Question

One common type of cation exchange resin is prepared by polymerization of a mixture containing styrene and 1,4-divinylbenzene (Problem 29.34). The polymer is then treated with concentrated sulfuric acid to sulfonate a majority of the aromatic rings in the polymer. (a) Show the product of sulfonation of each benzene ring. (b) Explain how this sulfonated polymer can act as a cation exchange resin.

Short answer

Answer: The product is a sulfonated crosslinked polymer with sulfonic acid groups (SO3H) attached to the benzene rings. This sulfonated polymer can function as a cation exchange resin due to the presence of negatively charged sulfonate groups (SO3-) that attract and bind to positively charged ions (cations) in a solution, enabling applications such as water softening and other ion exchange processes.

Step-by-step solution

Understand the polymer structure

The mixture containing styrene and 1,4-divinylbenzene makes a crosslinked polymer. Styrene has a chemical structure with a benzene ring and a vinyl group (C=C), while 1,4-divinylbenzene has two vinyl groups attached to a benzene ring. During the polymerization process, these vinyl groups can form covalent bonds with other vinyl groups, thus creating the crosslinked polymer structure.

The sulfonation of benzene rings

When the polymer is treated with concentrated sulfuric acid (H2SO4), the aromatic rings (benzene rings) in the polymer are sulfonated. The sulfonation process entails the replacement of a hydrogen atom on the aromatic ring with a sulfonic acid group (SO3H). The product for each benzene ring in styrene after sulfonation can be represented as: H2SO4 + C6H5CH=CH2 -> C6H4(SO3H)(CH=CH2) And the product for each benzene ring in 1,4-divinylbenzene after sulfonation can be represented as: H2SO4 + C6H4(CH=CH2)2 -> C6H3(SO3H)(CH=CH2) It is essential to note that not every benzene ring in the polymer would be sulfonated, but a majority of them are.

Sulfonated polymer as a cation exchange resin

The sulfonated polymer can act as a cation exchange resin due to the presence of negatively charged sulfonate groups (SO3-) in the polymer. These negatively charged groups can attract and bind to positively charged ions (cations) in a solution. This property of the sulfonated polymer makes it useful in various applications, including water softening, where it can remove hard water-causing cations like Ca2+ and Mg2+ from water. Additionally, it can be used in other ion exchange processes where the aim is to remove or capture specific cations from a solution.