Question

Meerwein's reagent, triethyloxonium tetrafluoroborate, is a powerful ethylating agent that converts alcohols into ethyl ethers at neutral pH. Show the reaction of Meerwein's reagent with cyclohexanol, and account for the fact that trialkyloxonium salts are much more reactive alkylating agents than alkyl iodides.

Short answer

Meerwein's reagent reacts with cyclohexanol to form ethyl cyclohexyl ether. Triethyloxonium salts are more reactive than alkyl iodides due to their potent leaving group, which enhances electrophilicity.

Step-by-step solution

Understand the Reaction

Meerwein's reagent, triethyloxonium tetrafluoroborate, \[\text{(C}_2\text{H}_5)_3\text{O}^+ \text{BF}_4^-\]is a reagent used to convert alcohols into ethers. When it reacts with cyclohexanol \[\text{C}_6\text{H}_{11}\text{OH}\]it donates an ethyl group to the hydroxyl group of cyclohexanol, resulting in the formation of an ethyl ether.

Write the Reaction Equation

The reaction of Meerwein's reagent with cyclohexanol can be described by the following chemical equation:\[\text{C}_6\text{H}_{11}\text{OH} + (\text{C}_2\text{H}_5)_3\text{O}^+ \rightarrow \text{C}_6\text{H}_{11}\text{OC}_2\text{H}_5} + (\text{C}_2\text{H}_5)_2\text{O} \rightarrow \text{C}_6\text{H}_{11}\text{OC}_2\text{H}_5 + \text{C}_2\text{H}_5\text{BF}_4\]In this reaction, cyclohexanol is converted to ethyl cyclohexyl ether.

Explain the Reactivity

Trialkyloxonium salts are more reactive alkylating agents than alkyl iodides because of the positive charge on the oxygen in the oxonium ion, which makes it a very good leaving group. This enhances the electrophilicity of the carbon atom bonded to the oxygen, facilitating nucleophilic attack by the alcohol. Alkyl iodides, in contrast, only possess a moderately good leaving group (iodide ion), making them less reactive in comparison. The positive charge in the oxonium ion is highly destabilizing, which drives the alkylation process to completion rapidly.

Key concepts

Triethyloxonium Tetrafluoroborate

Triethyloxonium tetrafluoroborate, often called Meerwein's reagent, is an organometallic compound with the formula \((\text{C}_2\text{H}_5)_3\text{O}^+ \text{BF}_4^-\). This reagent is unique due to its strong ability to transfer ethyl groups to different substrates like alcohols. The structure includes a positively charged oxonium ion, making it highly reactive. The ethyl transfer is facilitated by the tetrafluoroborate anion, \(\text{BF}_4^-\), which stabilizes the cationic counterpart without participating actively in the reaction.

Ethylating Agent

An ethylating agent, like Meerwein's reagent, plays a crucial role in organic synthesis by transferring an ethyl group to a nucleophile. In this case, triethyloxonium tetrafluoroborate donates an ethyl group quickly and efficiently. Meerwein’s reagent is especially valued because it reacts under mild conditions, often at neutral pH, which is advantageous for preserving sensitive functional groups in a molecule. The ethyl transfer is rapid, ensuring that unwanted side reactions are minimized.

Alcohols to Ethers Reaction

Converting alcohols to ethers is a common transformation in organic chemistry, and Meerwein's reagent is very effective in this task. When treating an alcohol like cyclohexanol with triethyloxonium tetrafluoroborate, the reagent donates an ethyl group to form an ether linkage. This reaction typically involves the displacement of the hydroxyl group in alcohol by an ethyl group, resulting in an ether, such as ethyl cyclohexyl ether. Such transformations are advantageous due to their selectivity and ease under benign reaction conditions.

Trialkyloxonium Salts Reactivity

Trialkyloxonium salts, such as triethyloxonium tetrafluoroborate, exhibit high reactivity due to the presence of a positively charged oxygen atom. This positive charge enhances the electrophilic nature of the attached carbon, making it susceptible to nucleophilic attack by alcohols or other nucleophiles. The oxonium ion serves as an excellent leaving group, contributing to its high reactivity and ability to facilitate alkylation reactions swiftly. This is in contrast to compounds with neutral leaving groups, which can be less reactive.

Comparison with Alkyl Iodides

Alkyl iodides are also used in alkylation reactions but are generally less reactive than trialkyloxonium salts. The difference in reactivity stems from the nature of the leaving groups. In alkyl iodides, the iodide ion is a moderate leaving group compared to the oxonium ion in trialkyloxonium salts. The oxonium ion, being positively charged, departs quickly, driving the reaction forward and completing the ethylation process faster. Consequently, trialkyloxonium salts offer advantages in reactions requiring high efficiency and speed.