Arenes

The story of benzene Write the electron configuration of Carbon. How many electrons are there in its outer shell? How many covalent bonds should carbon form?

A new hydrocarbon isolated by Michael Faraday in 1825 The empirical formula is CH and the molecular formula is C 6 H 6 suggesting that the molecule contained a large number of double bonds. Carbon = 92% Carbon (Atomic mass = 12) Hydrogen = ? % (Atomic mass = 1) Relative molecular mass = 78. Calculate the empirical formula then the molecular formula CH % % divided by gfm Divide by smallest Ratio Molecular formula Known RMM / RMM of empirical formula 78 / (12 + 1) = = 8 92 / 12 = / 1 = / 7.67 = 18 / 7.67 =

In 1865 after a dream about a snake biting its own tale, Kekulé suggested the following structure for benzene. 2 minute challenge: Draw as many possible structures for a hydrocarbon with the formula C 6 H 6 Make sure the structures satisfy the requirements of 4 bonds per carbon and 1 bond per hydrogen! Does this structure meet all the requirements of benzene?

Chemists at the time were convinced that benzene (like other alkenes) should react with bromine in the dark at room temperature = electrophilic addition. Problem 1: Lack of reactivity of benzene Observation: This did NOT happen with benzene. It only reacts by electrophilic substitution Conclusion: Benzene is not a normal alkene. Bromine water (brown) + alkene → Bromoalkane solution (colourless)

Problem 2: Only one isomer of 1,2- dibromobenzene Observation: This did NOT happen with benzene. Conclusion: Benzene has not alternating single and double bonds.

Bond Lengths/nm C-C cyclohexane0.154 C=C cyclohexane0.134 Problem 3: Bond lengths of benzene Compare the length of a single bond to a double bond Draw what benzene would look like if the different length double and single bonds are alternating to form a 6 member ring. Clue: Its not a perfect hexagon

What benzene would look like if it had fixed alternating double-single bonds… Different bond lengths in benzene would cause distortion Q. Who is this celebrity? A. Simon Cowell

Bond Lengths/nm C-C cyclohexane C=C cyclohexane0.134 C-C in benzene0.140 All the bonds in benzene have the same length... How does the carbon-carbon bond length of benzene compare to double and single carbon-carbon bonds? 1. All bonds are of equal length 2. 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! The resonance explanation suggests that Benzene is in such rapid equilibrium between the two forms; we detect a ‘blurred’ combination of the two forms. The electrons from the double bonds are therefore drawn as a circle shared equally between the carbon atoms. Resonance suggests the two structures rapidly alternate between the two forms.

Enthalpy of hydrogenation (addition of hydrogen) to cyclohexene was found to be -119kJmol -1 Predict the enthalpy of hydrogenation of benzene Problem 4: Thermodynamic stability of benzene

Since three double bonds are present in benzene, then the comparable reaction should liberate 3 times the energy of cyclohexene = -(3 × 119) = kJ mol -1 Is Benzene more or less stable than expected? Benzene is ( ) = 149 kJ mol -1 more stable than otherwise expected, or if it contained 3 ordinary C=C bonds. Theory verses experiment Experimentally determined value for the hydrogenation of benzene ΔH (hydrogenation) = -208kJ mol -1

-357kJ/mol (3 X –120) -208kJ/mol E progress Benzene (actual value) Structure ??? Benzene (predicted value) Difference = 149kJ/mol Cyclohexane

Current theory: Modern organic chemists use the word “delocalized π electron cloud” to describe how the electrons are spread across the whole molecule… The spreading of electrons STABILIZES the molecule: it lowers the internal energy of the molecule. The bonding in benzene is the result of the overlapping of the p-orbitals which forms a cloud of electron density above and below the molecule ( two donut- shaped rings )

1981 an atomic surface probing technique was developed called Scanning Tunneling Microscopy (STM). The first published STM image showed benzene with an undistorted hexagonal shape. Direct evidence

. Which drawing most accurately represents the structure and bonding in Benzene? Explain Are any of the pictures completely incorrect? (i.e. not Benzene) if so which and why? Which representation takes longest to draw? Which is quickest to draw? In your opinion which picture is best overall? Why? Drawing benzene 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

1.Describe the 3 problems that scientists faced when trying to explain the structure 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.What is the role of theory and experimentation in the advance of scientific knowledge? Use the story of benzene to support your answer. Questions

= electrophilic substitution The Nitration of benzene C 6 H 6 + HNO 3 C 6 H 5 NO 2 + H 2 O Benzene nitric acid = nitrobenzene Conc. H 2 SO 4

The Reduction of nitrobenzene = the path to phenylamine You cannot just put a NH 2 - group directly onto benzene by electrophilic substitution: phenylamine has to be synthesised in a 3 step process= synthetic route. Nitration Reduction 2 steps

Practice : Benzene + HNO 3  Nitrobenzene is reduced in acidic solution  Phenylammonium ion + NaOH 