Question

Following is a structural formula of the prescription drug famotidine, manufactured by Merck Sharpe \& Dohme under the name Pepcid. The primary clinical use of Pepcid is for the treatment of active duodenal ulcers and benign gastric ulcers. Pepcid is a competitive inhibitor of histamine \(\mathrm{H}_{2}\) receptors and reduces both gastric acid concentration and the volume of gastric secretions. (a) Complete the Lewis structure of famotidine showing all valence electrons and any formal positive or negative charges. (b) Describe each circled bond in terms of the overlap of atomic orbitals.

Short answer

Question: Complete the Lewis structure of famotidine, showing all valence electrons and any formal positive or negative charges. Describe the circled bonds in terms of the overlap of atomic orbitals. Answer: To complete the Lewis structure of famotidine, follow the steps in the solution provided. Once you have drawn its complete structure, analyze the circled bonds types and the corresponding overlap of atomic orbitals, such as sigma (σ) bonds, pi (π) bonds, or delocalized (resonance) bonds.

Step-by-step solution

Draw the Lewis structure of famotidine

To draw the Lewis structure of any compound, follow these general steps: 1. Determine the total number of valence electrons of the compound. 2. Sketch the backbone structure of the compound with single bonds between the atoms. 3. Complete the octets of the atoms by adding lone pairs of electrons. 4. Verify the structure using formal charges. Once you have drawn the Lewis structure of famotidine, make sure to denote any formal positive or negative charges on the atoms.

Describe the circled bonds in terms of the overlap of atomic orbitals

Analyzing the bonds in a compound generally involves looking at the type of bond formed in the molecule and describing the overlap of atomic orbitals that give rise to the specific bond. Three common types of bonds are: 1. Sigma (σ) bond: Formed by the head-to-head (axial) overlap of atomic orbitals. Common examples include s-s, s-p, and p-p overlap (e.g., single bonds in any molecule). 2. Pi (π) bond: Formed by the side-to-side (lateral) overlap of parallel p orbitals. An example includes double bonds, such as C=C, which consist of one σ bond and one π bond. 3. Delocalized (resonance) bonds: Occur when electrons are spread over multiple atoms, and atomic orbitals can overlap in multiple ways, creating resonance structures. For the circled bonds in the famotidine structure, describe each bond in terms of the type of bond formed and the overlapping atomic orbitals that give rise to the bond. Please note that since the actual structure of famotidine is not provided in the text format here, we cannot discuss specific circled bonds. However, the general guidelines in this solution should help you correctly answer the exercise based on the provided structure in your textbook or assignment.

Key concepts

Understanding Valence Electrons

Valence electrons play a pivotal role in the formation of chemical compounds, including therapeutic agents like famotidine. These are the electrons located in the outermost shell of an atom and are responsible for the chemical properties of the element. To draw a Lewis structure as seen in famotidine, it's crucial to determine the total number of valence electrons available for bonding.

In the context of famotidine, each atom brings a specific number of valence electrons. For instance, hydrogen has one valence electron, while carbon, nitrogen, and oxygen have four, five, and six, respectively. By knowing these numbers, we can calculate the total count of valence electrons in famotidine and distribute them in the structure to satisfy the octet rule for heavier atoms and duet rule for hydrogen.

The accurate arrangement of valence electrons in the Lewis structure is essential because it can reveal formal charges on atoms, which are crucial for understanding the stability and reactivity of the molecule. Formal charges indicate any electron surplus or deficit on atoms within the molecule, and they should be minimized to reflect the most stable structure. Famotidine’s activity as a histamine H2 receptor inhibitor is, in part, due to its specific arrangement of valence electrons contributing to its ability to interact with biological targets.

The Role of Atomic Orbital Overlap

The atomic orbital overlap is the foundation of chemical bond formation according to quantum mechanics. Orbitals are regions around an atom where electrons are likely to be found. The type of overlap determines the kind of bond that forms between atoms. When two atomic orbitals converge, they can either overlap directly or side-by-side, dictating the formation of sigma or pi bonds, respectively.

For example, in a simple molecule, a sigma bond () results from the head-on overlap of spherical s orbitals or the end-to-end overlap of dumbbell-shaped p orbitals. Famotidine is a complex molecule, and within its structure, there are single and possibly double bonds, each formed from sigma and pi bonds respectively. Single bonds are always sigma bonds, created by either s-s, s-p, or p-p overlap, forming the basic framework of the molecule.

The process of describing bonds in compounds, as required in the exercise, demands a close look at the molecule's structure to determine which atomic orbitals are merging. Since electron clouds are involved in these overlaps, the geometry and type of overlapping orbitals can significantly influence molecular shape and chemical behavior, factors quintessential in the drug's interaction with biological systems.

Fundamentals of Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules and compounds. The three main types of chemical bonds include ionic, covalent, and metallic bonds. Famotidine, as with most organic compounds, primarily features covalent bonds where electron pairs are shared between atoms.

In covalent bonding, electron sharing can vary, leading to single, double, or even triple bonds, with single bonds involving one electron pair and being the weakest. A double bond has two electron pairs and is stronger and shorter than a single bond. This pattern is conceptually important as it often corresponds to the real bond conditions in famotidine's molecular structure, with various atomic groupings linked by different types of covalent bonds.

Furthermore, some covalent bonds are polar, where electrons are shared unequally due to differences in electronegativity between the atoms. Famotidine may exhibit polarity in specific bonds which affects its solubility and hence its pharmacokinetics. Describing the chemical bonds in famotidine requires an understanding of these concepts, leading to an accurate depiction of its structure and contributing to the understanding of its inhibitory action on histamine H2 receptors.