Question

Suppose that you start with a diisocyanate and a diol. Show how their reaction can lead to a polymer called a polyurethane (Section 29.5D).

Short answer

Question: Explain the reaction between a diisocyanate and a diol leading to the formation of a polyurethane polymer. Answer: The reaction between a diisocyanate (having two NCO groups) and a diol (having two OH groups) involves the nucleophilic attack of the alcohol group on the isocyanate group, forming a urethane linkage. This reaction occurs multiple times, creating a long polymer chain called polyurethane. In simpler terms, R1-NCO + R2-OH react to form -[-R1-NHCOO-R2-]-, which is the polyurethane polymer.

Step-by-step solution

Identify the functional groups in the diisocyanate and diol

A diisocyanate is a compound containing two isocyanate functional groups (NCO), and a diol is a compound containing two alcohol functional groups (OH). Each functional group will participate in the reaction, forming a polyurethane polymer.

Understand the reaction mechanism

The reaction between a diisocyanate and a diol involves the nucleophilic attack of an alcohol group on an isocyanate group, resulting in the formation of a urethane linkage. This reaction is repeated multiple times to form a long polymer chain, which is called polyurethane.

Write the reaction

Let's represent the diisocyanate as R1-NCO and the diol as R2-OH. The reaction mechanism can be described as follows: 1. The oxygen atom in the alcohol group (R2-OH) acts as a nucleophile and attacks the electrophilic carbon atom in the isocyanate group (R1-NCO), breaking the C=N double bond. 2. A new bond between oxygen atom (from the alcohol group) and carbon atom (from the isocyanate group) is formed, and the nitrogen atom in the isocyanate group acquires a positive charge due to the loss of its bond with the carbon atom. 3. A hydrogen atom is transferred from the oxygen atom (alcohol group) to the nitrogen atom (isocyanate group), forming a urethane linkage and neutralizing the positive charge on the nitrogen atom. The overall reaction can be represented as: R1-NCO + R2-OH -> R1-NHCOO-R2

Represent the polymerization reaction

When multiple diisocyanate and diol molecules react with each other, they form a long polyurethane polymer chain. This can be represented as: R1-NCO + HO-R2 -> -[-R1-NHCOO-R2-]- (polyurethane polymer) This is a simplified representation of the reaction, but it shows how a diisocyanate reacts with a diol to form a polyurethane polymer. Please note that the actual polymer can have a more complex structure depending on the structure of the initial diisocyanate and diol molecules.