Question

Decamethylimidazolium Pillar[5]Arene contains a total of 10 positive charges, one on each of the 10 “arms” of the molecule. Often, it will be stated that these 10 charges are what allow the molecule to be water soluble. However, the truth is slightly more complicated. Intermolecular forces are actually what allow molecules to be soluble. Describe the intermolecular force(s) at play that allow the positively charged decamethylimidazolium pillar[5]arene to be water soluble. Compare this with the intermolecular forces that allow brominated pillar[5]arene to be soluble in organic compounds

Decamethylimidazolium Pillar[5]Arene ("R" represents the imidazole)

brominated pillar[5]arene

0 0 〉o 0 0 0 Br Br

Answer 1

For the case of decamethylimidazolium pillar[5]arene, the major intermolecular force comes from the hydrogen bonding with water. In this case, the positive charges of the molecule will interact with the lone pair of oxygen atoms of water. This will favor the solubility of the molecule in water. In addition to this, the other nitrogen atom of the imidazolium ion will also make hydrogen bonding with water. Due to the presence of the methyl group attached to this nitrogen, the electron density will be high in this atom. Hence it will donate its lone pair to water hydrogen to make hydrogen bonding. Due to this intermolecular forces, this molecule will be soluble in water.

In case of brominated pillar[5]arene, the favorable hydrogen bonding with water will be missing. Due to the large size of Br atoms, the stability gained through hydrogen bonding with water will be small. However the core of this molecule will be hydrophobic because of the presence of the 5 aromatic rings. Hence the interactions of these benzene rings with water will be unfavorable. Thus it will be insoluble in water.

However this molecule will be soluble in organic solvents. Organic solvents mostly contain hydrophobic parts. This parts of the solvents will interact favorably with the hydrophobic parts of the brominated pillar[5]arene through van der Waals interactions, thus making it soluble.