Question

In Kekule 's day, one puzzling aspect of his dynamic theory for benzene was provided by 1,2 -dimethylbenzene. According to his theory, there should be two distinct such compounds, one with a double bond between the two methyl- substituted carbons and one with a single bond in this position. Only a single 1,2 -dimethylbenzene is known, however. (a) Does Ladenburg's formula solve this problem? (b) Explain with modern resonance theory.

Short answer

Ladenburg's formula solves the problem faced by Kekule's theory, as it results in only one distinct 1,2-dimethylbenzene compound due to the single bonds in its representation. Modern resonance theory further explains this by suggesting that benzene is a resonance hybrid with delocalized electrons, making all carbon-carbon bonds equivalent and resulting in only one unique 1,2-dimethylbenzene compound.

Step-by-step solution

Problem with Kekule's Theory

Kekule's theory for benzene proposed alternating single and double bonds, making two different possible structures for 1,2-dimethylbenzene: 1. A double bond between the two methyl-substituted carbons. 2. A single bond between the two methyl-substituted carbons. However, only one 1,2-dimethylbenzene compound is known.

(a) Ladenburg's formula

Ladenburg's formula, also known as the prismane structure, represents benzene as a six-membered ring with three methylene bridges. In this representation, all carbon-carbon bonds are single bonds, and there is no difference in bond orders. Let's check if this formula solves the problem of two distinct 1,2-dimethylbenzene compounds. Using Ladenburg's formula, there is no difference between the two methylene bridge positions, meaning that when we substitute two methyl groups at positions 1 and 2, we obtain only one unique compound. Therefore, Ladenburg's formula solves the problem faced by Kekule's theory.

(b) Modern resonance theory

Modern resonance theory gives a better understanding of the structure of benzene. According to resonance theory, benzene is a combination of two equivalent structures, where the double bond positions are interchanged. The actual structure is a resonance hybrid of these two structures, and the electrons forming the double bonds are delocalized over the entire ring. When we consider the resonance hybrid, all the carbon-carbon bonds in benzene are equivalent, and their bond order is between a single bond and a double bond. Applying this understanding to 1,2-dimethylbenzene, when we substitute two methyl groups at positions 1 and 2 on the benzene ring, there is no difference in the bond between these two carbons since the electrons are delocalized. As a result, we obtain only one unique compound of 1,2-dimethylbenzene, which is consistent with the experimental evidence. In conclusion, modern resonance theory supports Ladenburg's formula and provides a better understanding of the actual structure of benzene and its derivatives, such as 1,2-dimethylbenzene.

Key concepts

Kekule's theory for benzene

Friedrich August Kekulé was a prominent figure in organic chemistry who proposed a groundbreaking model for the structure of benzene in the 19th century. Kekulé's theory depicted benzene as a six-membered ring with alternating single and double carbon-carbon bonds. This structure, often referred to as Kekulé's structure, implied that there should exist two distinct types of 1,2-dimethylbenzene due to the different potential placements of double bonds. However, only one type of compound for 1,2-dimethylbenzene has been observed, which brought about questions and confusion regarding the validity of the theory.

Kekulé's model struggled to explain the uniformity of benzene's carbon-carbon bond lengths, as measured by various experimental methods. Moreover, it did not account for the exceptional stability and symmetry suggested by benzene's observed chemical behavior.

Ladenburg's prismane structure

Albert Ladenburg was another influential chemist who proposed an alternative model to explain the structure of benzene. This proposal was known as Ladenburg's prismane structure and was representative of benzene as a three-dimensional structure characterized by three methylene bridges. In contrast to Kekulé's alternating single and double bonds, Ladenburg's model featured only single bonds, which was attractive because it could theoretically resolve the issue of the non-existing isomer of 1,2-dimethylbenzene posed by Kekulé's theory.

Ladenburg's structure illustrated all bonds in benzene as equivalent, thus predicting only one form of any disubstituted benzene derivative, including 1,2-dimethylbenzene. However, Ladenburg's model did not correspond with the planar shape of benzene observed through experimental methods, nor did it justify benzene's special stability, which is crucial in understanding benzene's unique chemistry.

1,2-dimethylbenzene

The compound 1,2-dimethylbenzene, also known as ortho-xylene, is a variant of benzene where two methyl groups replace hydrogen atoms on adjacent carbon atoms within the ring structure. The question arising from both Kekulé's and Ladenburg's theories was why, despite the theoretical possibility of having two distinct compounds (due to different possible bond placements between the two methyl groups), only one 1,2-dimethylbenzene is known to exist.

The traditional structural theories failed to predict this reality observed in experimentation. The classical depiction of fixed double bonds in Kekulé's theory does not correlate with the singular form of 1,2-dimethylbenzene encountered, thus necessitating a revision of the understanding of benzene's ring structure.

Delocalized electrons

Delocalized electrons are electrons in a molecule that are not associated with a single atom or a single covalent bond. This concept is integral to modern resonance theory, which states that the actual structure of a molecule is a resonance hybrid of multiple structures with delocalized electrons, rather than a single fixed configuration.

Within the benzene ring, the electrons originally involved in double bonds as per Kekulé's model are actually spread out or 'delocalized' across the entire ring. This results in a ring where all carbon-carbon bonds have the same intermediate bond order, neither a true single nor double bond. Delocalization confers extra stability to benzene, giving rise to its unique chemical behavior. Delocalized electrons are the true explanation behind the existence of only one 1,2-dimethylbenzene, as all positions on the ring are equivalent in the context of electron distribution.