Chem 2520 Final Study Guide Chapter 15 Benzene and Aromaticity Aromaticity Huckel s Rule electron count of a cyclic conjugated system planar must t the formula 4n 2 n 0 1 2 to be aromatic 4n are aromatic unstable Systems that do not apply are called nonaromatic Examples Electrophilic Aromatic Substitution EAS Benzene is stable electron rich and nucleophilic aromaticity is destroyed then restored Halogenation a catalyst is needed because halogens are weak electrophiles mechanism chlorinations are possible Cl2 FeCl3 but F2 and I2 are not Nitration The electrophile is a nitronium ion mechanism Sulfonation The electrophile is sulfur trioxide SO3 fuming H2SO4 about 8 SO3 mechanism sulfonation is reversible Friedel Craft Alkylation The electrophile is a metallated alkyl halide mechanism Example this reaction can create undesired products from polyalkylations and rearrangements the rearrangements occur because the reaction goes through a carbocation therefore carbocation rearrangements may occur propyl carbocation can rearrange to be more stable via a hydride shift in order to obtain products through the less stable carbocations Friedel Crafts acylations are useful Friedel Crafts Acylation more useful than alkylations and deactivate the ring AlCl3 is no longer a catalyst but it is needed to pull the Cl o the acyl halide to form the acylium ion the electrophile H2O is a work up step The electrophile is an acylium ion mechanism aqueous work up is required can use carboxylate anhydrides can remove the oxygen by the Clemmensen Reduction Chapter 16 Substituted Benzene Inductive E ects Donators Activators donate electron density Acceptors Deactivators withdraw electron density Resonance E ects will almost always trump induction donors activators are functional groups with a lone pair directly on the ring Acceptors deactivators contain polarized pi bonds directly on the ring Ortho Para Directors all activators and weak deactivators halogens are o p directors Strong activators have a lone pair next to the ring strong activation through resonance weak deactivation through induction Moderate activators have a lone pair next to the ring that is also tied up in resonance outside of the ring Weak activators are alkyl groups they have no resonance and are weak activators through induction Weak deactivators are halogens they are deactivating through induction and very weakly activating through resonance this is the exception to the rule Why are activators o p directors and not meta directors all has to do with resonance stabilization putting the electrophile at the o p positions is the most stable as the second resonance structure has all octets The meta position is not the most stable because it does not bene t from stabilization of the OMe Why are halogens o p directors when they are weak deactivators the explanation is the same as above in that the second o p resonance structure is stable because it contains all octets Meta Directors moderate strong deactivators are meta directors Strong deactivators have a pi bond to an electronegative atom next to the ring they are deactivating through resonance and induction Moderate strong deactivators are very powerfully electron withdrawing they have no resonance but have a strong induction e ect Why are strong moderate deactivators meta directors when the electrophile is o p to the deactivating group the structure is destabilized due to the inductive e ect of CF3 When the electrophile is meta to the group the structure is not stabilized and for this reason it is a meta director Predicting Directing E ects Two rules 1 Ortho para directors always beat meta directors 2 The strongest activator wins also the e ects are additive and sterics will dictate similar positions Examples Changing Directing Nature Aniline and Phenol are Superactivated meaning they result in multiple additions Reducing the Activity of Functional Groups some FGs can be protected to reduce activity EAS Reactions on Benzenoid Structures Napthalene will add at C1 Why C1 attack at C1 will maximize aromatic resonance structures C1 generates 5 total resonance structures with 2 being aromatic C2 generates 5 total resonance structures with 1 being aromatic activating substituents e ect the ring that they are attached to o p rules apply Deactivating substituents will add C5 and C8 C5 and C8 are C1 positions on other ring Larger benzenoid reaction sites are determined based on resonance structures Three main functional groups that will render a ring too deactivated to do a Friedel Craft s Chapter 17 Aldehydes and Ketones Oxidation of Alcohols Chromium Reagents Manganese Reagents selective allylic OH oxidation Ozonolysis the reductive work up makes it an aldehyde Hydration of Alkynes Markovnikov Anti Markovnikov Multiple Sites of Reactivity obey a general mechanism Nucleophilic Addition Protonation Mechanism basic Electrophilic Protonation Addition acidic Neutral Addition of Water Hydrate Formation mechanism base catalyzed aqueous OH H2O NaOH H cat mechanism H3O H2O HCl Reversibility 1 Alkyl groups stabilize carbonyls 2 Aldehydes are more electropositive and add faster than ketones Addition of Alcohols Hemiacetals and Acetals mechanism Acetals as Protecting Groups Thioacetals hydrolysis Addition of Amines and Ammonia Formation of Imines condensation reaction with a primary amine orNH3 mechanism acidic Imine Derivatives Formation of Enamines reaction with a secondary amine mechanism Wolf Kishner Reduction mechanism Cyanohydrins addition of CN to carbonyl The Wittig Reaction Addition of a phosphonium ylide to a carbonyl mechanism ylide formation alkene formation stabilized ylide stable products trans unstabilized ylide unstable products cis Baeyer Villiger Oxidation oxidizes ketones or aldehydes to esters COOH migratory aptitude cation like