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Stability of Benzene

Hydrocarbons are mainly composed of carbon and hydrogen atoms. They can be further classified as aliphatic and aromatic compounds. Aliphatic compounds are two types; open chain compounds and cyclic compounds. Open chain compounds have straight or branched carbon chain in parent molecule. They may have single, double or triple covalent bonds in the molecule. Aliphatic compounds with only single covalent bonds are called as alkanes whereas alkenes have double covalent bonds and alkynes have triple covalent bonds in the molecule. Cyclic aliphatic compounds have ring structure with single, double or triple covalent bonds. On that basis they are named as cycloalkane and cycloalkenes.

Aromatic compounds are hydrocarbons with conjugated pi-electron system in the molecule. They are well known for their unique aroma and extra stability of cyclic molecules.  Aromatic compounds are cyclic compounds with conjugated pi-system in which pi-bonds are located in alternate double and single bonds in the molecule. The presence of pi-bonds in alternate manner delocalized the pi-electron system and makes the molecule stable. Aromatic compounds can be classified as benzenoid and non-benzenoid compounds. Benzenoid compounds have at least a benzene molecule. Benzene is simplest aromatic compound with 6 carbon atoms and three pi-bonds in alternate manner. Non-benzenoid compounds are aromatic compounds without benzene ring and conjugated pi-electrons. They also have delocalized electrons system and exhibits aromaticity. Aromaticity can be defined as the phenomenon to exhibit extra stability due to conjugated pi-electrons in the molecule.

 

Resonance and Stability of Benzene

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Benzene is a simplest aromatic compound with molecular formula $C_{6}H_{6}$. It is a planer hexagonal ring with three pi-bonds in alternate manner. All the six carbon atoms are bonded with each other and have one H atom also. Each carbon atom of benzene is $sp^{2}$ hybridized with trigonal planer geometry. The $sp^{2}$ hybridize orbitals of carbon involves in sigma bond formation with other two carbon atoms and one H atom to make a hexagonal ring. Remaining one un-hybrid p-orbital involves in side-way overlapping with neighbor carbon atom to form pi-bond. Since there is equal probability of making pi-bond with either neighbor carbon atom so pi-electron remains delocalized over six carbon atoms of ring. 

Resonance Stability of Benzene

The delocalization of pi-electrons in benzene molecule provides it extra stability that is called as aromaticity. Due to aromaticity in the molecule, benzene is more stable compare to aliphatic alkenes and does not show specific addition reactions of alkenes. In other words, benzene is less reactive then alkene for addition reactions as addition reactions can be responsible for loss of aromaticity. Benzene prefers to give substitution reactions in which the bonded H-atom is replaced by some electrophile. In the planer molecule of benzene, the C-C bond length is around 1.39 Ã… that is intermediate value of C-C and C=C in alkanes and alkenes respectively.

The stability of benzene can be explained on the basis of resonance in the molecule. There are two possible resonating structures of benzene molecule that can be proved by equal bond length of all C-C bonds in molecule. The resonance hybrid of benzene molecule is represented with a circle at the center of hexagonal ring of carbon atoms.  

Resonance Hybrid of Benzene

Explain the Stability of Benzene

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In benzene molecule, the delocalization of the six p-orbital results an electron clouds above and below the aromatic ring. That makes the molecule stable compare to alkenes. Another measurement of stability of benzene is the tendency of benzene to undergo electrophilic substitution reactions in place of electrophilic addition reactions of alkenes. The regular-hexagonal planar ring benzene is attributed to resonance stabilization of this conjugated cyclic alkene. Two resonating structures of benzene prove the extra stability of molecule. 

Thermodynamic Stability of Benzene

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Let’s discuss the evidence of thermodynamic stabilty of benzene molecule with the help  of heat of hydrogenation. Heat of hydrogenation is the amout of heat released during hydrogenation of alkene to form alkane.  It is representation of stability of an alkene. As the amount of  heat of hydrogenation increases, the stability of molecule decreasaes. So we can say that less value of energy molecules are more stable compare to higher energy molecules.  With more substitution on double bonded carbon atoms in alkene, the heat of hydrogenation will decrease that makes the molecule more stable.

Let’s check the stability of benzene with the help of heat of hydrogenation. We know that benzene is also a cyclic conjugated triene. The heat of hydrogenation for cyclohexene is 28.6 kcal /mol where for 1,3-cycloexadiene it is 55.4 kcal /mol. So the hypothetical value of heat of hydrgenation for benzene must be 85.5 k cal/mol. For experimental determination of heat of hydrogenation for benzene gives the value as 49.8 k cal /mol that is around 36 kcal /mol less than expected value. In other words, benzene is 36 kcal/mol more stable compare to hypothitical cyclic conjugated trien. 

Thermodynamic Stability of Benzene

Stability of Benzene Resonance Energy

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The concept of resonance helps to predict the extra stability of molecule. Low heat of hydrogenation for benzene is used to determine the thermodynamic stability and resonance energy of molecule. Heat of hydrogenation for cyclohexene is around 28.6 kcal/mol that increases to 55.4 kcal/mol for cyclohexadiene. Hence for cyclohexatriene, it must be 85.8 kcal/mol but it is not so. The heat of hydrogenation of cyclic conjugated triene that is benzene is 49.8 kcal /mol that is even less than conjugated cyclohexadiene. The difference between hypothetical and experimental value is 36 kcal /mol that represents the extra stability of benzene and called as resonance energy. 

Stability of Benzene Resonance Energy

Stability of Benzene Diazonium Chloride

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Reaction of aniline with nitrous acid $(NaNO_{2} + HCl)$ at 273 K temperature to form benzene diazonium salts. The reaction equation is shown below. 

Benzene Diazonium Chloride
 
The reaction is known as diazotization reaction. The compound formed is called as diazonium salt as it has a –N=N- (azo group) in the molecule with +ve and –ve part. Diazonium compounds are stable at low temperature and are widely used in organic synthesis of other organic compounds. The extra stability of salt is due to resonance of azo (-N=N-) group with aromatic ring of molecule. Resonating structures of diazonium chloride are shown below. 

Resonating Structures of Diazonium Chloride
 
Aliphatic amines also form diazonium compounds with $NaNO_{2}$ and HCl but these diazonium compounds are not stable due to lack of resonance in the molecule. 
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