Week 2 Online Lecture Slides: Hydrocarbons, Aromaticity, Phenols & Anilines - PDF

W1 and 2 HYDROCARBONS Part 3. Aromaticity Learning objectives for Week 2 1. Draw benzene and understand its structure and unique stability. 2. Identify aromatic compounds and heterocycles and apply Hückel’s rule. 3. Identify the structure of phenols and anilines 4. Draw the resonance structures...

W1 and 2 HYDROCARBONS Part 3. Aromaticity Learning objectives for Week 2 1. Draw benzene and understand its structure and unique stability. 2. Identify aromatic compounds and heterocycles and apply Hückel’s rule. 3. Identify the structure of phenols and anilines 4. Draw the resonance structures of phenol. 5. Connect reactivity and pKa of phenols and anilines to resonance.  Original sample of benzene isolated by Faraday. Pre-workshop activity recap Isolated in 1825 by Michael Faraday, from distillation of “illuminating gas” residue, was an unknown material with the empirical formula C6H6 A number of possible structures for benzene were proposed in the following years all are C6H6 Claus Dewar Ladenburg 1867 1867 1869 Benzene – alternating single and double bonds? In 1865, Kekulé proposed a structure with three C−C single bonds and 3 C−C double bonds. Benzene – trouble with the model In 1865, Kekulé proposed a structure with three C−C single bonds and 3 C−C double bonds. BUT Kekulé’s structure does not fit the thermochemical evidence from hydrogenation studies. H2 ΔH° = −120 kJ mol -1 (experimental) three double bonds! Benzene is H2 more stable than predicted ΔH° = −360 kJ mol -1 for a simple triene! (prediction) ΔH° = −208 kJ mol -1 (experimental) Benzene – more trouble with the model In 1865, Kekulé proposed a structure with three C−C single bonds and 3 C−C double bonds. NOR does Kekulé’s structure explain how benzene reacts. Br Br2 Br Br Br2 HBr Benzene – Resonance structures In 1899, the proposal was refined with a model in which each bond is intermediate between single and double. HC CH or HC CH HC CH Benzene – a modern model This model can be refined further by considering orbital hybridisation to provide an explanation of benzene’s properties. Not isolated alkenes, but a cloud of electrons in a delocalised p-system. Determining aromaticity Benzene is not the only molecule to have stability due to such delocalisation. Molecules of this type we call aromatic. Hückel’s rules determine if a compound is aromatic: 1. Is the molecule a planar ring system with conjugated p electrons? 2. Does the number of p electrons = (4n + 2), where n is an integer? Note. The rule can be used with all molecules. Neutral, If Yes to both it is aromatic! positive or negative. Note. Analyse each ring separately. Hetero-aromatic compounds 6 π-electrons sp2 electron pair not N pyridine N involved in aromaticity. Why? O furan NH pyrrole 6 π-electrons 6 π-electrons O N H sp2 electron pair not involved in aromaticity. Summary Today we have: discussed the discovery of benzene. Introduced Hückel’s rules for determining if a molecule is aromatic. W1 and 2 HYDROCARBONS Part 4. Phenols and Anilines Learning objectives for Week 2 1. Draw benzene and understand its structure and unique stability. 2. Identify aromatic compounds and heterocycles and apply Hückel’s rule. 3. Identify the structure of phenols and anilines 4. Draw the resonance structures of phenol. 5. Connect reactivity and pKa of phenols and anilines to resonance. Phenol Phenols are commonly occurring materials with many uses. OH phenol OH O H 3CO CH3 N OH H CH3 resorcinol HO (Antibacterial used in acne treatment) capsaicin Phenol Phenol’s are strong acids. Phenol is about ~106 times more acidic than ethanol! OH O [A ][H ] Ka = [AH] H 2O H 3O = 1.02 X10-10 pK a = 9.99 AH A H [A ][H ] H 3C OH H 3C O Ka = H 2O H 3O [AH] = 1.3 X10-16 AH A H pK a = 15.9 Phenol Consider the resonance stabilisation of the phenoxide anion. δ O δ δ δ Resonance hybrid structure The negative charge has been stabilised by spreading it around. Resonance structures Resonance is a model used commonly to understand the structure and reactivity of molecules in which a single Lewis structure is inadequate. Consider the following example H 3C H 3C H 3C I O18 O18 O18 O18 I and CH3 C C C C H 3C O16 H 3C O16 H 3C O16 H 3C O16 Instead a mixture is observed. O O Notes on resonance C C H 3C O H 3C O 1) These are structures related through resonance (resonance structures) 1/2 O 2) They are hypothetical. or 3) Neither is real, a mix (the resonance hybrid) C H 3C O can be useful to explain reactivity. 1/2 Resonance structures When must we consider resonance structures? Below is a common case that will come across a few times in CHM1022. A A O O B B i.e. C C C C C C 1. Atom A has a lone pair (can be an anion or neutral). This is a resonance arrow. 2. Atoms B and C are connected One line with an arrow at each end. via a double bond. Aniline Anilines are common in the dyeing industry and have properties explainable by resonance NH2 Badische Anilin und Soda Fabrik (BASF) Largest chemical manufacturing company in the world. 120 000 employee’s worldwide. 4.7 billion euro revenue in 2018 Aniline is a weak base. Aniline is ~106 times less basic than cyclohexylamine! NH 2 NH 3 [B ][OH ] Kb = [B] H 2O OH = 4.5 X10-10 pKb = 9.36 B B OH NH 2 NH 3 [B ][OH ] Kb = [B] H 2O OH = 4.5 X10-4 pKb = 3.34 B B OH Aniline Consider the resonance structures of aniline. NH2 NH2 NH2 NH2 δ NH2 δ δ δ Resonance hybrid structure  Summary Today we have: Discussed how resonance structures can help predict reactivity. Building upon this concept we have looked at the acidity of phenol. Building upon this concept we have looked at the basicity of aniline.