Question

Ethers undergo an acid-catalyzed cleavage reaction when treated with the Lewis acid \(\mathrm{BBr}_{3}\) at room temperature. Propose a mechanism for the reaction.

Short answer

Ethers react with \(\text{BBr}_3\) via cleavage of the C-O bond, forming an alkyl bromide and an alcohol.

Step-by-step solution

Identify Reactants and Products

Begin by noting that the reactants are an ether and boron tribromide (\(\text{BBr}_3\). The final products will include an organobromine compound and an alcohol. Let's denote the ether as R-O-R', where R and R’ are alkyl groups.

Initiation of the Reaction

The reaction starts with the interaction between the Lewis acid \(\text{BBr}_3\) and the ether. The boron atom in \(\text{BBr}_3\) is electron-deficient and acts as an electrophile. It coordinates with the lone pair of electrons on the oxygen atom of the ether, forming a coordinate covalent bond and generating a complex.

Formation of the Onium Ion

The bond between the oxygen and the boron strengthens, leading to the generation of an oxonium ion (R-O^(+)-R') and a correspondingly released bromide ion (Br^-). This step is crucial for the cleavage of the C-O bond.

Cleavage of the Carbon-Oxygen Bond

The formed oxonium ion is unstable. The lone pair of electrons on the bromide ion attacks the more sterically hindered or relatively more positive carbon atom, leading to the cleavage of one of the carbon-oxygen bonds. This results in the formation of an alkyl bromide (R-Br) and an alcohol (R'-OH).

Repeat for the Other Alkyl Group

The process can repeat if the ether is symmetrical, resulting in the continuous production of alkyl bromide and alcohol until the ether is completely cleaved or the \(\text{BBr}_3\) reagent is exhausted.

Key concepts

Acid-Catalyzed Cleavage

Acid-catalyzed cleavage is a type of organic reaction where an acid is used to break down molecules. In the context of ether chemistry, it refers to the process where an ether is broken up into smaller components. This reaction typically involves a Lewis acid acting as a catalyst to facilitate the breaking of bonds. For example, when ethers are treated with boron tribromide (BBr₃), an acid-catalyzed cleavage occurs, resulting in the formation of different products such as alcohols and alkyl halides. The acid acts to make the ether molecule more reactive, allowing it to be cleaved more easily. In this process:

• The ether molecule is activated by the Lewis acid.
• The acid accepts electron pairs, facilitating bond breaking.
• New bonds form, yielding the reaction products.

This reaction is important in synthetic chemistry for modifying ether groups in compounds.

Lewis Acid

A Lewis acid is a chemical species that can accept an electron pair from a donor, which is often a Lewis base. In the realm of ether chemistry, Lewis acids like boron tribromide (BBr₃) play a crucial role in facilitating bond cleavages. A Lewis acid has certain characteristics:

• They tend to have an incomplete octet of electrons, making them eager to accept electron pairs.
• They often possess a positive charge or are neutral atoms with an electron-deficient region.
• They can interact with various substrates, including ethers, to initiate chemical reactions.

In the context of the acid-catalyzed cleavage of ethers, a Lewis acid such as BBr₃ coordinates with the oxygen in an ether. This coordination makes the ether more susceptible to cleavage.

Ether Chemistry

Ether chemistry revolves around the study and manipulation of ether compounds. Ethers are organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups (R-O-R').

• Ethers are generally considered stable, but they can undergo specific reactions under suitable conditions.
• They are nonpolar, making them good solvents for a wide range of reactions.
• In acid-catalyzed cleavages, ethers react with acids like BBr₃ to break down into smaller compounds.

The stability of ethers can be overcome using powerful reagents such as Lewis acids. As such, ether chemistry has its importance in developing various chemical pathways in organic synthesis.

Oxonium Ion

An oxonium ion is a type of cation with a structure like R-O⁺-R', where an oxygen atom bears a positive charge. The formation of an oxonium ion is a critical step in the acid-catalyzed cleavage of ethers. Here's what happens:

• The ether's oxygen atom donates an electron pair to the Lewis acid, creating a bond.
• This results in the oxygen temporarily taking on a positive charge, forming the oxonium ion.
• The oxonium ion is an intermediate; it is highly reactive and crucial for the process of cleavage.

This ion provides a route for breaking the carbon-oxygen bond, eventually leading to the formation of different organic products. Its instability helps drive the reaction forward.

Carbon-Oxygen Bond Cleavage

Carbon-oxygen bond cleavage is a pivotal part of the mechanism in breaking down ethers into their product components. Specifically, during acid-catalyzed reactions, the cleavage of C-O bonds leads to the formation of alcohols and alkyl halides. Here’s how it occurs:

• The Lewis acid makes the oxygen atom electron-deficient, setting the stage for bond cleavage.
• The positive charge on the oxonium ion draws a nucleophile, like a bromide ion, towards the carbon center.
• This leads to the breaking of the C-O bond, resulting in the formation of new products such as alcohols and organohalides.

This process is crucial for transforming ethers into other useful synthetic compounds, illustrating the dynamic nature of carbon-oxygen bonds in organic chemistry.