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Chapter 29: Problem 18

Propose a mechanism for the formation of this polyphenylurea. To simplify your presentation of the mechanism, consider the reaction of one \(-\mathrm{NCO}\) group with one \(-\mathrm{NH}_{2}\) group.

Short Answer

Expert verified
Answer: The mechanism for the formation of polyphenylurea involves nucleophilic attack of the amine group on the isocyanate group, formation of an unstable intermediate, proton transfer, and the formation of the urea linkage, which is a key component of the polyphenylurea polymer.

Step by step solution

01

Identify the reacting groups

First, we need to identify the functional groups involved in the reaction. In this case, we have one \(-\mathrm{NCO}\) group and one \(-\mathrm{NH}_{2}\) group.
02

Nucleophilic attack of amine group on isocyanate group

The \(-\mathrm{NH}_{2}\) group is a nucleophile, which means it has a lone pair of electrons on the nitrogen atom that can form a bond with an electrophilic site. The \(-\mathrm{NCO}\) group is electrophilic, as the carbon atom in the isocyanate group has a partial positive charge due to the oxygen atom's electronegativity. The nitrogen atom in the amine group will attack the electrophilic carbon atom in the isocyanate group, forming a bond and breaking the carbon-oxygen double bond.
03

Formation of an unstable intermediate

As the bond between the nitrogen atom of the amine group and the carbon atom of the isocyanate group forms, the oxygen atom's double bond is broken, and the oxygen atom acquires a negative charge. This forms an unstable intermediate, with the carbon atom is now bonded to both the nitrogen atom from the amine group and the oxygen atom from the isocyanate group.
04

Proton transfer to oxygen atom

In the unstable intermediate, the oxygen atom, which has acquired a negative charge, will get closer to one of the hydrogen atoms on the nitrogen from the amine group, forming a bond with the hydrogen atom. The bond between the nitrogen and hydrogen will break, transferring the proton (H\(^+\)) to the oxygen atom, converting its negative charge into a neutral charge.
05

Urea linkage formation

After the proton transfer, we are left with a urea linkage between the two functional groups, which is the final product of the reaction between one \(-\mathrm{NCO}\) group and one \(-\mathrm{NH}_{2}\) group. This urea linkage is a key component of the polyphenylurea polymer. In summary, the mechanism for the formation of polyphenylurea involves nucleophilic attack of the amine group on the isocyanate group, formation of an unstable intermediate, proton transfer, and the formation of the urea linkage.

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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.

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