Question

Which of the following compound does not exhibit aromaticity?

• A. Cyclohexene
• B. Benzene
• C. Thiophene
• D. Pyridine
• E. Naphthalene

Hint:

Remember the four criteria for aromaticity: cyclic, planar, fully conjugated, and (4n+2) π electrons.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept of Aromaticity:
Aromaticity is a special property of certain cyclic compounds that makes them unusually stable. To be aromatic, a compound must satisfy all of Hückel's criteria:
1. It must be cyclic.
2. It must be planar.
3. It must have a continuous ring of p-orbitals (i.e., be fully conjugated).
4. It must contain (4n+2) \(\pi\) electrons, where n is an integer (0, 1, 2, ...).
A compound that fails one or more of these criteria is considered non-aromatic or anti-aromatic.
Step 2: Analysis of Options:
Let's evaluate each compound against the criteria for aromaticity:
(B) Benzene: It is cyclic, planar, fully conjugated (alternating double and single bonds), and has 6 \(\pi\) electrons (which fits 4n+2 for n=1). It is the archetypal aromatic compound.
(C) Thiophene and (D) Pyridine: These are heterocyclic compounds. Both are cyclic, planar, fully conjugated, and have 6 \(\pi\) electrons participating in the aromatic system. They are aromatic.
(E) Naphthalene: It consists of two fused benzene rings. It is cyclic, planar, fully conjugated, and has 10 \(\pi\) electrons (which fits 4n+2 for n=2). It is aromatic.
(A) Cyclohexene: It is a six-membered cyclic compound, but it is not fully conjugated. The molecule contains four sp\(^3\)-hybridized carbon atoms and only one double bond. The presence of these sp\(^3\) carbons breaks the continuous ring of p-orbitals. Since it fails the conjugation criterion, it is non-aromatic.
Step 3: Final Answer:
Cyclohexene does not have a continuous conjugated system of \(\pi\) electrons and is therefore non-aromatic. This corresponds to option (A).