Organic chemistry Notes Chapter 17 and 18
Organic chemistry Notes Chapter 17 and 18 Chem 372-0
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This 15 page Class Notes was uploaded by Megan steltz on Friday October 7, 2016. The Class Notes belongs to Chem 372-0 at Eastern Michigan University taught by Gregg M. Wilmes in Fall 2016. Since its upload, it has received 6 views. For similar materials see Organic Chemistry in Chemistry at Eastern Michigan University.
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Date Created: 10/07/16
Chapter 17 part 2 Reactions of benzene Benzene is unexpectedly stable Or Measure relative energy of unsaturated compounds by hydrogenation C=C +H2 = C-C Lose C-C π gain 2x C-H σ H – H σ ● Products more stable than reactants ● Exothermic reaction ΔH = -120 KJ/mol ΔH = -232 KJ/mol (8 KJ/mol of stabilization from conjugation) ΔH = - 208 KJ/mol (152 KJ/mol of stabilization from conjugation) Extra stabilization results from aromaticity Aromatic molecules (or groups) 1. cyclic 2. planar 3. conjugated 4. has specific # of π electrons examples of not aromatic molecules hexatriene not aromatic not cyclic cyclooctatetraene Not planar not aromatic For a molecule to be conjugated all atoms must have unhybridized p-orbital sp^2 or sp Huckels Rule All aromatic groups have 4n+2 π electrons where n is an integer 2,6,10,14,18,22… All possible numbers of π electrons an aromatic compound can have Cyclic, planar, conjugated molecules with 4n π electrons are unstable antiaromatic Aromatic = cyclic, planar, conjugated 4n + 2 π e- Antiaromatic = cyclic, planar, conjugated, 4n π e- Non-aromatic = fails any one of all 4 criteria  - annulene 14 π e- Polycyclic aromatic hydrocarbons Aromatic molecules with heteroatom (O, N, S) Things to remember ● look for Sp^2 hybridized carbons ● count π bonds in 4n + 2 pattern Chapter 18 Electrophlic aromatic substitution ● Aromatic compounds are electron rich o Many electrons in π orbital o π bonds are less stable than σ bonds o aromaticity is retained in the product Mechanism of electrophlic aromatic substitution There are two more possible resonance structures for the above molecule 5 different types of reactions with aromatic compounds 1) Halogenation 2) Nitration 3) Sulfonation 3) Friedel-Crafts Alkylation 4) Friedel-Crafts Acylation Review of Lewis Dot/Resonance Structures ● H’s attached to carbons are not shown but they are still there ● No more than 8e- on C, N, O ● Lone pairs are optional, charges are required ● Resonance structures, differ in the location of o Lone pairs o π bonds o atoms and σ bonds do not change ● Shows movement of e- always start the arrow at the bond (σ or π) or a lone pair. ● End of arrow must point to atom or bond making new π bond Halogenation ● Reacts with Cl2 or Br2 ● Catalyst is FeCl3 or FeBr3 ● Reaction forms electrophile Nitration and Sulforation ● HNO3 reagent ● H2SO4 catalyst Sulfonation ● Reactions contains SO3 with H2SO4 ● Always going to be SO3 + H2O = H2SO4 fuming sulfuric acid Friedel-Crafts Alkylation 1º alkyl halide = no carbocation Limitations of F-C Alkyation Does not work if halogen is attached to Sp2 or Sp3 hybridized carbons. Carbocations in F.C. alkylation are prone to rearrangement problems with 1º or 2º R-Cl EAS or Substituted compounds ● The identity of the z, controls whether ortho, meta or para product(s) are formed ● Group already on the ring decides whether the reaction happens ● Some substitutions (activators) speed up the rate of EAS (relative to benzene) some (deactivators) slow down the rate. Activating/ deactivating directing effects ● Determined an electron withdrawing r donating capabilities of substance. ● Electron donating groups are activators Examples. Electron withdrawing groups are deactivating Electron donating or withdrawing can occur by ● Inductive effects resulting in differences in electronegativty ● Occur through σ bonds Alkyl groups are inductively donating Heteroatoms (O, N, X) are more electronegative than H so they are inductively withdrawing. Resonance effects ● Groups allow resonance structures that put + or – into ring ● Resonance donating group places negative charge in ortho and para position ● For O, N resonance donating is stronger than inductive withdrawing With -F, -Cl, -Br, -I inductive effects are stronger than resonance donating Resonance withdrawing groups Example + is ortho or para to substituent
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