Question

Caprolactam, the monomer from which nylon 6 is synthesized, is prepared from cyclohexanone in two steps. In Step 1, cyclohexanone is treated with hydroxylamine to form cyclohexanone oxime. Treatment of the oxime with concentrated sulfuric acid in Step 2 gives caprolactam by a reaction called a Beckmann rearrangement. Propose a mechanism for the conversion of cyclohexanone oxime to caprolactam.

Short answer

Answer: The proposed mechanism involves four major steps: 1. Protonation of cyclohexanone oxime by concentrated sulfuric acid 2. Nucleophilic attack of water on the carbonyl carbon 3. Rearrangements through the migration of the alkyl group, leading to a nitrilium ion 4. De-protonation of the nitrilium ion by the bisulfate anion and the formation of caprolactam.

Step-by-step solution

Protonation of the oxime

The first step involves the protonation of the nitrogen atom in cyclohexanone oxime by concentrated sulfuric acid (H2SO4). This increases the electrophilicity of the oxime and makes it more susceptible to nucleophilic attack. Cyclohexanone oxime + H2SO4 -> Protonated cyclohexanone oxime + HSO4^- #Step 2: Nucleophilic attack of water#

Attack by water on the carbonyl carbon

After protonation, a water molecule (present in the reaction mixture) acts as a nucleophile and attacks the carbonyl carbon of the protonated cyclohexanone oxime. This forms a tetrahedral intermediate with the nitrogen atom still bearing a positive charge. Protonated cyclohexanone oxime + H2O -> Tetrahedral intermediate #Step 3: Rearrangement of the intermediate#

Beckmann rearrangement

In the Beckmann rearrangement step, the migration of the alkyl group (R) linked to the nitrogen atom of the oxime occurs. This migration results in the breaking of the C-N bond and the formation of a new C-N bond. The intermediate formed at this stage is called a nitrilium ion, having a positive charge on the nitrogen atom and a double bond with a carbon atom. Tetrahedral intermediate -> Nitrilium ion #Step 4: Proton loss and caprolactam formation#

De-protonation and caprolactam formation

The final step is the de-protonation of the nitrilium ion by the bisulfate anion (HSO4^-) which leads to the formation of a cyclic amide, caprolactam. Nitrilium ion + HSO4^- -> Caprolactam + H2SO4 Thus, the proposed mechanism for the conversion of cyclohexanone oxime to caprolactam via Beckmann rearrangement involves protonation of the oxime, nucleophilic attack by water, rearrangement of the intermediate, and de-protonation to form the final product, caprolactam.

Key concepts

Beckmann rearrangement

The Beckmann rearrangement is a chemical reaction that involves the conversion of an oxime to an amide. This reaction is named after the German chemist Ernst Otto Beckmann. It is a well-known example of a rearrangement reaction, where the structure of the compound is changed by shifting some of its atoms around. This reaction is particularly useful in organic chemistry for converting ketoximes into amides.
In the Beckmann rearrangement, specific conditions are required for the transformation to happen, such as the presence of an acid, often sulfuric acid. The acid helps to protonate the oxime, enabling a rearrangement where the alkyl group migrates from carbon to nitrogen, forming an intermediate nitrilium ion. This ion then undergoes further reactions to produce the final amide product.

Cyclohexanone oxime

Cyclohexanone oxime is the crucial starting material in the synthesis of caprolactam. It is formed from the reaction of cyclohexanone with hydroxylamine. This substance is an oxime, a type of compound characterized by the presence of the functional group C=N-OH.
The structure of cyclohexanone oxime includes a six-membered cyclohexane ring with an attached oxime group. This structure is susceptible to rearrangement under the action of acids, which is precisely what happens during its conversion to caprolactam in the Beckmann rearrangement.
Understanding the structure and reactivity of cyclohexanone oxime is essential for mastering its role in organic synthesis and its transformation into more complex products.

Caprolactam synthesis

Caprolactam is a widely used chemical, most notably as a precursor to nylon 6, a synthetic polymer. It is produced commercially through the Beckmann rearrangement of cyclohexanone oxime.
The synthesis process involves multiple steps starting from cyclohexanone. First, cyclohexanone is converted to cyclohexanone oxime through reaction with hydroxylamine. The oxime is then rearranged into caprolactam using an acidic catalyst, typically concentrated sulfuric acid.
This chemical transformation is an excellent example of how organic reactions are used to prepare industrially relevant compounds, facilitating the production of materials with everyday applications, such as fabrics and textiles.

Nucleophilic attack

Nucleophilic attack is a key step in many organic reactions, including the Beckmann rearrangement. It refers to the approach of a nucleophile, which is a species containing a lone pair of electrons, towards an electrophile, which is electron-deficient.
In the case of cyclohexanone oxime's conversion to caprolactam, after protonation, the oxime becomes susceptible to nucleophilic attack by water. Water acts as a nucleophile, attacking the carbonyl carbon of the protonated oxime, forming a tetrahedral intermediate. This intermediate is a crucial step before the actual rearrangement occurs.
Understanding nucleophilic attack helps in predicting the outcomes of various organic reactions, as it influences how molecules interact and transform.

Protonation reaction

Protonation is the initial step in the Beckmann rearrangement, setting the stage for the subsequent chemical transformations. It involves the addition of a proton (H⁺) to a molecule, increasing its reactivity.
In this mechanism, cyclohexanone oxime is protonated by sulfuric acid. This reaction makes the nitrogen atom more electrophilic, enhancing its propensity to rearrange. The protonation increases the molecule's ability to accept a nucleophile, in this case, water, which will later attack during the nucleophilic step.
Protonation is a fundamental concept in acids and bases chemistry, and in organic chemistry, it is a vital process that can substantially alter the chemical properties of compounds, leading to various transformation pathways.