The structure of acetylsalicylic acid (aspirin) is shown...

Name: Problem 88, Chapter 11: Chemistry A Molecular Approach
Uploaded: 2021-02-10T22:37:46-08:00
Duration: 4 min 11 s
Channel: Ronald Prasad
Description: The structure of acetylsalicylic acid (aspirin) is shown here. How many pi bonds are present in acetylsalicylic acid? How many sigma bonds? What parts of the molecule are free to rotate? What parts are rigid?

The structure of acetylsalicylic acid (aspirin) is shown here. How many pi bonds are present in acetylsalicylic acid? How many sigma bonds? What parts of the molecule are free to rotate? What parts are rigid?

Key Concepts

Aromaticity and Conjugated Systems

Aromatic systems are cyclic, planar structures with delocalized pi electrons, which provide extra stability and rigidity. Conjugation, the overlap of p orbitals across adjacent bonds, often leads to reduced rotational flexibility and defines many aspects of molecular geometry and chemical reactivity.

Sigma Bonds

Sigma (?) bonds are the primary, single covalent bonds formed by the head?on overlap of atomic orbitals. They serve as the framework for a molecule’s structure, allowing for free rotation around the bond axis in most cases. This rotational flexibility, however, is subject to other factors such as steric hindrance or adjacent rigid structures.

Pi Bonds

Pi (?) bonds are formed by the side-by-side overlap of p orbitals, typically accompanying sigma bonds in double or triple bonds. These bonds contribute to regions of electron density above and below the internuclear axis and restrict rotation around the bond due to the fixed orientation of the overlapping orbitals.

Bond Rotation and Flexibility

The concept of bond rotation involves understanding that single bonds (sigma bonds) generally allow free rotation, whereas bonds with a pi component (as in double bonds or aromatic systems) do not. This difference in rotational freedom is key in determining which segments of a molecule are flexible and which are rigid.

Rigid Structures in Molecules

Rigid parts of a molecule are typically those that involve double bonds or aromatic rings. The presence of pi bonds in these structures prevents free rotation, fixing the relative spatial positions of the atoms, and contributing to the overall shape and reactivity of the molecule.

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