To solve this problem, we need to determine which term best describes a molecule that is cyclic, planar, completely conjugated, and has four π electrons.
1. Understanding the Criteria:
- Cyclic: The molecule forms a closed ring.
- Planar: All atoms in the molecule lie in the same plane.
- Completely Conjugated: The π electrons are delocalized around the ring.
- Four π Electrons: The molecule must have a total of four π electrons in its conjugated system.
2. Hückel's Rule:
- Hückel's rule states that a molecule is aromatic if it has [tex]\(4n + 2\)[/tex] π electrons, where [tex]\( n \)[/tex] is a non-negative integer (0, 1, 2, 3, ...). Typical aromatic compounds have 2, 6, 10, 14, etc., π electrons.
- Molecules that do not follow this rule and have [tex]\( 4n \)[/tex] π electrons (such as 4, 8, 12, etc.) tend to be antiaromatic.
3. Evaluating the Given Options:
- Aromatic: For a molecule to be aromatic, it must follow the [tex]\( 4n + 2 \)[/tex] rule. Thus, with four π electrons, it would not be aromatic.
- Semiaromatic: This term is less commonly used in chemistry and typically not well-defined. Without a concrete definition, it’s safe to exclude this as well.
- Nonaromatic: This term refers to molecules that aren’t cyclic, planar, or completely conjugated. Since our molecule meets all those criteria, it isn't nonaromatic.
- Antiaromatic: For a molecule with [tex]\( 4n \)[/tex] π electrons, such as four π electrons, it fits the criteria of antiaromatic compounds.
4. Conclusion:
- Based on Hückel's rule and the criteria provided, a molecule with four π electrons fits the definition of being antiaromatic.
Thus, the correct term to define a molecule that is cyclic, planar, completely conjugated, and has four π electrons is:
5. antiaromatic
