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Section isomers of benzene took several decades after the initial discovery of benzene before people understood that its molecular formula was CgH6 Once this was known, the next problem was working out the arrangement of the atorns Several different possible structures were suggested. H H In one paper on benzene, British scientist James Dewar suggested constructing models using thin strips of brass to show the arrangement of bonds, and black discs to represent the carbon atoms. His representation of the correct structure of benzenei is shown on the right. H (c) Draw skeletal structural formula for each of the three isomers represented by the models A B. and shown below. H. H H H X H H HF H H H B A y H H H c Page One of the key features of benzene is that allthe C-C bond lengths are exactly the same with all the carbon atorns exactly the same environment. This is instantly revealed using the modern technique of 13C nuclear magnetic resonance spectroscopy, or 13C NMR. The 13CNMR spectrum of benzene shows one peak since there only one unique carbon environment in contrast, in the isomers of C8H6 shown below, some of the carbon atoms are in equivalent environments, others are not. In each structure, the different carbon environments are numbered; atoms with the same number are in the same environment The number of peaks the 13C NMR spectrum corresponds to the number of different carbon environments ina given molecule 1 2 1 1 3 3 2 1 just one peak the three peaks in the 13C NMR spectrum 13C NMR spectrum 1 4 1 4 four peaks the 13C NMR spectrum Section III historic structures benzene Dewar benzene Ladenburg benzene or 'prismane The correct structure of benzene and two other structures which were considered are shown above So- called Dewar benzene is 252 kJ moF┬╣ higher in energy than normal benzene (both species in the gas phase). Prismane is even higher in energy and explosive (e) Use the thermodynamic data given to calculate: (i) the standard enthalpy change of formation of benzene (g) (ii) the standard enthalpy change of combustion, AcHf, of Dewar benzene (g) value mol A,H benzene (g) -3301 ArH CO2(g) -393.5 AFH H2O (I) -285.8

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