In 1865, the german chemist August Kekulé proposed a cyclic structure for benzene, an aromatic–smell — Organic Chemistry Chemistry Question
Kekulé, Benzene and the Problem of Aromaticity
In 1865, the german chemist August Kekulé proposed a cyclic structure for benzene, an aromatic–smelling hydrocarbon with the empirical formula C6H6, that was discovered in 1825 by Michael Faraday. Kekulé proposed that carbon has four valences and that it can form carbon–carbon single bonds (1/4 overlap) or double bonds (2/4 overlap). In his model, benzene has alternating single and double bonds. The remaining 6 valences are saturated with bonds to the six hydrogen atoms. These are copies of his original work: [VISUAL]
However, at that time it was already known that there is only one isomer of ortho di–substituted benzenes. If benzene had alternating single and double bonds there would be two isomers, one with a double bond between the substituents and one with a single bond. Kekulé solved this contradiction by assuming that the single and double bonds in benzene are “somehow combined in a common benzene nucleus”.
Now, we know that benzene is a planar, regular hexagon with all the C–C bonds of equal lengths and that its chemical reactivity is different from that of a normal olefin.
Draw resonance structures that explain the electronic structure of benzene.
Model Answer
Kekulé originally suggested two equilibrating structures with alternating single and double bonds. According to Kekulé, the single bonds would be longer than the double bonds and the structures would have irregular hexagonal shapes. Spectroscopy, however, has shown that benzene has a planar ring, with all the carbon–carbon bond distances having the same length of 1.397 Å (C–C typically 1.48 Å, C=C typically 1.34 Å). Since there are equal distances between the atoms, and the locations of the electrons in the two Kekulé structures are the only difference, they are in fact resonance structures. [VISUAL]
Draw the structures of all conceivable disubstituted benzene isomers bearing two identical substituents (C6H4R2).
Model Answer
Two substituents attached to a benzene ring can be positioned in three different ways: ortho, meta, and para substituted isomers. [VISUAL]
An alternative benzene structure was proposed by Staedeler. Nowadays it is known as the Dewar benzene structure:
= [VISUAL]
How many isomers of Dewar benzene will be conceivable if it is substituted with two identical substituents? Draw the structures.
Model Answer
Dewar benzene was one of the structures proposed for benzene in the early days of organic chemistry. There are six different structural isomers of a disubstituted Dewar benzene, three of them are chiral and occur in two enantiomeric forms. Because only three benzene isomers C6H4R2 have been found by experiment, the Dewar benzene structure cannot be correct. However, Dewar benzene can be synthesized but it is much less stable than benzene because of its considerable angle strain and its lack of aromatic stabilization. [VISUAL]
Shortly after, A. Ladenburg, who used to be Professor for Organic Chemistry here in Kiel, proposed the so–called Ladenburg benzene structure (now called prisman):
1 1
2 6
3 5
4
4
2
3
6
5
[VISUAL]
According to Prof. Ladenburg, the benzene model is in agreement with the fact that there are three disubstituted benzene isomers:
R R R
R
R
R
[VISUAL]
Ladenburg was wrong. The list above is not complete. There is a 4th isomer. What does it look like?
Model Answer
The missing fourth isomer is an enantiomer of one of the structures that Ladenburg originally suggested. So he did not notice that one of his proposed structures is chiral. [VISUAL]
Aromatic compounds are more stable than their non–aromatic counterparts. There are different ways to measure the so–called aromatic stabilization energy. The following experiment was performed to compare the stabilization energy of benzene with naphthalene:
[VISUAL]
The equilibrium constants Kb and Kn were measured for both reactions at 300 K. Calculate the free enthalpies of reaction ΔrG for both reactions.
Model Answer
The free enthalpies of reaction ΔrG can be calculated from the equilibrium constant K according to the following equation:
ΔrG = – RT ln K
For Kb = 4.9:
ΔrGb = – 8.314 J mol–1 K–1 × 300 K × ln(4.9) = –3964 J mol–1 (or –3.964 kJ mol–1)
For Kn = 970:
ΔrGn = – 8.314 J mol–1 K–1 × 300 K × ln(970) = –17154 J mol–1 (or –17.154 kJ mol–1)
Calculate the enthalpy of reaction ΔrH for each reaction assuming that for both reactions ΔS is –125 J mol–1 K–1 and the temperature is 300 K.
Model Answer
Using the relation ΔrG = ΔrH – T ΔS:
For the styrene reaction:
ΔrHb = ΔrGb + T ΔSb
ΔrHb = –3.964 kJ mol–1 + 300 K × (–0.125 kJ mol–1 K–1) = –41.464 kJ mol–1 (stated as –41.464 J mol–1 in the original solution text)
For the vinyl naphthalene reaction:
ΔrHn = ΔrGn + T ΔSn
ΔrHn = –17.154 kJ mol–1 + 300 K × (–0.125 kJ mol–1 K–1) = –54.654 kJ mol–1 (stated as –54.654 J mol–1 in the original solution text)
Why is the second reaction more exothermic than the first? Write down all resonance structures of the starting materials and products and count those having favourable benzene resonances.
Model Answer
In this Diels–Alder reaction, the aromatic π–system of styrene is completely destroyed. The product is not aromatic any more. Consequently, this loss in aromatic stabilization reduces the reaction enthalpy ΔrH. Vinyl naphthalene makes the resulting product still be aromatic, only a part of the aromatic system is destroyed. Hence, the energetic loss in this case is less than compared to styrene (naphthalene is not twice as stable as benzene) and the reaction is therefore more exothermic. [VISUAL]
What do you think are the products of the following reactions (use the same arguments)?
Br2
Br2
[VISUAL]
Fill in the structures of the reaction products.
Model Answer
Bromine is electrophilically added to the (formal) middle double bond in the phenanthrene molecule. In anthracene, it is added to the opposing carbon atoms. These products contain two aromatic benzene rings. The aromatic stabilization is hence larger than in the alternative products with naphthalene rings. [VISUAL]