Final answer:
Aromatic compounds must have a planar structure, a cyclic structure, and a conjugated pi system. The statement that an aromatic compound must have an odd number of pi electrons is not true. Option 4 is correct.
Step-by-step explanation:
Aromatic compounds are characterized by certain features. Let's evaluate each statement:
It must have a planar structure: This statement is true because aromatic compounds are cyclic and planar, which allows for the delocalization of pi electrons.
It must have a cyclic structure: This statement is true as aromatic compounds have a closed ring structure.
It must have a conjugated pi system: This statement is true because aromatic compounds have a continuous system of alternating single and double bonds (pi bonds).
It must have an odd number of pi electrons: This statement is not true. Aromatic compounds can have both odd and even numbers of pi electrons, as long as they follow the rule of 4n+2, where n is a whole number.
Therefore, the correct answer is:
The compound does not need to have an odd number of pi electrons to be considered aromatic.