ARENES

1. ARENES

2. Part 6 Aromatic chemistry • Bonding • understand the nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double. • Delocalisation stability • should understand that delocalisation confers stability to the molecule, and be able to use thermochemical evidence from enthalpies of hydrogenation to illustrate this principle. • Electrophilic substitution • understand that electrophilic attack in arenes results in substitution (mechanisms limited to the monosubstitutions given below).

3. Part 6 Aromatic chemistry • Nitration •   understand that nitration is an important step in synthesis (e.g. explosive manufacture and formation of amines from which dyestuffs are manufactured). • understand the mechanism of nitration, including the generation of the nitronium ion. • Friedel–Crafts reactions •  understand that Friedel–Crafts alkylation and acylation reactions are important steps in synthesis. • understand the mechanism of alkylation and acylation using AlCl3 as catalyst. • know that industrially ethylbenzene is manufactured from benzene and ethene using HCl/AlCl3. • know that ethylbenzene is an important intermediate in the manufacture of polystyrene (details of processes not required).

4. STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of C1H1and a molecular mass of 78 and a molecular formula of C6H6 Kekulésuggested that benzene was... PLANAR CYCLICand HADALTERNATING DOUBLE AND SINGLE BONDS

5. STRUCTURE OF BENZENE HOWEVER... • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed X Y X Y X and Y on carbons joined by a single bond X and Y on carbons joined by a double bond So the Kekuléformula is at odds with the evidence!

6. STRUCTURE OF BENZENE HOWEVER... • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed • all six C—C bond lengths were similar; C=C bonds are shorter than C-C • the ring was thermodynamically more stable than expected

7. STRUCTURE OF BENZENE • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed • all six C—C bond lengths were similar; C=C bonds are shorter than C-C • the ring was thermodynamically more stable than expected Suggested that the structure oscillated between the two Kekulé forms but was represented by neither of them. It was a RESONANCE HYBRID.

8. THERMODYNAMIC EVIDENCE FOR STABILITY Reducing unsaturated hydrocarbons to the corresponding saturated compound, releases energy. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g)  C6H12(l) - 120 kJ mol-1 - 120 kJ mol-1

9. THERMODYNAMIC EVIDENCE FOR STABILITY Reducing unsaturated hydrocarbons to the corresponding saturated compound, releases energy. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g)  C6H12(l) Theoretically, if benzene contained three separate C=C bonds it would release 3x(120) =360kJ per mole when reduced to cyclohexane C6H6(l) + 3H2(g)  C6H12(l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale It is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY. MORE STABLE THAN EXPECTED by 152 kJ mol-1 - 120 kJ mol-1 Theoretical - 360 kJ mol-1 (3 x -120) - 360 kJ mol-1 Experimental - 208 kJ mol-1 - 120 kJ mol-1

10. 2p 2 2s 1 1s HYBRIDISATION OF ORBITALS The electronic configuration of a carbon atom is 1s22s22p2

11. 2p 2p 2 2 2s 2s 1 1 1s 1s HYBRIDISATION OF ORBITALS The electronic configuration of a carbon atom is 1s22s22p2 A bit of energy you can promote one of the s electrons into a p orbital. The configuration is now 1s22s12p3 The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

12. HYBRIDISATION OF ORBITALS The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent. 2s22p2 2s12p3 4 x sp3 HYBRIDISE sp3HYBRIDISATION

13. HYBRIDISATION OF ORBITALS Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged. 2s22p2 2s12p3 3 x sp2 2p HYBRIDISE sp2HYBRIDISATION

14. STRUCTURE OF ALKENES In ALKANES, the four sp3orbitals repel each other into a tetrahedral arrangement. In ALKENES, the three sp2orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them

15. STRUCTURE OF ALKENES Covalent bonds are formed by overlap of orbitals. An sp2 orbital from each carbon overlaps to form a single C-C bond. The resulting bond is called a SIGMA (δ) bond.

16. STRUCTURE OF ALKENES The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond. For maximum overlap and hence the strongest bond, the 2p orbitals are in line. This gives rise to the planar arrangement around C=C bonds.

17. STRUCTURE OF ALKENES two sp2 orbitals overlap to form a sigma bond between the two carbon atoms two 2p orbitals overlap to form a pi bond between the two carbon atoms s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds the resulting shape is planar with bond angles of 120º

18. STRUCTURE OF BENZENE The theory suggests that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planarstructure. 6 single bonds one way to overlap adjacent p orbitals another possibility delocalised pi orbital system This final structure was particularly stable and resisted attempts to break it down through normal electrophilic addition. However, substitution of any hydrogen atoms would not affect the delocalisation.

19. STRUCTURE OF BENZENE

20. STRUCTURE OF BENZENE

21. ELECTROPHILIC ATTACK ON THE RING The high electron density of the ring makes it open to attack by electrophiles but... The mechanism involves an initial disruption to the ring, so... ... electrophiles will have to be more powerful than those which react with alkenes. A fully delocalised ring is stable so will resist attack.

22. ELECTROPHILIC SUBSTITUTION Addition to the ring would upset the delocalised electron system STABLE DELOCALISED SYSTEM ELECTRONS ARE NOT DELOCALISED AROUND THE WHOLE RING - LESS STABLE Substitution of hydrogen atoms on the ring does not affect the delocalisation Overall there isELECTROPHILIC SUBSTITUTION

23. ELECTROPHILIC SUBSTITUTION Summary: • High electron density of the ring makes it open to attack by electrophiles • Addition to the ring would upset the delocalised electron system • Substitution of hydrogen atoms on the ring does not affect the delocalisation • Because the mechanism involves an initial disruption to the ring electrophiles must be more powerful than those which react with alkenes Overall there is ELECTROPHILIC SUBSTITUTION

24. ELECTROPHILIC SUBSTITUTION Mechanism • a pair of electrons leaves the delocalised system to form a bond to the electrophile • this disrupts the stable delocalised system and forms an unstable intermediate • to restore stability, the pair of electrons in the C-H bond moves back into the ring • overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION

25. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION conc. nitric acid and conc. sulphuric acid(catalyst) Reagents Conditions Equation Mechanism Electrophile Use reflux at 55°C C6H6 + HNO3 C6H5NO2 + H2O nitrobenzene NO2+ ,nitronium ionor nitryl cation; it is generated in an acid-base reaction 2H2SO4 + HNO3Ý 2HSO4¯ + H3O+ + NO2+ acid base Nitration of benzene is the first step in a chain of reactions that lead to the formation of dyes, and explosives.

26. ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION Reagents Conditions Mechanism Electrophile chlorine and a halogen carrier(catalyst) reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3) chlorine is non polar so is not a good electrophile the halogen carrier is required to polarise the halogen C6H6 + Cl2 C6H5Cl + HCl chlorobenzene Cl+it is generated as follows... Cl2 + FeCl3Ý FeCl4¯ + Cl+ Lewis acid

27. ELECTROPHILIC SUBSTITUTION REACTIONS – FRIEDEL CRAFTS ALKYLATION Overview Reagents Conditions Mechanism Electrophile General alkylation involves substituting an alkyl (methyl, ethyl) group a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3 room temperature; dry inert solvent (ether) C6H6 + C2H5Cl  C6H5C2H5 + HCl ethylbenzene a carbocation ion R+ (e.g. CH3+) C2H5Cl + AlCl3Ý AlCl4¯ + C2H5+ A catalyst is used to increase the positive nature of the Electrophile and make it better at attacking benzene rings. AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.

28. FRIEDEL-CRAFTS REACTIONS - INDUSTRIALALKYLATION IndustrialAlkenes are used instead of haloalkanes but an acid must be present Phenylethane, C6H5C2H5 is made by this method Reagentsethene, anhydrous AlCl3 , conc. HCl ElectrophileC2H5+ (an ethyl carbonium ion) EquationC6H6 + C2H4 ———> C6H5C2H5 (ethyl benzene) Mechanismthe HCl reacts with the alkene to generate a carbonium ion electrophilic substitution then takes place as the C2H5+ attacks the ring Useethyl benzene is dehydrogenated to produce phenylethene (styrene); this is used to make poly(phenylethene) - also known as polystyrene

29. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATION Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) group Reagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3 Conditionsreflux 50°C; dry inert solvent (ether) ElectrophileRC+= O ( e.g. CH3C+O ) Equation C6H6 + CH3COCl ———> C6H5COCH3 + HCl Mechanism ProductA carbonyl compound (aldehyde or ketone)

30. STRUCTURAL ISOMERISM RELATIVE POSITIONS ON A BENZENE RING 1,2-DICHLOROBENZENE ortho dichlorobenzene 1,3-DICHLOROBENZENE meta dichlorobenzene 1,4-DICHLOROBENZENE paradichlorobenzene Compounds have similar chemical properties but different physical properties