17.4
Structure of Benzene: Kekulé Model
In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formul…
In 1865, August Kekulé proposed the first definite structure for benzene.
He suggested that benzene has a cyclic structure of six carbon atoms with alternating single and double bonds. Each carbon atom is attached to one hydrogen atom.
This model satisfied the tetravalency of carbon, making all the hydrogens equivalent.
Kekulé's model predicted two forms of 1,2 -dibromobenzene, which differed in the positions of the double bonds. But, only one form of 1,2-dibromobenzene was isolated.
Kekulé defended it by proposing a rapid equilibrium between the two inseparable forms of benzene, now known as Kekulé structures. However, it is now known that no such equilibrium exists.
Moreover, it failed to explain the unusual chemical behavior of benzene, which undergoes a substitution reaction with bromine, as opposed to alkenes, which undergo an addition reaction.
View the full transcript and gain access to JoVE Core videos
Q1: What structure did August Kekulé propose for benzene in 1865?
Kekulé proposed that benzene has a cyclic structure of six carbon atoms with alternating single and double bonds, each attached to one hydrogen atom. This model satisfied carbon's tetravalency, making all hydrogens equivalent and accounting for benzene's molecular formula C6H6.
Q2: Why did Kekulé's model predict two isomers of 1,2-dibromobenzene?
According to Kekulé's alternating double bond structure, the two bromine atoms in 1,2-dibromobenzene could be connected by either a double bond or a single bond, creating two distinct isomers. However, only one form was experimentally isolated, contradicting the model's prediction.
Q3: How did Kekulé explain the existence of only one 1,2-dibromobenzene isomer?
Kekulé proposed that benzene exists in rapid equilibrium between two interconvertible forms, making the two isomers inseparable. These rapidly interchanging structures, now called Kekulé structures, would appear as a single compound. However, this equilibrium does not actually exist in reality.
Q4: What chemical behavior did Kekulé's model fail to explain?
Kekulé's model failed to explain why benzene undergoes substitution reactions with bromine rather than addition reactions like alkenes do. Ethylene adds bromine to form ethylene dibromide, but benzene substitutes a hydrogen atom to form bromobenzene, revealing benzene's unusual reactivity compared to other unsaturated hydrocarbons.
Q5: How many degrees of unsaturation does benzene contain according to Kekulé's model?
Benzene has four degrees of unsaturation: one from the six-membered ring and three from the alternating pi bonds. Despite this high degree of unsaturation, benzene does not undergo typical addition reactions characteristic of other unsaturated hydrocarbons like alkenes and alkynes.
Q6: What conditions are required for benzene to react with bromine?
Benzene reacts with bromine to form bromobenzene through a substitution reaction in the presence of a Lewis acid catalyst, specifically ferric bromide. This requirement for a catalyst and the substitution mechanism distinguish benzene's reactivity from typical unsaturated compounds like alkenes.
Q7: Why is Kekulé's alternating bond model considered incomplete?
Kekulé's model failed to account for benzene's exceptional stability and unusual preference for substitution over addition reactions. The model could not explain why benzene behaves differently from other unsaturated hydrocarbons despite having comparable degrees of unsaturation, indicating the need for alternative structural explanations like the criteria for aromaticity and the Hückel 4n + 2 rule.