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What is the problem with the Kekule structure?

Kekule had an idea of assigning that certain atoms are applied to certain positions within the molecule structure and hence it does connect to the each other and it was thus called affinity units and also valence or Bonds. They were largely based on some evidence from certain chemical reactions and not only on instrumental methods that could see the molecule directly such as X ray crystallography. Some physical methods of determining the structures were not developed and so chemists of earlier days had to rely on early chemical experiments. Some chemists heavily criticised the use of formulas to determine their structure and thus most of the Chemists followed Kekule lead in thinking and going forward with calling the Classical structural theory and these were modified later after electron discovery and Quantum mechanisms. These were pivotal in determine the current chemical structures for molecules since all the experiments were based on some assumptions and theories pertaining to that time.

Here we have an idea that the number of valencies of an element was not variant and it was a key component of Kekule structure for the Chemistry. This particular generalisation did suffer from many exceptions and was replaced by the fact that valencies did have fixed oxidation states and for example in the periodic acid according to Kekulean structure had chain of I-O-O-O-O-H. However by contrast the modern structure was that the periodic acid has all the four oxygen atoms that surrounds the iodine in a geometrical tetra hedral structure in the field of Chemistry.

Benzene Structure

Kekulé structure of benzene with alternating double bonds –

  1. This is the most famous structure from Kekule. It was in the year 1865 and suggested that the Benzene structure has a six member ring of Carbon atoms with single and double bonds which are alternating. The structural formula of benzene is C6H6.
  • He published couple of papers to prove that. The Empirical formula for Benzene has been known but it is highly unsaturated and this was not easily proven. In 1861 it was told that the structure contained multiple bonds or rings to say but its Study of the aromatic compounds in the earliest years. The Chemists could not prove any particular structure.
  • Kekule had mentioned that in 1890 of a vision he had of the atoms and molecules which are dancing and which led to the theory of the structure as described by him and it is anecdotal as it reflects the accurate memory of a real event and circumstances led him to develop the theory.
  • The Benzene structure became so popular that it proved the aromatic compounds properties and hence the German Chemical Society organised the Kekule honour and it celebrated the twenty   fifth anniversary for it and it was for his benzene paper.
  • Kekule did argue that for his proposed structure that the isomers number was a derivative of Benzene. For every mono derivative of Benzene only one isomer is found and all six carbons are same so that the carbon substituent gives only one possible product. For some derivatives we get some isomers and for which Kekule proposed some structures and it had two carbon atoms which were separated by one, two and three carbon bonds and they are named as ortho, meta, para isomers respectively.
  • The counting was however criticised by some chemist and also by Albert Ladenburg who was a student of Kekule and argued that two distinct oath structures means the substituted carbons are separated by single to double bonds.
  • Kekule suggested that Benzene molecules will oscillate between tow structures of same type and single bond and double bond will change positions continuously. This implies that all the six carbon carbon bonds are same and each of the single is half the time and double half the time. A theoretical basis for this was later also proposed in 1928 By Linus Pauling who replaced the assumption of Kekule by the concept of resonance between quantum structures.

PROBLEM WITH THE BENZENE STRUCTURE:

Here the main problem was that the Carbon Carbon Single and Double bonds are of different lengths. Which means the Hexagons would be of irregular shape and structure if they followed the Kekule structure and it would have alternating shorter and longer sides.

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