Chapter 22 Flashcards
Reactions of Benzene
•The most characteristic reaction of aromatic compounds is substitution at a ring carbon.
Electrophilic Aromatic Substitution
•Electrophilic aromatic substitution:Electrophilic aromatic substitution: A reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile.
•We study several common electrophiles (“E+”)
–how each is generated.
–the mechanism by which each replaces hydrogen.
Chlorination
Step 1: Formation of a chloronium ion.
Step 2: Attack of the chloronium ion on the ring.
Step 3: Proton transfer regenerates the aromatic character of the ring.
Nitration
•Generation of the nitronium ion, NO2+
–Step 1: Proton transfer to nitric acid.
–Step 2: Loss of H2O gives the nitronium ion, a very strong electrophile.
Step 1: Attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile).
Step 2: Proton transfer regenerates the aromatic ring.
•A particular value of nitration is that the nitro group can be reduced to a 1° amino group
Sulfonation
•Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide.
-Sulfonation can be reversed by heating in H22O
Role ofAlCl3
Acts as a Lewis acid to promote ionizationof the alkyl halideof the alkyl halid
Friedel-Crafts Alkylation
Step 1: Formation of an alkyl cation as an ion pair.
Step 2: Attack of the alkyl cation on the aromatic ring.
Step 3: Proton transfer regenerates the aromatic ring.
- Carbocation rearrangements are common
- They are tough to stop!
- alkylation alkylation fails fails on benzene rings bearing one or on benzene rings bearing one or more strongly electron-withdrawing groups
•Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group.
•The electrophile is an acylium ion
–An acylium ion is represented as a resonance hybrid of two major contributing structures.
•Friedel-Crafts acylations are free of a major limitation of Friedel-Crafts alkylations; acylium ions do not rearrange
•A special value of F-C acylations is preparation of unrearranged alkylbenzenes.
Other Aromatic Alkylations
•Carbocations are generated by
–treatment of an alkene with a proton acid, most commonly H2SO4, H3PO4, or HF/BF3.
–and by treating an alcohol with H2SO4 or H3PO4.
Di- and Polysubstitution
•Orientation:–Certain substituents direct preferentially to ortho & para positions; others to meta positions.–Substituents are classified as either ortho-para directingortho-para directingor meta directing meta directing toward further substitution.•Rate–Certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower.–Substituents are classified as activatingactivating or deactivatingdeactivatingtoward further substitution.
–-OCH3 is ortho-para directing.–-COOH is meta directing.
•From the information in this Table, we can make these generalizations:–Alkyl, phenyl, and all other substituents in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other substituents are meta directing.–All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating.`
Theory of Directing Effects
•The rate of EAS is limited by the slowest step in the reaction.
•For almost every EAS, the rate-determining step is attack of E+on the aromatic ring to give a resonance-stabilized cation intermediate.
•The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction.
•For ortho-para directors, ortho-para attack forms a more stable cation than meta attack.
–Ortho-para products are formed faster than meta products.
•For meta directors, meta attack forms a more stable cation than ortho-para attack.
–Meta products are formed faster than ortho-para products.
–-OCH3; assume meta attack.
–-OCH3: assume ortho-para attack. Here only para attack is shown.
–-NO2; assume meta attack.
–-NO2: assume ortho-para attack.
•The rate of EAS is limited by the slowest step in the reaction.•For almost every EAS, the rate-determining step is attack of E+on the aromatic ring to give a resonance-stabilized cation intermediate.•The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction.
•For ortho-para directors, ortho-para attack forms a more stable cation than meta attack.–Ortho-para products are formed faster than meta products.•For meta directors, meta attack forms a more stable cation than ortho-para attack.–Meta products are formed faster than ortho-para products.
Activating-Deactivating
•Any resonance effectAny resonance effect, such as that of -NH2, -OH, and -OR, that delocalizes the positive charge on the cation intermediate lowers the activation energy for its formation, and has an activating effect toward further EAS.•
•Any resonance or inductive effectAny resonance or inductive effect, such as that of -NO2, -CN, -C=O, and -SO3H, that decreases electron density on the ring deactivates the ring toward further EAS.
•Any inductive effectAny inductive effect, such as that of -CH3 or other alkyl group, that releases electron density toward the ring activates the ring toward further EAS.•
•Any inductive effectAny inductive effect, such as that of halogen, -NR3+, -CCl3, or -CF3, that decreases electron density on the ring deactivates the ring toward further EAS.
–For the halogens, the inductive and resonance effects run counter to each other, but the former is somewhat stronger.
–The net effect is that halogens are deactivating but ortho-para directing.
What we really have here is..
–The good (strongly activating; o,p directing (e.g., -OCH3, -NH2, etc.)
–The bad (strongly deactivating; m directing (e.g., -NO2, -acyl, etc.)
–and, the ugly (halogens; weakly deactivating, but o,p directing)
Activating-Deactivating - Redux
What we have here is..
–The good (strongly activating; o,p directing (e.g., -OCH3, -NH2, etc.)
–The bad (strongly deactivating; m directing (e.g., -NO2, -acyl, etc.)
–and, the ugly (halogens; weakly deactivating, but o, directing)
Nucleophilic Aromatic Substitution
•Aryl halides do not undergo nucleophilic substitution by either SN1 or SN2 pathways.•They do undergo nucleophilic substitutions, but by mechanisms quite different from those of nucleophilic aliphatic substitution.–Nucleophilic aromatic substitutions are far less common than electrophilic aromatic substitutions.
Addition-Elimination
–When an aryl halide contains electron-withdrawing NO2groups ortho and/or para to X, nucleophilic aromatic substitution takes place readily.
Meisenheimer Complex
–Reaction involves formation of reactive intermediate called a Meisenheimer complex.
