MCQ Detailed View

Aromatic compounds are a special class of organic molecules that show unusual stability due to the delocalization of π electrons. The main criterion for a compound to be aromatic is the presence of 4n+2 π electrons in a cyclic, planar, and fully conjugated system. This rule is known as Hückel’s rule, where n is any whole number (0, 1, 2, 3…).

The simplest and most common example is benzene (C₆H₆). Benzene is a planar, cyclic compound with six π electrons. Substituting n = 1 into the formula 4n+2 gives 6, which matches the number of π electrons in benzene. This explains why benzene is aromatic and highly stable.

Looking at the given options:

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  Unsaturations alone do not make a compound aromatic. Many unsaturated compounds like alkenes are not aromatic because they lack cyclic delocalization.

  
  


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  Cyclic structure is necessary but not sufficient. For example, cyclobutadiene is cyclic and conjugated, but it has 4 π electrons, making it antiaromatic and unstable.

  
  


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  Presence of 4nπ electrons is a condition for antiaromaticity, not aromaticity. Compounds with 4, 8, or 12 π electrons in a planar conjugated ring are highly unstable.

  
  


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  Presence of 4n+2π electrons is the correct criterion. This condition ensures continuous delocalization and extra stability due to resonance.

  
  

Other examples of aromatic compounds include naphthalene (10 π electrons), anthracene (14 π electrons), and heteroaromatic compounds like pyridine and furan, all of which follow Hückel’s rule.

Therefore, the main criterion for aromaticity is the presence of 4n+2 π electrons in a cyclic conjugated system, which explains the unique stability and reactivity pattern of aromatic compounds in organic chemistry.