Problem 75 - Chemistry The Central Science

Question

Adenine and guanine are members of a class of molecules known as purines; they have two rings in their structure. Thymine and cytosine, on the other hand, are pyrimidines, and have only one ring in their structure. Predict which have larger dispersion forces in aqueous solution, the purines or the pyrimidines.

Step-by-Step Solution

Answer

In an aqueous solution, purines, which include adenine and guanine, are predicted to have larger dispersion forces compared to pyrimidines, such as thymine and cytosine. This is due to their larger molecular size, greater number of electrons, and increased hydrophobicity.

Step 1: 1. Compare the molecular structures of purines and pyrimidines

Adenine and guanine, the purines, have two fused rings, while thymine and cytosine, the pyrimidines, have only one ring each. The purines are larger due to the presence of an extra ring.

Step 2: 2. Analyze polarizability and molecular size

Purines have more electrons due to their larger size, meaning that there are more opportunities for temporary electron shifts to occur and create dispersion forces. The pyrimidines, with fewer electrons, are less polarizable.

Step 3: 3. Consider hydrophobicity of the molecules

In an aqueous solution, the more hydrophobic the molecule, the larger the dispersion forces. Since purines have a larger molecular structure and are more hydrophobic than pyrimidines, they are more likely to experience dispersion forces in an aqueous solution

Step 4: 4. Predict which class of molecules has larger dispersion forces in an aqueous solution

Taking into account the molecular size, polarizability, and hydrophobicity of the molecules, we can predict that purines, with their larger structure, greater number of electrons, and increased hydrophobicity, will have larger dispersion forces in an aqueous solution compared to pyrimidines.