Problem 62 - Chemistry: The Central Science

Question

Acetylsalicylic acid, better known as aspirin, has the Lewis structure (a) What are the approximate values of the bond angles labeled 1,2 , and 3 ? (b) What hybrid orbitals are used about the central atom of each of these angles? (c) How many \(\sigma\) bonds are in the molecule?

Step-by-Step Solution

Answer

The bond angles in acetylsalicylic acid are approximately 120° for angles 1 and 3 (O-C-O and C-C-O, respectively) and 109.5° for angle 2 (C-C-C). The hybrid orbitals used by the central atoms in these bond angles are sp2 for carbons in angles 1 and 3 and sp3 for carbon in angle 2. There are a total of 17 $\sigma$ bonds in the molecule.

Step 1: Drawing the Lewis structure of acetylsalicylic acid

The first step is to draw the Lewis Structure of acetylsalicylic acid. Here is the structure: 

Step 2: Determine bond angles 1, 2, and 3

Here are the bond angles: 1. The O-C-O bond angle (1) is approximately 120° since the carbon is sp2-hybridized, and there are three regions of electron density around it in a planar trigonal arrangement. 2. The C-C-C bond angle (2) is approximately 109.5° since the carbon is sp3-hybridized, and there are four regions of electron density around it in a tetrahedral arrangement. 3. The C-C-O bond angle (3) is approximately 120° since the carbon is sp2-hybridized, and there are three regions of electron density around it in a planar trigonal arrangement.

Step 3: Determine the hybrid orbitals used by the central atom in each bond angle

1. The carbon atom in the O-C-O bond angle (1) is using sp2-hybrid orbitals. 2. The carbon atom in the C-C-C bond angle (2) is using sp3-hybrid orbitals. 3. The carbon atom in the C-C-O bond angle (3) is using sp2-hybrid orbitals.

Step 4: Count the number of σ bonds in the molecule

To count the sigma bonds in the molecule, we will count each single bond between two atoms. Double bonds consist of one sigma bond and one pi bond, so they will also be counted as one sigma bond. There are a total of 14 single bonds and 3 double bonds in the molecule. Therefore, there are a total of 14 + 3 = 17 sigma bonds in acetylsalicylic acid.

Key Concepts

Bond Angles

When examining a molecule like acetylsalicylic acid, understanding bond angles is crucial to predict the molecule's shape and behavior.
Bond angles provide insight into how atoms are arranged in three-dimensional space. Here, we focus on three particular bond angles:

• The O-C-O bond angle is approximately 120°. This occurs because carbon is sp2-hybridized in this scenario. Such hybridization results in a trigonal planar geometry, with three regions of electron density equally spaced around the central carbon atom
• The C-C-C bond angle is approximately 109.5°, as the carbon here is sp3-hybridized. This hybridization leads to a tetrahedral shape where four regions of electron density are symmetrically arranged.
• The C-C-O bond angle again is roughly 120°, owing to the sp2 hybridization of carbon, similar to the O-C-O angle.

Each bond angle reveals how the atoms are situated, highlighting the structural characteristics inherent to this important molecule.

Hybrid Orbitals

Hybrid orbitals explain how atomic orbitals merge to form new orbitals that can accommodate electron pairs in a molecule. In acetylsalicylic acid, the type of hybrid orbitals used by the carbon atoms significantly influences the molecular geometry and bond angles.
Here's how different hybridizations correspond to homogenous geometries:

• For the O-C-O bond angle, the carbon atom uses sp2-hybrid orbitals. This hybridization results in a flat, trigonal planar configuration, suitable for angles of 120°.
• The carbon involved in the C-C-C bond angle employs sp3-hybrid orbitals, achieving a tetrahedral shape. In this case, four orbitals hybridize, allowing for bond angles of about 109.5°.
• Similar to the O-C-O bond, the C-C-O bond angle utilizes sp2-hybrid orbitals, again exploiting the trigonal planar geometry leading to the 120° angle.

By understanding these hybrid configurations, we can gain comprehensive insights into how atoms bond and the resulting molecular structure.

Sigma Bonds

Sigma bonds (\(\sigma\) bonds) constitute a fundamental bond type in molecular chemistry, formed by the head-on overlapping of atomic orbitals. In acetylsalicylic acid, sigma bonds hold the framework together by linking atoms effectively.
Here's how it works:

• Every single bond between two atoms invariably constitutes one sigma bond. These are the strongest type of covalent bond due to the direct orbital overlap.
• Double bonds also contain one sigma bond alongside a pi bond. Therefore, when counting sigma bonds, each double bond contributes one additional sigma bond.
• In acetylsalicylic acid, a total of 17 sigma bonds exist, derived from 14 single bonds and 3 double bonds in its structure.

This understanding of sigma bonds helps in visualizing the atomic arrangements within the molecule, providing a clearer picture of its chemical architecture.