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If something is more exothermic why is it less stable?

Discussion in 'Science' started by msb1983, Dec 13, 2011.

  1. I'm refering to benzene being more stable than a cyclic structure with 3 double bonds

    Can anyone help please
     
  2. I'm refering to benzene being more stable than a cyclic structure with 3 double bonds

    Can anyone help please
     
  3. phlogiston

    phlogiston Star commenter

    Describing something as being "more exothermic" is a bit vague.
    If we were to be able to make cylohexatriene - we would expect an exothermic reaction as it converts to benzene (with the delocalised structure). This means that the bonds are now stronger, and energy has been released to the surroundings.
    Alternatively, if we set fire to benzene we get some energy released (-3273kJ/mol according to Wikipedia). If we set fire to the triene, we get more than that. It's bonds are weaker and more energy is released when the C=O and O-H bonds are formed.
    Best wishes
    P
     
  4. Ssn77

    Ssn77 New commenter

    When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

    When cyclohexene (one C=C bond) is reduced to cyclohexane, 120 kJ of energy is released per mole.



    If benzene contained three separate C=C bonds, you would then expect it to release 360 kJ per mole when reduced to cyclohexane.


    Actual benzene releases only 208 kJ per mole when reduced.


    It is 152 kJ per mole more stable than expected. This value is known as the resonance energy, and comes about from the delocalisation of electrons (in p orbitals) in the benzene ring.
     

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