert-butyl ether and regenerating the acid catalyst.

Key concepts

Mechanism of Reaction

In organic chemistry, understanding the mechanism of a reaction is crucial because it illustrates the step-by-step process by which reactants transform into products. In the synthesis of tert-butyl ethers from 2-methylpropene and an alcohol using an acid catalyst, the key mechanism consists of a series of defined steps:

1. **Protonation of Alkene:** The reaction kicks off with the protonation of the alkene, specifically the 2-methylpropene. Here, the electron-rich double bond of the alkene attacks a proton (H⁺) provided by the acid catalyst. This protonation is essential as it increases the electrophilicity of the molecule, making the resulting product more reactive.

2. **Formation of Carbocation:** After protonation, a carbocation forms. This intermediate is very reactive, creating a positive charge on one of the carbon atoms. This entity is essential for the next step, where a nucleophile can attack.

3. **Nucleophilic Attack and Deprotonation:** The alcohol, acting as a nucleophile, attacks this carbocation, sharing an electron pair to form an oxonium ion, which is then deprotonated to yield the ether. These steps help regenerate the acid catalyst, staying in line with the catalytic cycle.

Carbocation Formation

Carbocations are ionic species featuring a carbon atom with a positive charge. They play a pivotal role in many reaction mechanisms, including ether synthesis.

During the synthesis of tert-butyl ethers, the carbocation forms right after the protonation of the alkene. In this scenario, the electron pair in the double bond of 2-methylpropene is transferred to bind with a proton (H⁺) from the acid. This leaves one of the carbon atoms with a deficiency of electrons, thus holding a positive charge.

Because of this charge, the carbocation is highly unstable and reactive. It will seek stability either by undergoing further chemical reactions or by structural adjustments, such as shifting electrons or rearranging its bonds. In the context of this reaction, the carbocation provides a key intermediate that allows a nucleophile — here the alcohol — to make an effective attack, facilitating the creation of new products.

Nucleophilic Attack

A nucleophilic attack is a critical step in many organic reactions, including the formation of ethers. It involves a nucleophile, a species rich in electrons or having lone pair(s) of electrons, targeting a positively charged center, typically a carbocation.

In the reaction mechanism for synthesizing tert-butyl ethers, the alcohol acts as the nucleophile. The oxygen atom, with its lone pairs, effectively targets and attacks the positively charged carbocation created in the previous step. This attack results in the formation of a new bond between the oxygen and the carbon, converting the carbocation into an oxonium ion.

This interaction is crucial because it creates the primary framework of the final product. Once the nucleophilic attack occurs, subsequent steps involve structural reconfigurations, such as deprotonation, that stabilize and finalize the formation of the ether. Thus, through nucleophilic attack, the complex transitions effectively from charged intermediates into stable final products.