Which of the following compounds is not an aromatic hydrocarbon?

Which of the following compounds is not an aromatic hydrocarbon?

Step 1: Understand what an aromatic hydrocarbon is. Aromatic hydrocarbons are compounds that have a ring structure, are cyclic, and follow Huckel's rule.
Step 2: Recall Huckel's rule, which states that a compound is aromatic if it has (4n + 2) π electrons, where n is a non-negative integer.
Step 3: Identify the compounds in the question. One of them is Cyclooctatetraene.
Step 4: Check the structure of Cyclooctatetraene. It has 8 π electrons, which does not fit Huckel's rule (it should have 6, 10, 14, etc. π electrons).
Step 5: Determine if Cyclooctatetraene is planar. Aromatic compounds must be planar to allow for delocalization of electrons. Cyclooctatetraene is not planar.
Step 6: Conclude that Cyclooctatetraene is not an aromatic hydrocarbon because it does not satisfy Huckel's rule and is not planar.

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