The story of benzeneWrite the electron configuration of Carbon. How many electrons are there in Write the electron configuration of Carbon. How many electrons are there inits outer shell? How many covalent bonds should carbon form? its outer shell? How many covalent bonds should carbon form?
A new hydrocarbon isolated by Michael Faraday in 1825 Carbon = 92% Carbon (Atomic mass = 12) Hydrogen = ? % (Atomic mass = 1) Relative molecular mass = 78. Calculate the empirical formula then the molecular formula C H Molecular formula% 92 100-92 = 8 Known RMM / RMM of% divided by RAM 92 / 12 = 7.67 8/1=8 empirical formulaDivide by smallest 7.67 / 7.67 = 1 8 / 7.67 = 1.04 78 / (12 + 1) = 6Ratio 1 1The empirical formula is CH and the molecular formula is C 6H66suggesting that the The empirical formula is CH and the molecular formula is C 6H suggesting that themolecule contained aalarge number of double bonds. molecule contained large number of double bonds.
2 minute challenge: Draw as many possible structures for a hydrocarbon with the formula C6H6 Make sure the structures satisfy the requirements of 4 bonds per carbon and 1 bond per hydrogen!In 1865 after a dream about a snake biting its own tale, Kekulé suggested thefollowing structure for benzene. Does this structure meet all the requirements of benzene?
Problem 1: Lack of reactivity of benzeneChemists at the time were convinced that benzene (like other alkenes) should reactwith bromine in the dark at room temperature. Bromine water (brown) + alkene → Bromoalkane solution (colourless) Observation: This did NOT happen with benzene. Conclusion: Benzene is not a normal alkene
Problem 2: Thermodynamic stability of benzeneEnthalpy of hydrogenation (addition of hydrogen) to cyclohexenewas found to be -119kJmol-1 Predict the enthalpy of hydrogenation of benzene Predict the enthalpy of hydrogenation of benzene
Theory verses experimentSince three double bonds are present in benzene, then the comparable reactionshould liberate 3 times the energy of cyclohexene= -(3 × 119) = - 357 kJ mol-1 Experimentally determined value for the hydrogenation of benzene ΔH (hydrogenation) = -208kJ mol-1Is Benzene more or less stable than expected?Benzene is (360-208) = 149 kJ mol-1 more stable than otherwise expected, or if itcontained 3 ordinary C=C bonds.
E Benzene (predicted value) Difference = 149kJ/mol -357kJ/mol (3 Benzene (actual value) X –120) Structure ??? Cyclohexane -208kJ/mol progress
Problem 3: Bond lengths of benzene Bond Lengths /nm C-C cyclohexane 0.154 C=C cyclohexane 0.134•Compare the length of a single bond to a double bond•Draw what benzene would look like if the different length double andsingle bonds are alternating to form a 6 member ring.Clue: Its not a perfect hexagon
What benzene would look like if it had fixedalternating double-single bonds… Different bond lengths in benzene would cause distortion Q. Who is this celebrity? A. Simon Cowell
Direct evidence1981 an atomic surface probing technique was developed called ScanningTunneling Microscopy (STM). The first published STM image showed benzenewith an undistorted hexagonal shape. www.newton.ex.ac.uk
All the bonds in benzene have the same length... Bond Lengths /nm C-C cyclohexane 0.154 C=C cyclohexane 0.134 C-C in benzene 0.140•How does the carbon-carbon bond length of benzene compare todouble and single carbon-carbon bonds?1. All bonds are of equal length2. The bond length is between a double bond and a single bond.• What do these two facts suggest about the structure of benzene?
The double-single bonds can’t be fixed in position!Resonance suggests the two structures rapidly alternate between the two forms.The resonance explanation suggests that Benzene is in such rapid equilibriumbetween the two forms; we detect a ‘blurred’ combination of the two forms.The electrons from the double bonds are therefore drawn as a circle sharedequally between the carbon atoms.
Current theory:•Some text books still use the word “resonance” to describe the structure ofbenzene.•There is NO evidence to support two rapidly changing forms of benzene.•Modern organic chemists use the word “conjugation” to describe how theelectrons are delocalized (spread) across the whole molecule… Valence bond theory explains the bonding in benzene as a series of unhybridized p-orbitals which overlap forming a cloud of electron density above and below the molecule.The spreading of electrons STABILIZES the molecule.
Drawing benzeneWhich drawing most accurately representsthe structure and bonding in Benzene?ExplainAre any of the pictures completelyincorrect? (i.e. not Benzene) if so whichand why?Which representation takes longest todraw? Which is quickest to draw?In your opinion which picture is bestoverall? Why? •D is incorrect (cyclohexane not benzene!) •A doesn’t show delocalized electrons and is time consuming to draw •Organic chemists tend to use B or C if they want to draw mechanisms
Questions1.Describe the 3 problems that scientists faced when trying to explain thestructure and bonding in Benzene.2.What is the current understanding of the structure and bonding of Benzene?(answer as if you were talking to a chemist who had never heard of Benzene)3.How does this current knowledge Q2 better explain the problems youmentioned in Q1?4.“We now fully understand benzene” Do you agree or disagree with this claim?Explain your answer.5.What is the role of theory and experimentation in the advance of scientificknowledge? Use the story of benzene to support your answer.