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.
The enthalpy of hydrogenation of cyclohexene is `-119.5 "kJ mol"^(-1)`.If resonance energy of benzene is `-150.4 "kJ mol"^(-1)`, its enthalpy of hydrogenation would be

Options

A. `-208.1 "kJ mol"^(-1)`
B. `-269.9 "kJ mol"^(-1)`
C. `-358.5 "kJ mol"^(-1)`
D. `-508.9 "kJ mol"^(-1)`

Answer

To find the enthalpy of hydrogenation of benzene, we can follow these steps:

### Step 1: Understand the Process of Hydrogenation
Hydrogenation is the addition of hydrogen (H2) to a molecule. For benzene (C6H6), hydrogenation will convert it into cyclohexane (C6H12).

### Step 2: Use Given Data
We are given:
- The enthalpy of hydrogenation of cyclohexene is -119.5 kJ/mol.
- The resonance energy of benzene is -150.4 kJ/mol.

### Step 3: Calculate the Enthalpy of Hydrogenation for Benzene
To calculate the enthalpy of hydrogenation for benzene, we need to consider that three moles of hydrogen are required to convert benzene into cyclohexane. Therefore, we can express the enthalpy of hydrogenation of benzene as:

\[ \text{Enthalpy of hydrogenation of benzene} = 3 \times (\text{Enthalpy of hydrogenation of cyclohexene}) \]

Substituting the values:

\[ \text{Enthalpy of hydrogenation of benzene} = 3 \times (-119.5 \text{ kJ/mol}) = -358.5 \text{ kJ/mol} \]

### Step 4: Adjust for Resonance Energy
Next, we need to adjust this value by subtracting the resonance energy of benzene:

\[ \text{Enthalpy of hydrogenation of benzene} = -358.5 \text{ kJ/mol} - (-150.4 \text{ kJ/mol}) \]

Calculating this gives:

\[ \text{Enthalpy of hydrogenation of benzene} = -358.5 \text{ kJ/mol} + 150.4 \text{ kJ/mol} = -208.1 \text{ kJ/mol} \]

### Final Answer
Thus, the enthalpy of hydrogenation of benzene is:

\[ \text{Enthalpy of hydrogenation of benzene} = -208.1 \text{ kJ/mol} \]

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