Problem 71 - 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 aqueous solution, purines, such as adenine and guanine, are predicted to have larger dispersion forces compared to pyrimidines, such as thymine and cytosine. This is because purines have a larger molecular surface area due to their two-ring structure, while pyrimidines have only one ring. London dispersion forces generally increase with the size and complexity of molecules, leading to greater dispersion forces for purines.

Step 1: Recall the definition of London dispersion forces

London dispersion forces, also known as van der Waals forces or induced-dipole-induced-dipole interactions, are weak intermolecular forces that arise due to temporary, instantaneous fluctuations in the electron distribution around molecules. Dispersion forces generally increase with the size and complexity of molecules.

Step 2: Understand the structures of purines and pyrimidines

Purines, such as adenine and guanine, have two rings in their structure, whereas pyrimidines, such as thymine and cytosine, have only one ring. As a result, purines have more atoms and a larger molecular surface area compared to pyrimidines.

Step 3: Analyze the relationship between molecular size and dispersion forces

Since London dispersion forces increase with the size and complexity of molecules, we can predict that the larger surface area of purines will lead to greater dispersion forces compared to those of pyrimidines.

Step 4: Predict which has larger dispersion forces in aqueous solution

Based on our analysis, we can predict that purines, with their larger molecular surface area, will have larger dispersion forces in aqueous solution compared to pyrimidines, which are smaller and less complex.