Aromatic Chemistry
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What is an arene? (arom. hydroc. - w/ b. rings)
An aromatic hydrocarbon containing one or more benzene rings
What is the molecular formula of benzene?
C6H6
Describe the basic structure of benzene (6C - hex. ring - 1H - 1C)
Six carbon atoms in a hexagonal ring, with one hydrogen atom bonded to each carbon
The benzene molecule is p______?
planar
What is the actual C-C bond length in benzene?
An intermediate between a single and double bond
How is each carbon atom bonded in benzene? (ech. C bndd. 2C + 1H by sngle. cov. sgm. bnd.)
Each carbon is bonded to two other carbons and one hydrogen by single covalent sigma bond
What happens to the unused electron on each carbon atom? (occ. - p. orb. perp. - plane of ring)
It occupies a p orbital perpendicular to the plane of the ring
How are π (pi) bonds formed in benzene?
Sideways overlap of p orbitals on adjacent carbon atoms
Describe the delocalisation of electrons in the benzene structure (six π e- - shared ovr. 6C - ctng. ring neg. chrg. - abv/blw - plne ring)
There are six π electrons that are shared over all six carbon atoms, creating a ring of negative charge above and below the plane of the ring
Draw benzene’s structure skeletally
Hexagon with a circle inside
What was Kekulé’s key contribution to benzene’s structure? (1st sugg. - b. ring struc. 6C - 1C to 1H)
He was the first to suggest benzene had a ring structure of six carbon atoms, each bonded to one hydrogen atom
What name would Kekulé’s structure give benzene? Draw it too
Cyclohexa-1,3,5-triene
What mistake did Kekulé make about benzene’s bonding? (thght. B cntnd. 3C=C and 3C-C)
He thought benzene contained three C=C double bonds and three C-C single bonds
What are three pieces of evidence that support the delocalised structure rather than the Kekulé’s “triene” structure? • C-C bnd. lng. • bhvr. drng. add. rctns. • vle. enthlpy. hydrogntn.
• The C-C bond length • Behaviour during addition reactions • The value of enthalpy of hydrogenation
Why does bond length evidence disprove Kekulé’s structure? (strctr. prdcts. = alt. bond lngt. - not obsve. - C-C sme.)
His structure predicts alternating bond lengths (some long and short), however this is not observed, all the C-C bonds are the same
How does benzene’s C-C bond length compare to single and double bonds? (intermediate)
Benzene has a C-C bond length intermediate between a C-C single bond and C=C double bond
How do addition reactions provide evidence to disprove Kekulé’s structure? /rdly. undrg. add rctns. - dsnt. dclrs. brme. wtr. - exp. if tri.)
Benzene does not readily undergo addition reactions, so it does not decolourise bromine water, which would be expected if it was triene
What enthalpy change would be expected for a triene, based on the enthalpy of hydrogenation of cyclohexene?
–360 kJ mol⁻¹ (3 × –120 kJ mol⁻¹)
What is the actual enthalpy of hydrogenation of benzene?
–208 kJ mol⁻¹
How does benzene’s enthalpy of hydrogenation provide evidence to disprove Kekul’s structure? (B only/ rlss. -208 instd. -360 shwng. B 152 >stble. thn. trne.)
Benzene only releases –208 kJ mol⁻¹ instead of -360 kJ mol⁻¹, showing that benzene is 152kJmol⁻¹ more stable than triene
What does benzene’s enthalpy of hydrogenation show about its stability? (shws. B +stab. - due delo. e-. knwn deloc. stab.)
Shows benzene has extra stability due to the delocalisation of electrons, known as delocalisation stability
What causes delocalisation stability in benzenes? (dlcls. pi. elctrns. sprd. ovr. whle. rng.)
The delocalised π electrons that spread over the whole ring