Chemistry 345—Test 2

Chapter 16 * In an electrophilic aromatic substitution reaction, a hydrogen of an aromatic ring is substituted by an electrophile (Lewis acid) * Rate limiting step = formation of the carbocation intermediate * Substituents containing atoms with unshared electron pairs are ortho/para directing because their electron pairs can be involved in resonance stabilization of the carbocation intermediate * Substituents containing positive charges adjacent to the aromatic ring are meta directing because meta substitution gives the carbocation intermediate in which like charges are farther apart * Note: not all meta directors have positive charge. HOWEVER, all of them have bond dipoles that place a substantial amount of positive charge next to the benzene ring * Activation/deactivation is dependent on two things: * Resonance effect: the ability of a substituent to stabilize the carbocation intermediate in electrophilic substitution by delocalization of the electrons from the substituent to the ring * Activating * Polar effect: the tendency of a substituent (because of its electronegativity) to pull electrons from the ring * Deactivating * Halogens * DEACTIVATING because orbitals of halogens and 2p oribtals of benzene do not overlap well (weak resonance effect and strong polar effect) * ORTHO/PARA DIRECTING because donates electron pair to ring

Chapter 17: Allylic and Benzylic Reactivity * Allylic group: a group on a carbon adjacent to a double bond * Benzylic group: a group on a carbon adjacent to a benzene ring * Allylic and benzylic groups are unusually reactive! * Benzylic and allylic carbocations are resonance-stabilized Allylic/benzylic intermediates make reactions faster because charge can be delocalized onto substituent Reactions involving allylic/benzylic carbocations often have multiple products * Radical reactions allylic/benzylic carbon will be substituted * Halogen addition to a double bond and allylic substitution are competing reactions * If you want addition product: * Carry out reaction in polar solvent that promote ionic mechanism ex: CH2Cl2 * If you want allylic substitution product: * Add heat, light or free-radical initiators * Carry out reaction in an apolar solvent ex: CCl4 * Add bromine SLOWLY so that concentration remains low * Use N-bromosuccinimide (NBS) to keep Br concentration low NBS reacts with HBr * Allylic and benzylic anions are also more stable for two reasons: * Resonance stability * Polar effect of the double bond (allyl anion) or phenyl ring (benzene) Grignard and E2 reactions involve species that have carbanion character * Allylic rearrangement: the simultaneous movement of a substituent and a double bond so that one allylic isomer is converted into another * Note: NOT the same as resonance structures * Seen in Grignard reagent reactions * E2 Eliminations involving allylic or benzylic hydrogens * Recall: in E2 elimination, beta-hydrogens are used to form adjacent double bond * An allylic or benzylic hydrogen is favored to be used to form double bond because it is more acidic than its non-allylic/non-benzylic counterparts * SN2 Reactions involving allylic or benzylic atoms * Allylic/benzylic SN2 reactions involve a reactive carbocation intermediate and thus multiple products are often observed * In contract, analogous non-allylic/non-benzylic SN2 reactions do not involve reactive intermediate, and thus only one product is often observed * Allylic and Benzylic Oxidation * Allylic and benzylic alcohols are oxidized by MnO2 (solvent: CH2Cl2) * ONLY oxidizes allylic benzylic alcohols * Primary allylic alcohols are oxidized to aldehydes; secondary allylic alcohols are oxidized to ketones *