Question

If the Favorskii rearrangement of 2-chlorocyclohexanone is carried out using sodium ethoxide in ethanol, the product is ethyl cyclopentanecarboxylate.

Short answer

Question: Briefly outline the steps involved in the Favorskii rearrangement of 2-chlorocyclohexanone and sodium ethoxide to produce ethyl cyclopentanecarboxylate. Answer: The Favorskii rearrangement involves the following steps: 1. Nucleophilic attack of ethoxide ion on 2-chlorocyclohexanone. 2. Formation of a tetrahedral intermediate. 3. Migration of the chlorine atom to form a cyclic intermediate. 4. Ring contraction and chloride anion departure. 5. Protonation of the negatively charged oxygen atom to form ethyl cyclopentanecarboxylate as the final product.

Step-by-step solution

Understand the reactants and conditions

The given reactants are 2-chlorocyclohexanone, which is a cyclohexanone with a chlorine atom attached to the 2-position, and sodium ethoxide (NaOEt), which is a strong base and nucleophile in this reaction. The solvent used is ethanol.

Nucleophilic attack on 2-chlorocyclohexanone

The reaction occurs by the nucleophilic attack of the ethoxide ion (EtO^-) on the carbonyl carbon of 2-chlorocyclohexanone. This step generates a tetrahedral intermediate and the negative charge residing on the oxygen atom.

Formation of an intermediate

The next step involves the migration of the chlorine atom from the 2-position to the initially negatively charged oxygen atom, leading to a cyclic intermediate. The cyclic intermediate consists of an ester-like moiety with a chloride anion as a leaving group.

Ring contraction and chloride anion departure

In this step, the negatively charged oxygen atom attacks the neighboring carbon atom bonded to the chlorine. The chloride anion (Cl^-) departs as a leaving group, causing a ring contraction from a six-membered cyclohexanone to a five-membered cyclopentanone.

Formation of the final product, ethyl cyclopentanecarboxylate

The last step involves the protonation of the negatively charged oxygen atom (from the EtO^- of sodium ethoxide) to form the final product, ethyl cyclopentanecarboxylate. This occurs by transferring a hydrogen (proton) from ethanol to the negatively charged oxygen atom in the cyclopentanone ring.

Key concepts

Cyclohexanone

Cyclohexanone is an organic compound with a six-membered carbon ring containing a ketone group. It belongs to the class of compounds known as ketones, which are characterized by a carbonyl group (a carbon atom double-bonded to an oxygen atom). The general structure of cyclohexanone implies it has a cyclic shape, making it a stable and common intermediate in chemical reactions.

In the context of the Favorskii rearrangement, cyclohexanone serves as the starting point, specifically in its derivative form, 2-chlorocyclohexanone. The presence of the chlorine atom makes the molecule more reactive, ready to undergo further transformations like ring contraction.

When we talk about 2-chlorocyclohexanone, we are referring to a cyclohexanone with chlorine substituting a hydrogen atom on the second carbon of the ring, which plays a crucial role in the rearrangement process. This structure sets up the initial stage for the subsequent nucleophilic attack by making the molecule susceptible to nucleophiles.

Nucleophilic Attack

A nucleophilic attack is a key step in many organic chemistry reactions. It involves a nucleophile, which is a molecule or ion with an electron pair to donate, attacking a positively charged or electron-deficient site on another molecule. This usually leads to the formation of a new chemical bond.

In the case of the Favorskii rearrangement, the nucleophile is the ethoxide ion (EtO^-), a strong base, which attacks the carbonyl carbon in 2-chlorocyclohexanone. This carbon is partially positive due to the electron-withdrawing nature of the oxygen in the carbonyl group, making it a suitable target for nucleophilic attack.

• The nucleophilic attack by EtO^- leads to the formation of a tetrahedral intermediate.
• This intermediate has a negatively charged oxygen, which is stabilized temporarily.

The nucleophilic attack is an essential part of the reaction mechanism, driving the setup for subsequent steps such as ring contraction and product formation.

Sodium Ethoxide

Sodium ethoxide (NaOEt) is a common strong base and nucleophile used in organic chemistry reactions. It is composed of sodium (Na) and ethoxide (EtO^-) ions. The ethoxide ion, a conjugate base of ethanol, is a potent nucleophile due to its negatively charged oxygen, which can attack electron-deficient sites.

In the Favorskii rearrangement, sodium ethoxide serves two main functions:

• It acts as a nucleophile that initiates the reaction by attacking the carbonyl carbon of 2-chlorocyclohexanone. This starts the transformation process that eventually leads to the contraction of the carbon ring.
• As a strong base, it facilitates the deprotonation steps throughout the reaction, particularly aiding in the final formation of ethyl cyclopentanecarboxylate by transferring hydrogen from ethanol to the negatively charged oxygen.

Thus, sodium ethoxide is crucial in this mechanism, enabling both the structural rearrangement of the molecule and stabilization of intermediates.

Organic Synthesis

Organic synthesis is the process of constructing complex organic compounds from simpler ones through chemical reactions. It is a cornerstone of chemistry, allowing chemists to design molecules for pharmaceuticals, materials, and various applications.

The Favorskii rearrangement is a classic example of organic synthesis. It involves a series of well-defined steps to transform 2-chlorocyclohexanone into ethyl cyclopentanecarboxylate. This transformation demonstrates several principles of organic synthesis:

• Selective reaction conditions are used to bring about a specific molecular change.
• Key intermediates are formed and stabilized through steps like nucleophilic attack and base-mediated reactions.
• Final products are constructed by precise control over reagent interactions and conditions.

Understanding the mechanisms behind such organic synthesis processes can help chemists innovate and develop new methods for creating complex molecules efficiently.