Aromatic Chemistry - A Level Chemistry PDF

A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY STRUCTURE & BONDING IN BENZENE Benzene has the formula C6H6, with the 6 carbons arranged in a cyclical structure. H H C H C C...

A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY STRUCTURE & BONDING IN BENZENE Benzene has the formula C6H6, with the 6 carbons arranged in a cyclical structure. H H C H C C However, the skeletal structure is more often C C seen and used… H C H H Given that we now that, in organic molecules, each carbon atom must have 4 bonds around it, this structure seems odd, as each carbon atom only has 3 bonds around it! Also, what’s with the circle in the middle? Each carbon uses 3 electrons to form 3 single covalent bonds: - 2 x C-C bonds - 1 x C-H bond All three are sigma (𝛔) bonds. Very stable. Very strong. The 4th, unpaired, outer electron in each carbon atom in the ring occupies a ‘p’ orbital. Since the carbon atoms are arranged in a ring structure, these p orbitals overlap with each other both above and below the carbon ring! Remember, ‘p’ orbitals are 3D figure of 8’s. Like two balloons end to end! C C C C—C—C—C C—C—C—C C C C From the side, showing from above from the side that the ‘p’ orbitals merge All 6 ‘p’ orbitals overlap creating 6 pi (∏) bonds, that means the 6 electrons (1 from each C atom) can jump between them. Hence they are DELOCALISED and form what is known as the “Pi ring structure”. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY Benzene is a perfect PLANAR hexagonal structure. This is because: 1. All C-C bonds are the same length. Shorter than a regular C-C covalent bond due to the ‘p’ orbital overlap, but longer than a C=C double bond. 2. The bond angle around each C atom is 120o (Trigonal planar) If asked in a exam to describe the bonding & shape of benzene, here’s your model answer: Molecular Formula = C6H6 Each C has 4 outer (valence) electrons. 3 are used to form 3 covalent (sigma) bonds with two neighbouring C atoms and a H. Each 4th electron occupies a ‘p’ orbital which overlaps with neighbouring p orbitals to form a delocalised pi system of 6 electrons. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY STABILITY OF BENZENE Previously, Kekulé proposed that the structure of benzene could have alternating single and double bonds. This has since been proven to be incorrect due to the following: 1. Benzene is a perfect hexagonal structure. So all the C- C bond lengths must be the same length. C-C bonds are longer than C=C bonds, so Kekulé’s structure would actually be a warped hexagon. 2. Kekulé’s model contains C=C bonds, which you would expect to undergo electrophilic addition. It would decolourise Br2(aq). However, in practise, benzene does NOT do this. It undergoes electrophilic substitution reactions. 3. Benzene is more thermodynamically stable than the Kekulé model Kekulé + 3H2 ΔH = -360 kJ.mol-1 Benzene + 3H2 ΔH = -208 kJ.mol-1 As benzene’s ΔH is less negative, it shows that benzene has greater stability than the other model. How To Calculate the Both reactions require 6 new C-H bonds to form. ΔH Stability of Benzene = -2490 kJ.mol-1. Kekulé: -360 + 2490 = +2130 kJ.mol-1 Benzene: -208 + 2490 = +2282 kJ.mol-1 The value in bold is the amount of energy required to break bonds in the molecule to accept the 6 H atoms. Benzene requires more energy to break bonds, so is therefor more stable. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY ELECTROPHILIC SUBSTITUTION As benzene is such a stable molecule, it only undergoes electrophilic substitution. Addition reactions would mean disrupting the delocalised pi system, and this requires a large amount of energy. During substitution reactions, a H atom is replaced by an electrophile. There are two reactions involving different electrophiles that you need to know. Both have the same mechanism. For each, you also need to know the equations for the formation of the electrophile and for the reforming of the catalyst. NITRATION OF BENZENE Reagent: conc. HNO3(aq) Catalyst: conc. H2SO4(aq) Forming the Electrophile: HNO3 + H2SO4 NO2+ + HSO4- + H2O (nitronium ion) Electrophilic Substitution: NO2+ NO2 NO2 + H + H+ nitrobenzene Reforming the Catalyst: HSO4- + H+ H2SO4 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY FRIEDEL-CRAFTS ACYLATION OF BENZENE Reagent: acyl chloride (R-COCl) Catalyst: AlCl3 This reaction forms a ketone group between the benzene ring and the R group. Forming the Electrophile: O O R C + AlCl3 R C + [AlCl4]- + Cl Electrophilic Substitution: O O O R C C R + C R + H + H+ phenyl____anone Reforming the Catalyst: length of R group [AlCl4]- + H+ AlCl3 HINTS | TIPS | HACKS The mechanism for both reactions is the How To Answer Electrophilic same. Substitution Questions Learn the two equations linked with each reaction. These actions also take place when there are other groups bonded to the benzene ring. Pay attention to the position of the substitution. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.10 AROMATIC CHEMISTRY PHENYLAMINE Phenylamine is essentially a benzene ring with an amine group bonded and is formed by the reduction of nitrobenzene using conc. HCl + Sn(s) catalyst, followed by excess NaOH(aq). Overall Equation: NO2 NH2 conc.HCl + 6[H] + 2H2O Sn The excess NaOH is added at the end because the amine group is a weak base and reacts with the conc. HCl to form an amine salt. NO2 NH3 + Cl- NH2 conc.HCl + NaOH Sn + NaCl As the amine group is a weak base, it exists in equilibrium. This means we can reform the amine group by adding an alkali. Random Fact to Recall! Phenylamine and other aromatic amines are used in the manufacture of dyes. AQA www.chemistrycoach.co.uk © scidekick ltd 2024