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Structure of Benzene
Transcript of Structure of Benzene
-empirical formula: CH -RFM: 78; molecular formula: C6H6
-distilled from benzoic acid and calcium oxide
-named it benzin 1845 - Charles Mansfield Many came up with possible structures, but were proved wrong by the way it reacted -first large scale industrial production
-produced from coal tar E
N 8/10: First isolation is important as it allows the chemical to be tested without other chemicals voiding the results. This allows the functional groups to be identified - and from that, some of the bonds and atoms present. A mass spectra can also be taken to determine the molecular formula and fragmentation patterns, which could shed more light on the structure.
Already from the empirical formula of CH, a number of hydrocarbons can be eliminated. It also leads to the possibility of double and triple bonds. 9/10: The relative formula mass and the empirical formula allow the allow the molecular formula to be derived. This can then be used to create some ideas of the structure of benzene. eg. Although these were incorrect, structures that benzene was not were eliminated. 3/10: Important in an industrial and economical sense, but not in terms of identifying its structure. KEKULÉ'S MODEL 1865 Kekulé believed that benzene was a hexagonal hydrocarbon, made up of alternating double and single bonds. He believed that the bonds switched between two isomers. Kekulé belived the single C-C bond was 147pm and the double C=C bond was 135pm.
As he believed the bonds were not all equal, the hexagon was thought to be irregular but this is not the case. DELOCALISED MODEL OF BENZENE The delocalisted model of benzene has now replaced Kekulé's model for a variety of reasons: Although incorrect, Kekulé's structure is still used in skeletal drawings. Bond Length Kathleen Lonsdale used x-ray crystallography on benzene crystals to discover that benzene was planar and was a regular hexagon. This meant all the bonds had to be the same length and disproves Kekulé's model of alternating bonds of different lengths.
The actual bond length was 140pm, between the lengths of double (135pm) and single (147pm) bonds. Bond Enthalpy When cyclohexene is hydogenated, the enthalpy change is -120kJ/mol. Kekulé's model has three double bonds, so the enthalpy change when benzene is hydrogenated is expected to be -360kJ/mol. However, this is not the case. The actual enthalpy of reaction is -208kJ/mol - a lot less than the expected value. This is means that more energy has been put into the breaking the bonds than expected, making the actual structure of benzene more stable than the model.
Benzene is more stable because the delocalised electrons are evenly spread out. Reactivity The most common form of reaction expected by an alkene is an addition reaction. As benzene supposedly has three double bonds it is expected to participate in electrophilic addition reactions. However, this is not the case, benzene rarely partakes in addition reactions and usually is involved in electrophilic substiution, as there is a region of high electron density above and below the plane.
Benzene wants to preseve the stability of its pi system, so it exchanges the hydrogen rather than breaking the pi system. In benzene, one of the electrons in the 2s orbital in carbon is promoted to the empty 2p orbital, giving 4 half filled orbitals. The s-orbital and two of the p-orbitals hybridise giving three sp2 orbitals. These form sigma bonds with the two adjourning carbons and hydrogen. The remaining p-orbitals overlap to form a pi-system, which consists of delocalised electrons shared equally between the carbon atoms.