Question

All rearrangements we have discussed so far have involved generation of an electrondeficient carbon followed by a 1,2 -shift of an atom or group of atoms from an adjacent atom to the electron-deficient carbon. Rearrangements by a 1 ,2-shift can also occur following the generation of an electron-deficient oxygen. Propose a mechanism for the acid-catalyzed rearrangement of cumene hydroperoxide to phenol and acetone.

Short answer

Question: Describe the acid-catalyzed rearrangement of cumene hydroperoxide into phenol and acetone, mentioning the key intermediate species formed during the process. Answer: The acid-catalyzed rearrangement of cumene hydroperoxide into phenol and acetone involves a 1,2-shift mechanism with oxygen as an electron-deficient species. The key steps include the formation of a protonated cumene hydroperoxide, initiating the 1,2-shift to produce an oxonium intermediate, deprotonation of the oxonium ion to form phenol, and finally, protonation of a water molecule to generate acetone.

Step-by-step solution

Identify the structures of the reactants and products

First, draw the structures of cumene hydroperoxide (reactant), phenol, and acetone (products). Cumene hydroperoxide has a t-butyl hydrogen peroxy structure, while phenol has a hydroxyl group attached to a benzene ring and acetone is a simple ketone with the formula (CH3)2C=O.

Formation of the protonated cumene hydroperoxide

In the presence of an acid, cumene hydroperoxide can abstract a proton from the acid. The oxygen (O) of the hydroperoxide group (-OOH) picks up the proton (H+) from the acid. This protonation generates a more reactive species with an -OOH2+ group.

Initiate the 1,2-shift with oxygen acting as an electron-deficient species

Now that we have a protonated cumene hydroperoxide, the 1,2-shift can occur. In this mechanism, the oxygen adjacent to the electron-deficient oxygen (the one with two H+ atoms) acts as a nucleophile. This oxygen attacks the carbon adjacent to it, which is bearing a methyl group. Simultaneously, the methyl group detaches with the O-O bond breaking. This migration ultimately generates an oxonium ion as an intermediate species.

Deprotonation of the oxonium ion

The oxonium ion is unstable, and to become more stable, it loses a proton to form a carbonyl group. It abstracts a proton from the hydroxonium oxygen atom, converting it into a carbonyl group and forming phenol and a proton.

Acetone formation

Finally, we have one last fragment to account for, which is the acetone. The proton, which was just removed from the oxonium ion, can now protonate the O-H group of the released water molecule, leading to the formation of a positively charged oxygen ion. This oxygen ion can depart as a stable water molecule, resulting in the formation of acetone molecule. Now we have detailed the overall proposed mechanism of acid-catalyzed rearrangement of cumene hydroperoxide to phenol and acetone, involving a 1,2-shift with the oxygen acting as an electron-deficient species.

Key concepts

Cumene Hydroperoxide and its Role in Acid-Catalyzed Rearrangement

Cumene hydroperoxide is an organic compound that serves as a crucial intermediate in the production of other chemicals. In the context of acid-catalyzed rearrangements, cumene hydroperoxide is transformed into useful compounds like phenol and acetone. Let's understand this compound more deeply.

Structurally, cumene hydroperoxide is derived from cumene (isopropylbenzene) by the addition of a hydroperoxy group (-OOH). When exposed to acid, this hydroperoxy group becomes highly reactive. The addition of a proton from an acid to the oxygen atom of the -OOH group results in the formation of an oxonium ion, setting the stage for the ensuing rearrangement process. This key step increases the electrophilicity of the oxygen, which subsequently leads to a crucial 1,2-shift mechanism that eventually breaks the O-O bond, forming phenol and acetone.

Understanding the structure and reactivity of cumene hydroperoxide is fundamental in grasping how the 1,2-shift mechanism occurs in its acid-catalyzed rearrangement. This knowledge is not just of academic interest, but also of great industrial relevance as phenol and acetone are important chemicals used widely in the production of polymers, resins, and other materials.

The 1,2-Shift Mechanism Explained

The 1,2-shift mechanism plays a pivotal role in many chemical rearrangements and is particularly important in understanding the transformation of cumene hydroperoxide into phenol and acetone. This mechanism involves the migration of an atom or group of atoms from one position to an adjacent position in a molecule. When facilitated by acid catalysis, this process can lead to significant molecular transformations.

In the specific case of cumene hydroperoxide, the 1,2-shift mechanism features the migration of a methyl group from one carbon to a neighboring electron-deficient oxygen. This shift is key to breaking the somewhat weak O-O bond in the protonated cumene hydroperoxide, which is an inherently unstable structure prone to decomposition. The rearrangement through 1,2-shift is often facilitated by the stabilizing effect of forming a carbocation or, in this instance, an oxonium ion, which is a highly reactive intermediate step in the reaction pathway.

The understanding of this mechanism is crucial for students as it is a fundamental concept in organic chemistry that explains the behavior of many rearrangement reactions. It showcases how the molecular structure can dramatically change with a simple shift of atomic groups, leading to diverse chemical species.

Electron-Deficient Oxygen in Chemical Reactions

Electron-deficient oxygen atoms are a recurring theme in many chemical reactions, especially in those involving peroxides like cumene hydroperoxide. In the case of the acid-catalyzed rearrangement, the electron-deficient oxygen is the linchpin for the reaction to proceed.

Upon protonation by an acid, the oxygen in the hydroperoxy group of cumene hydroperoxide becomes electron-deficient. This deficiency is due to the strong electronegative nature of oxygen which, when bonded to a proton, becomes even more eager to attract electrons. The electron-deficiency is further heightened by the presence of an adjacent oxygen atom that can share electron density, thus allowing the formation of an intermediate oxonium ion.

The concept of electron-deficiency is not only important to understand the specifics of the cumene hydroperoxide rearrangement but also has broader implications in organic chemistry. It explains how certain functional groups become reactive under acidic conditions and participate in bond-forming and breaking processes. This forms the basis for many reactions in synthetic chemistry, showcasing the vast importance of understanding electron-deficient species.

Formation of Phenol and Acetone via Rearrangement

The formation of phenol and acetone from cumene hydroperoxide is an excellent example of how a specific chemical reaction can lead to the creation of two distinct and widely used compounds. This reaction not only illustrates the ingenuity of chemical processes but also their practical applications in the industrial production of materials.

Phenol, characterized by a hydroxyl group attached to a benzene ring, is a compound with diverse uses, including in the manufacture of plastics and pharmaceuticals. Acetone, a simple ketone with the formula \(CH_3)2C=O\), is a solvent used in various industries and is known for its ability to dissolve many organic substances. The transition of the unstable oxonium ion to the stable compounds of phenol and acetone showcases the beauty of equilibrium in chemical reactions.

By exploring the final steps of this acid-catalyzed rearrangement, students can appreciate how the deprotonation of the oxonium ion leads to carbonyl formation, resulting in the cleavage of the O-O bond and subsequent release of acetone. Understanding this aspect of the reaction provides insight into the mechanism of bond rearrangement and how it is utilized to synthesize substances of significant commercial value.