Arene Reactions Past Paper 2024 PDF

Success Criteria - State four pieces of evidence that suggest arenes had resonance - State the reagents, conditions & products of two reactions involving arenes  10: Arene Reactions Unit 3: Organic Chemistry Kognity S2.2.12 & R.3.4.13 Aromatic Hydrocarbons (arenes)...

Success Criteria - State four pieces of evidence that suggest arenes had resonance - State the reagents, conditions & products of two reactions involving arenes  10: Arene Reactions Unit 3: Organic Chemistry Kognity S2.2.12 & R.3.4.13 Aromatic Hydrocarbons (arenes) historically proposed structures for benzene Contain a benzene ring C6H6 Arene Overview Despite being drawn as alternating single and double bonds, evidence suggests that arenes are better described by resonance structures: 1. If alternating single and double bonds existed, then there would be two isomers of structures like 1,2-dichlorobenzene with different properties (not observed) 2. All arene C-C bonds are equal in length at 140 pm, where average C-C single bonds are 154 pm and C=C bonds are 134 pm Arene Overview 3. Arenes do not undergo reactions that alkenes do (hydration, hydrogenation, halogenation, etc.) under the same conditions 4. Less energy is released than expected when hydrogenating benzene: hydrogenating three C=C bonds should be -360 kJ mol-1, but for arenes it is only -208 kJ mol-1 This suggests arenes are very stable due to resonance →  Electrophilic Substitution Benzene or substituted benzene rings (arenes) undergo electrophilic substitution: - benzene is an electron rich molecule - E+ is the electrophile (electron loving species) - a C-H bond on the benzene ring is replaced with a C-E bond  Electrophilic Substitution Mechanism (FYI / HL) To get arenes to react, the delocalized electrons in the arene must be disrupted: - one of the shared pairs of electrons from a double bond react with the electrophile - a positively charged intermediate is formed - the hydrogen bonded to the same carbon as the electrophile leaves & resonance is reestablished The Ea is very high when breaking resonance, leading to this being the slow step in the reaction mechanism. Electrophilic Substitution: Halogenation Starting Reagents Conditions Product(s) Material(s) (chemical) (non-chemical) benzene Cl2 / AlCl3 chlorobenzene/ (arene) (Br2 / AlBr3) bromobenzene makes Cl+ electrophile Electrophilic Substitution: Nitration of Benzene Starting Reagents Conditions Product(s) Material(s) (chemical) (non-chemical) benzene HNO3 heat Nitroarene (arene) conc. H2SO4 (50 oC) (NO2 = nitro group) makes NO2+ electrophile Organic Reactions Organic Synthesis The art of Chemical Synthesis: https://www.youtube.com/watch?v=pEJhB7TsTNM&list=PL6fmwNvY2XXo3hv1wz9lb3vdie q1w6ONR&index=6