The valuable polymer polyurethane is made by a condensa- tion reaction of alcohols (ROH) with compounds that con- tain an isocyanate group (RNCO). Two reactions that can generate a urethane monomer are shown here: (i)



(ii)
(c) If you wanted to promote the formation of the isocyanate intermediate in each reaction, what could you do, using Le Châtelier's principle?

Verified step by step guidance

1. Identify the reactions involved in the formation of the isocyanate intermediate. The first step involves the reaction of a primary amine (RNH2) with an aldehyde or ketone to form an imine (R2C=NR) and water (H2O).
2. The second step involves the reduction of the imine using sodium cyanoborohydride (NaBH3CN) to form a secondary amine (R2CH-NHR).
3. To promote the formation of the isocyanate intermediate, apply Le Châtelier's principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.
4. In the first reaction, to shift the equilibrium towards the formation of the imine, you can remove the water (H2O) produced in the reaction. This can be done by using a dehydrating agent or by continuously removing water from the reaction mixture.
5. In the second reaction, ensure that there is an excess of the reducing agent (NaBH3CN) to drive the reduction of the imine to the secondary amine, thus promoting the formation of the desired intermediate.

Key Concepts

Le Châtelier's Principle

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. In the context of chemical reactions, this means that if you increase the concentration of reactants or remove products, the system will shift towards the formation of more products to restore equilibrium.

Isocyanate Formation

Isocyanates are reactive compounds containing the isocyanate functional group (RNCO). They are typically formed from the reaction of amines with carbonyl compounds, such as aldehydes or ketones, followed by dehydration. Understanding the conditions that favor isocyanate formation is crucial for synthesizing polyurethanes effectively.

Condensation Reactions

Condensation reactions involve the combination of two molecules with the loss of a small molecule, often water. In the formation of polyurethanes, the reaction between alcohols and isocyanates is a key example of a condensation reaction, where the formation of urethane bonds occurs alongside the release of water, driving the reaction forward.