An internal mirror plane of symmetry is a plane that cuts through a molecule such that one half of the molecule is a perfect mirror image of the other half, lying on the opposite side of the plane.
In chemistry, symmetry elements, such as mirror planes, are crucial for determining a molecule's properties, including its chirality. A mirror plane (σ) is an imaginary plane through which reflection of a molecule results in an identical molecule.
The provided reference, "Determining Chirality Using Planes of Symmetry," highlights the role of mirror planes in this determination:
- As stated in the reference: "Method. We concluded that a molecule was a chiral. If reflection through an external mirror plane generated. The same molecule." This means that if reflecting a molecule across a mirror plane (even an external one) produces the exact same molecule, the molecule is considered achiral (not chiral).
Internal vs. External Mirror Planes
While the reference specifically mentions external mirror planes, the principle of reflection holds for all types of mirror planes.
- An external mirror plane is a mirror plane that does not pass through any atoms but reflects one part of the molecule across the plane to match another part.
- An internal mirror plane, which is the focus of the question, is a mirror plane that passes through one or more atoms and/or bonds within the molecule.
Regardless of whether the plane is internal or external, if a molecule possesses any mirror plane of symmetry (σ), it is achiral.
Characteristics of Internal Mirror Planes
- Location: Passes directly through the molecule's structure.
- Function: Divides the molecule into two halves that are mirror images of each other.
- Result of Reflection: Reflection of the molecule across this plane results in the original, indistinguishable molecule.
- Impact on Chirality: The presence of an internal mirror plane of symmetry is a sufficient condition for a molecule to be achiral.
Examples of Molecules with Internal Mirror Planes
Many common molecules have internal mirror planes. Here are a few simple examples:
- Water (H₂O): Has two internal mirror planes. One passes through the oxygen atom and bisects the H-O-H angle. Another passes through the oxygen atom and is perpendicular to the plane formed by the three atoms.
- Methane (CH₄): Has multiple internal mirror planes, each containing a carbon atom and two hydrogen atoms, bisecting the angle between the other two hydrogens.
- Benzene (C₆H₆): Has numerous internal mirror planes, including one that contains all six carbon atoms and one that passes through opposite carbon atoms and bisects the molecule.
Summary Table
| Feature | Description | Significance |
|---|---|---|
| Definition | Plane within a molecule where one side is mirror of other. | Key symmetry element. |
| Location | Passes through atoms and/or bonds. | Distinguishes it from external plane. |
| Reflection Test | Reflection across plane yields identical molecule. | Principle noted in reference for achirality. |
| Chirality | Presence makes a molecule achiral (not chiral). | Crucial for stereochemistry determination. |
Understanding internal mirror planes is fundamental to recognizing symmetry in molecular structures and predicting properties like optical activity. The ability for reflection through such a plane to generate the same molecule (as highlighted by the reference for external planes) is the defining characteristic linked to achirality.
