Cleavage of Phenyl Ethers: Videos & Practice Problems

Acidic cleavage of phenyl ethers involves treating them with strong acids like HBr or HI, resulting in the formation of phenol and an alkyl halide. The ether's oxygen becomes protonated, making it susceptible to nucleophilic attack, leading to the cleavage of the ether bond. Unlike regular ethers, phenyl ethers do not yield further alkyl halides due to the stability of phenol. This reaction highlights the unique properties of phenols and their resistance to further reactions under acidic conditions.

Cleavage of Phenyl Ethers Concept 1

Acidic cleavage of phenyl ethers involves the reaction of the ether with strong acids such as HBr or HI, leading to the formation of phenol and an alkyl halide. This process is distinct from the cleavage of regular ethers due to the unique properties of phenol, which does not undergo further reactions to form additional alkyl halides.

During the reaction, the ether's oxygen atom is protonated by the acid, making it more susceptible to nucleophilic attack. The alkyl halide then attacks the carbon atom of the ether that is not part of the benzene ring. This results in the cleavage of the ether bond, allowing the oxygen to bond with a hydrogen atom, thus forming phenol. The overall reaction can be summarized as:

Phenyl Ether + HBr/HI → Phenol + Alkyl Halide

It is crucial to remember that regardless of the amount of strong acid used, the products will always include phenol and an alkyl halide when phenyl ethers are involved. This understanding is essential for predicting the outcomes of reactions involving acidic cleavage of phenyl ethers.

Cleavage of Phenyl Ethers Example 1

In the process of acidic cleavage of propoxybenzene, a phenyl ether, the reaction typically involves a strong acid such as HBr. The mechanism begins with the oxygen atom of the ether group (–O–) protonating by grabbing a hydrogen ion (H⁺) from the acid, resulting in a positively charged protonated oxygen. This step is crucial as it prepares the ether for cleavage.

In this reaction, the nucleophile, which in this case is bromide (Br^-), will specifically attack the ether carbon that is not part of the benzene ring. This is a key characteristic of phenyl ethers; the cleavage occurs at the carbon atom adjacent to the oxygen, leading to the breaking of the C–O bond. As a result, the bond shifts to the oxygen, allowing it to stabilize as a phenol (C₆H₅OH) and forming an alkyl halide (in this case, propyl bromide).

Thus, the products of the acidic cleavage of propoxybenzene are phenol and the corresponding alkyl halide. This reaction highlights the predictable nature of phenyl ether cleavage, where the outcome consistently yields a phenol and an alkyl halide, making it a straightforward process in organic chemistry.

Predict the product formed from the following acidic cleavage reaction. 

In this practice question, we explore the products formed from an acidic cleavage reaction involving a phenyl ether and excess hydroiodic acid (HI). When phenyl ethers undergo this reaction, they yield two primary products: phenol and an alkyl halide. The phenol is produced from the ether carbon that is part of the benzene ring, resulting in a hydroxyl group (–OH) attached to the aromatic structure.

The alkyl halide is formed when the ether carbon gains a halogen, specifically iodine in this case. The attachment of iodine can occur in two distinct orientations, leading to the formation of two different alkyl halides. This results in a racemic mixture, as the iodine can attach either from the top or the bottom of the chiral center, creating two stereoisomers. One configuration will have the iodine bond represented as wedged, while the other will be dashed.

Thus, the final products of this reaction are phenol and two alkyl halides, reflecting the chiral nature of the ether carbon after the iodine attachment. This understanding of the reaction mechanism and stereochemistry is crucial for predicting the outcomes of similar reactions in organic chemistry.

Determine which of the following choices represents the major product from the following reaction. 

• A.
• B.
• C.
• D.

Answer: (B)

Choose the correct chemical sequence necessary for the following synthesis. 

• A. 1. HBr (xs)/ heat2. KOH3. Benzyl bromide
• B. 1. HI (xs)/ heat2. KOH3. Bromobenzene
• C. 1. H₃O⁺2. Br₂ / FeBr₃3. NaOEt
• D. 1. HI (xs)/ heat2. Benzyl bromide3. KOH

Answer: (A)

Here’s what students ask on this topic:

What is the mechanism of acidic cleavage of phenyl ethers?

The mechanism of acidic cleavage of phenyl ethers involves several steps. First, the ether oxygen is protonated by a strong acid like HBr or HI, making it more susceptible to nucleophilic attack. The protonated ether then undergoes nucleophilic attack by the halide ion (Br^- or I^-), targeting the carbon atom bonded to the oxygen that is not part of the benzene ring. This attack leads to the cleavage of the C-O bond, resulting in the formation of phenol and an alkyl halide. The phenol remains stable and does not undergo further reaction under these conditions.

Why does phenol not undergo further reaction in the acidic cleavage of phenyl ethers?

Phenol does not undergo further reaction in the acidic cleavage of phenyl ethers due to its stability. The aromatic ring in phenol provides resonance stabilization, making the phenol less reactive towards further nucleophilic attack. Additionally, the hydroxyl group in phenol is less likely to be protonated under the reaction conditions, preventing further cleavage. This stability ensures that phenol remains as one of the final products in the reaction.

What are the products of the acidic cleavage of phenyl ethers?

The products of the acidic cleavage of phenyl ethers are phenol and an alkyl halide. When a phenyl ether is treated with a strong acid like HBr or HI, the ether bond is cleaved, resulting in the formation of phenol (C₆H₅OH) and an alkyl halide (R-X, where R is the alkyl group and X is the halide, either Br or I). The phenol remains stable and does not undergo further reaction.

How does the presence of a benzene ring affect the cleavage of phenyl ethers?

The presence of a benzene ring in phenyl ethers significantly affects the cleavage process. The aromatic ring provides resonance stabilization to the phenol product, making it less reactive towards further nucleophilic attack. This stabilization ensures that once phenol is formed, it does not undergo further reaction to form additional alkyl halides. The benzene ring's stability is a key factor in the unique behavior of phenyl ethers during acidic cleavage.

What role does the protonation of the ether oxygen play in the cleavage of phenyl ethers?

The protonation of the ether oxygen is a crucial step in the cleavage of phenyl ethers. When the ether oxygen is protonated by a strong acid like HBr or HI, it becomes more electrophilic and susceptible to nucleophilic attack. This protonation increases the positive charge on the oxygen, weakening the C-O bond and facilitating its cleavage. The nucleophilic halide ion (Br^- or I^-) then attacks the carbon atom bonded to the oxygen, leading to the formation of phenol and an alkyl halide.