Benzene and Aromatic Compounds Chem-160 4/6/2019 D rSeemal Jelani

The Structure of Benzene In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å). 4/6/2019 D rSeemal Jelani

Interesting Aromatic Compounds Benzene and toluene, the simplest aromatic hydrocarbons obtained from petroleum refining, are useful starting materials for synthetic polymers. They are also two of the components of the BTX mixture added to gasoline to boost octane ratings. Compounds containing two or more benzene rings that share carbon—carbon bonds are called polycyclic aromatic hydrocarbons (PAHs). Naphthalene, the simplest PAH, is the active ingredient in mothballs. 4/6/2019 D rSeemal Jelani

KMnO4 oxidation no reaction Br2/CCl4 addition HI H2/Ni reduction Reagent Cyclohexene Benzene KMnO4 oxidation no reaction Br2/CCl4 addition HI H2/Ni reduction 4/6/2019 D rSeemal Jelani

Benzene + 3 H2, Ni, room temp.  NR Benzene + 3 H2, Ni, 200oC, 1500 psi  cyclohexane Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead. 4/6/2019 D rSeemal Jelani

Freidel-Crafts alkylation C6H6 + RX, AlCl3  C6H5R + HX substitutions Reactions of benzene: Nitration C6H6 + HNO3, H2SO4  C6H5NO2 + H2O Sulfonation C6H6 + H2SO4, SO3  C6H5SO3H + H2O Halogenation C6H6 + X2, Fe  C6H5X + HX Freidel-Crafts alkylation C6H6 + RX, AlCl3  C6H5R + HX substitutions 4/6/2019 D rSeemal Jelani

The bonds in benzene are all equal and 1.39 Å e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds. C—C single bonds  1.50 Å C = C double bonds  1.34 Å The bonds in benzene are all equal and 1.39 Å but 1,3,5-cyclohexatriene has three double bonds and three single bonds! 4/6/2019 D rSeemal Jelani

Key Ideas on Benzene Unusually stable heat of hydrogenation 150 kJ/mol lower than a cyclic triene Planar hexagon: bond angles are 120° carbon–carbon bond lengths 139 pm Undergoes substitution not addition Resonance hybrid One more important factor is the number of electrons in the cyclic orbital 4/6/2019 D rSeemal Jelani

Aromaticity E Huckel (1931) Aromaticity is a property of certain molecules Chemistry would be similar to benzene Meet the following criteria Planar Mono cyclic system Conjugated pi system Contains 4n + 2  electrons Can apply rules to variety of compounds and determine aromatic nature. 4/6/2019 D rSeemal Jelani

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4/6/2019 D rSeemal Jelani

Aromaticity and the 4n + 2 Rule Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2  electrons (n is 0,1,2,3,4) For n=1: 4n+2 = 6 benzene is stable and the electrons are delocalized 4/6/2019 D rSeemal Jelani

Number of π electrons = 4 n + 2 6 = 4 n + 2 n = 1 For example, the benzene molecule, which has 3 π bonds or 6 π electrons, is aromatic. Number of π electrons = 4 n + 2 6 = 4 n + 2 n = 1 4/6/2019 D rSeemal Jelani

For n = 0, we get (4 × 0 + 2) = 2 For n = 1, we get (4 × 1 + 2) = 6 4/6/2019 D rSeemal Jelani

The letter “n” is not a characteristic of the molecule! Molecules that have the 3 characteristics listed above (cyclic, conjugated, flat) and have this number of π electrons [4n +2] will be aromatic. The letter “n” is not a characteristic of the molecule! 4/6/2019 D rSeemal Jelani