Problem 75 - Chemistry: The Central Science in SI Units, Global Edition

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

Purines (adenine and guanine) will have larger dispersion forces in aqueous solution compared to pyrimidines (thymine and cytosine), due to their larger size and more complex structures, which result in more atoms and electrons in the molecules.

Step 1: Understand the concept of dispersion forces

Dispersion forces, also known as van der Waals forces or London dispersion forces, are weak intermolecular forces that occur between molecules due to the temporary fluctuations in electron distribution which create temporary dipoles. The more electrons a molecule has, the more significant these forces are. These forces are particularly important for larger molecules with many atoms and electrons.

Step 2: Compare purines and pyrimidines

Purines (adenine and guanine) are larger molecules with two ring structures, while pyrimidines (thymine and cytosine) are smaller molecules with only one ring structure. Due to their larger size and more complex structures, purines have more atoms and electrons than pyrimidines. As a result, purines will have stronger dispersion forces than pyrimidines.

Step 3: Determine which class of molecules will have larger dispersion forces in aqueous solution

Since purines (adenine and guanine) have more atoms and electrons due to their larger size and more complex structures, they will experience stronger dispersion forces than pyrimidines (thymine and cytosine).

Step 4: Conclusion

The purines (adenine and guanine) will have larger dispersion forces in aqueous solution compared to pyrimidines (thymine and cytosine), due to their larger size and more complex structures, which result in more atoms and electrons in the molecules.

Key Concepts

Purines: Structure and Dispersion Forces

Purines, such as adenine and guanine, are fundamental components of DNA and RNA. They are characterized by having a dual-ring structure, which distinguishes them from pyrimidines. This structure means that purines are larger molecules, consisting of more atoms and electrons.
The size and complexity of purines contribute to their stronger dispersion forces when compared to smaller molecules. Dispersion forces, also known as van der Waals forces, are weak forces of attraction caused by the random movement of electrons. In purines, the higher number of electrons means that temporary dipoles are created more easily, leading to stronger dispersion forces.
These forces are crucial in biological systems as they help stabilize molecular formations, both in aqueous solutions and within the complex structures of nucleic acids. Thus, purines like adenine and guanine significantly influence the stability and properties of genetic material.

Pyrimidines and Their Physical Properties

Pyrimidines are simpler in structure compared to purines, as they include only a single ring. Thymine and cytosine are typical examples of pyrimidines found in DNA. Due to their smaller size, pyrimidines have fewer atoms and electrons, leading to weaker dispersion forces.
Although the dispersion forces in pyrimidines are weaker compared to purines, these molecules still play a critical role in the formation and stability of DNA and RNA structures. Their specific shapes and chemical properties allow them to pair precisely with complementary purines (adenine pairs with thymine, and guanine pairs with cytosine), ensuring the correct encoding and transmission of genetic information.
In aqueous solutions, pyrimidines will generally exhibit weaker intermolecular forces than purines because fewer electrons contribute to the less pronounced temporary dipoles formed in such environments.

Aqueous Solutions and Molecular Interactions

In an aqueous solution, which is a solution where water is the solvent, molecules like purines and pyrimidines interact in unique ways. Water molecules are polar, having a slight positive charge on one side and a slight negative charge on the other. This polarity leads to specific interactions like hydrogen bonding, significantly affecting how other molecules behave in water.
For purines and pyrimidines, being in an aqueous solution means their dispersion forces are affected by the presence of water. Purines, with their larger size, have stronger dispersion forces and are more influenced by the solvent properties of water compared to pyrimidines.

• Water may disrupt some of the weaker dispersion forces present in pyrimidines.
• The large number of electrons in purines allows them to maintain stronger intermolecular attractions even in the presence of water, although these can also be affected by water's solvent action.

Understanding these interactions is essential in fields like biochemistry, where aqueous environments play a pivotal role in molecular functionality and stability.