The Story of Benzene
Kekule’s Benzene vs Actual Benzene
On a cold winter afternoon, sitting in an armchair and sipping hot cocoa, August Kekule was lost in thoughts. He was in deep thought about what the structure of benzene would be like. He soon fell asleep, dreaming about benzene. In his dreams, he saw a “snake biting its own tail.” He woke up with a start, grabbed a pen and a piece of paper, and started drawing the structure of benzene.
He described the structure of benzene as something like this-
“ Benzene is a six-carbon molecule where all the six carbons are arranged in the form of a hexagon with alternating single and double bonds. Each carbon is further attached to one hydrogen each.”
Thus, Kekule’s reverie formed the basis of the structure of benzene. Sure, it has some discrepancies, but this gave the scientific community, which was perplexed for many years, a new insight into the structure of benzene.
Haltopub, CC BY-SA 3.0, via Wikimedia Commons
In this article, we will discuss the following:
- What is benzene?
- The structure of benzene, as given by Kekule.
- Discrepancies in Kekule’s structure.
- The delocalised model of benzene.
INTRODUCTION TO BENZENE
What is Benzene?
We know that the broad classification of organic compounds gives us two categories:
- Aliphatic compounds
- Aromatic compounds
Benzene belongs to the class of aromatic compounds. Michael Faraday, in the year 1825, isolated benzene as an oily residue from the gas lines of London.
Benzene is a colourless compound with a melting point of 6°C and a boiling point of 80°C. The molecular formula of benzene is C₆H₆ . It is a planar molecule.
Benzene is a pretty stable organic molecule resistant to the characteristic reactions of unsaturated hydrocarbons like addition and oxidation. It gives substitution reactions instead.
Let us now explore Kekule’s structure of Benzene and the reasons for the failure of Kekule’s structure.
Kekule’s structure
Before August Kekule, many scientists tried to understand the structure of benzene. Some of them even thought of benzene to be an open chain structure but failed to explain why benzene gives only one mono-substituted product. For example, if benzene was supposed to be an open-chain compound like- 1,5-hexadiene-3-yne, it should give three different substitution products, yet benzene gives a single mono-substituted product.
As discussed in the introduction, Kekule explained that benzene is a hexagon with six carbon atoms and alternating double and single bonds. Based on this structure, he explained that all six carbons are at similar positions, hence giving a mono-substituted product.
Despite cracking it, Kekule could not explain many questions related to the peculiar behaviour of benzene.
Evidence against Kekule’s benzene
Kekule’s benzene failed in answering the following questions as evidence suggested otherwise. Let us see what they are and why Kekule’s benzene could not give satisfactory answers.
- Why benzene prefers substitution over addition?
If benzene was like the other unsaturated compounds(alkenes and alkynes), it should give addition reactions. That means it should decolourise bromine water. But, evidence shows that it doesn’t behave that way, despite having 3 double bonds. Why?
2. Why all the bonds in benzene are of equal length?
The C-C bond length is 0.154 nm while the bond length of C=C is 0.134 nm. If Kekule’s structure was accurate, the structure of benzene should be a distorted hexagon.
But, the X-ray diffraction studies proved otherwise. It became evident that all the bonds in benzene are equivalent- meaning have the same bond length that is neither equal to 0.154 nm nor 0.134 nm. Instead, the actual bond length of true benzene was found to be 0.140 nm, an intermediate value between the C-C and C=C.
Thus, the true benzene is not distorted, but a regular hexagon. This evidence suggests that Kekule’s ‘localised’ model of benzene is inaccurate.
3. Why is benzene’s enthalpy of hydrogenation less than expected?
Let us recall what the enthalpy of hydrogenation is.
“The enthalpy of hydrogenation is nothing but the enthalpy change that occurs when one mole of an unsaturated compound reacts completely with an excess of hydrogen under standard conditions to form a saturated compound.”
Let us predict the enthalpy of the hydrogenation of benzene using the enthalpy of the hydrogenation of cyclohexene(molecular formula: C₆H₁₀).
When one mole of cyclohexene gets converted to cyclohexane, the enthalpy of hydrogenation is -120 kJ /mol¹. Based on this, as benzene has 3 double bonds, the enthalpy of hydrogenation should be:
3(-120) = -360 kJ/mol¹
C₆H₆ + 3H₂ → C₆H₁₂ ΔH = -208 kJ/mol¹
But, evidence suggested that the actual enthalpy of hydrogenation of benzene is
-208 kJ/mol¹. This means it is around -152 kJ/mol¹ lower than expected. Kekule’s benzene failed to explain the reason for lower hydration enthalpy than expected.
4. Why is the benzene more stable than the predicted stability of Kekule’s benzene?
Based on the hydrogenation enthalpy, we can say that benzene is 152 kJ/mol¹ more stable than predicted. Why?
All these questions are later answered by the delocalised model of benzene, which we will be discussing for the next half of the article.
The delocalised model of benzene
Contrary to what Kekule established, the three double bonds are not localised-not fixed in their positions. They are equally shared among all the carbons forming an electron cloud above and below the plane of benzene. This is called as delocalisation of pi-electrons. This model of benzene is popularly described as the delocalised model of benzene. Delocalisation simply means that the electrons are not particularly situated between a fixed set of atoms, but rather move around. They do not belong to one.
Debunking the delocalised structure…
- All the carbon atoms in benzene are sp² hybridised. Each sp²carbon forms a sigma bond with the adjacent sp² carbon and another sigma bond with hydrogen (sp²- s overlap). Benzene is a planar molecule as the carbons are sp² hybridised, and the geometry of the carbons is trigonal planar.
- Based on experimental data, the C-C bond length in benzene is 0.140 nm. All the bonds are equal in length because all the bonds have some pi-character as the pi-electron cloud is equally distributed among all the carbons.
- The pi-bonds are formed by the side-wise overlap of the unhybridized p-orbital which contains one electron each (the other two p-orbitals participate in sp² hybridisation).
- The bond angles <CCC and <CCH is equal to 120° thus forming a regular hexagon.
The most common representation of benzene is a hexagon with a circle in the centre. This circle represents the continuous delocalisation of pi-electrons.
Let us answer the questions, based on this delocalised model, that Kekule’s benzene could not answer.
- Benzene prefers substitution over addition unlike other unsaturated compounds because it is stabilised by the delocalisation of pi-bonds which is extremely difficult to disrupt. Hence, benzene does not decolourise bromine water.
- Benzene is stabilised by resonance(delocalisation of pi-electrons in the case of benzene). Hence, owing to its stability, the enthalpy of hydration is less. This is because more energy is needed to disrupt the delocalisation, hence the energy evolved is less.
- All the bonds in benzene are equal in length because all the bonds have a partial double-bond character due to the delocalisation of the pi-electron cloud. Hence, all the bonds have an equal bond length of 0.140nm.
- The benzene ring is stabilised by resonance energy. Hence, it is resistant to oxidation, reduction and addition reactions. It undergoes them reluctantly.
Thus, the delocalised model solved the discrepancies that existed in Kekule’s model.
Benzene is an example of why you should never stop dreaming.
