Aromatic hydrocarbons are highly unsaturated molecules but behave like saturated hydrocarbons. Those which contain a benzene ring are called benzenoids but those which contain some highly unsaturated ring other than benzene are called non-benzenoids. Their aromatic character can be ascertained through Hückel rule. Aromatic hydrocarbons are, however, more reactive than alkanes but are less reactive than alkenes and alkynes. The lower reactivity of arenes is because of the extra stability associated with these molecules due to delocalization of -electrons. Their stability is measured in terms of resonance energy which can be estimated from either heat of combustion or heat of hydrogenation data. They normally undergo electrophilic substitution reactions. In presence of a Lewis acid catalyst, nuclear halogenation occurs but in absence of Lewis acid catalyst and in presence of light, halogens add to the benzene ring. If an aromatic hydrocarbon contains an alkyl side chain, then in presence of heat/light side chain halogenation occurs in preference to addition of halogens to the benzene ring. The reactivity of aromatic hydrocarbons towards electrophilic substitution reactions depends upon the electron density in the benzene ring. Electron-donating groups favour while electron-withdrawing groups retard these reactions. Orientation of electrophilic substitution reactions is governed by the nature of the substituent already present in the ring. Although aromatic hydrocarbons are resistant to oxidising agents `(KMnO_4, K_2Cr_2O_7`, etc.) they do undergo ozonolysis.
Which of the following is not a non-benzenoid aromatic compound ?

Options

A. Tropylium cation
B. Azulene
C. Tropolone
D. Diphenyl

Answer

To determine which of the following is not a non-benzenoid aromatic compound, we need to analyze the characteristics of each compound mentioned in the options. Let's go through the steps to arrive at the solution:

### Step 1: Understand the Definitions
- **Aromatic Hydrocarbons**: These are compounds that contain a benzene ring (benzenoids) or other unsaturated rings (non-benzenoids).
- **Benzenoid Compounds**: Compounds that contain a benzene ring.
- **Non-Benzenoid Compounds**: Compounds that do not contain a benzene ring but can still exhibit aromaticity.

### Step 2: Identify the Compounds
We need to look at the options provided (though not explicitly listed in the question, we can infer based on the transcript):
1. Tropylium ion
2. Azulene
3. Tropolone
4. Biphenyl

### Step 3: Analyze Each Compound
1. **Tropylium Ion**: This is a non-benzenoid aromatic compound. It has a cyclic structure with 7 π-electrons, satisfying Hückel's rule (4n + 2).

2. **Azulene**: This compound consists of a 5-membered ring fused to a 7-membered ring. It is non-benzenoid but aromatic due to its structure and electron count.

3. **Tropolone**: This compound has a non-benzenoid structure with a 7-membered ring and is also aromatic. It does not contain a benzene ring.

4. **Biphenyl**: This compound consists of two benzene rings connected by a single bond. It is a benzenoid compound because it contains benzene rings.

### Step 4: Conclusion
Based on the analysis, the compound that is **not a non-benzenoid aromatic compound** is **Biphenyl** because it is a benzenoid aromatic compound.

### Final Answer
**Biphenyl is not a non-benzenoid aromatic compound.**

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