SNS Organic Chemistry - Cliff's 2 Flashcards
Polycyclic benzenoid aromatic compounds
Compounds which have two or more benzene rings fused together.
A typical example of this type of molecule is naphthalene, C10H8.
Other benzenoid structures include anthracene, phenanthrene, and pyrene.
Heterocyclic compound
An organic compound in which one or more of the carbon atoms in the backbone of the molecule has been replaced by an atom other than carbon.
Typical hetero atoms include nitrogen, oxygen, and sulfur.
Heterocyclic Aromatic Compounds
Huckel’s Rule
- Because these compounds are monocyclic aromatic compounds, they must obey Hückel’s Rule.
- Hückel’s Rule requires 4 n + 2 π electrons, so the simplest aromatic compound should contain 6 π electrons ( n = 1).
- Pyrrole, furan, and thiophene appear, however, to have only 4 π electrons (2 π bonds).
- In systems such as these, the extra electrons needed to produce an aromatic condition come from the unshared electron pairs in sp2 hybrid orbitals around the hetero atom.
Aromatic Compounds
Electrophilic Aromatic Substitution Reactions
- Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions.
- Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete π electron delocalization (resonance).
- Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved.
- For example, benzene reacts with bromine to form bromobenzene.
Aromatic Compounds
Electrophilic Aromatic Substitution Reactions
Mechanism
- An electrophile is generated. For the bromination of benzene reaction, the electrophile is the Br+ ion generated by the reaction of the bromine molecule with ferric bromide, a Lewis acid.
- The electrophile attacks the π electron system of the benzene ring to form a nonaromatic carbocation.
- The positive charge on the carbocation that is formed is delocalized throughout the molecule.
- The aromaticity is restored by the loss of a proton from the atom to which the bromine atom (the electrophile) has bonded.Finally, the proton reacts with the FeBr4− to regenerate the FeBr3 catalyst and form the product HBr.
Aromatic Compounds
Electrophilic Substitution
Nitration
Mechanism
- Sulfuric acid ionizes to produce a proton.
- Nitric acid accepts the proton in an acid-base reaction.
- The protonated nitric acid dissociates to form a nitronium ion (+NO2).
- The nitronium ion acts as an electrophile and is attracted to the π electron system of the benzene ring.
- The nonaromatic carbocation that forms has its charge delocalized around the ring.
- The aromaticity of the ring is reestablished by the loss of a proton from the carbon to which the nitro group is attached.
Aromatic Compounds
Electrophilic Substitution
Sulphonation
Mechanism
The reaction of benzene with concentrated sulfuric acid at room temperature produces benzenesulfonic acid.
- The sulfuric acid reacts with itself to form sulfur trioxide, the electrophile.
- The sulfur trioxide is attracted to the π electron system of the benzene molecule.
The remaining steps in the mechanism are identical with those in the bromination and nitration mechanisms: The charge around the ring is delocalized, and then the loss of a proton reestablishes the aromaticity of the ring
Aromatic Compounds
Electrophilic Substitution
Birch Reaction
- The fully delocalized π electron system of the benzene ring remains intact during electrophilic aromatic substitution reactions.
- However, in the Birch reduction, this is not the case.
- In the Birch reduction, benzene, in the presence of sodium metal in liquid ammonia and methyl alcohol, produces a nonconjugated diene system.
- This reaction provides a convenient method for making a wide variety of useful cyclic dienes.
- The production of the less stable nonconjugated diene instead of the more stable conjugated diene occurs because the reaction is kinetically controlled rather than thermodynamically controlled.
Aromatic Compounds
Electrophilic Substitution
Birch Reaction
Kinetic vs Thermodynamic Control
- In general, reactions that aren’t easily reversible are kinetically controlled because equilibrium is rarely established.
- In kinetically controlled reactions, the product with the lowest-energy transition state predominates.
- Reactions that are easily reversible are thermodynamically controlled, unless something occurs that prevents equilibrium.
- In thermodynamically controlled reactions, the lowest-energy product predominates.
Aromatic Compounds
Electrophilic Substitution
Friedel Crafts
Mechanism
- An electrophile is formed by the reaction of methylchloride with aluminum chloride.
- The electrophile attacks the π electron system of the benzene ring to form a nonaromatic carbocation.
- The positive charge on the carbocation that is formed is delocalized throughout the molecule.
- The aromaticity is restored by the loss of a proton from the atom to which the methyl group has bonded.
- Finally, the proton reacts with the AlCl4− to regenerate the AlCl3 catalyst and form the product HCl.
Aromatic Compounds
Electrophilic Substitution
Friedel Crafts
Carbocation Rearrangement
- Carbocations can rearrange during the Friedel-Crafts alkylation reaction, leading to the formation of unpredicted products.
- One example is the formation of isopropyl benzene by the reaction of propyl chloride with benzene.
- The isopropyl benzene results from a rearrangement of the initially formed propyl carbocation to the more stable isopropyl carbocation.
- This rearrangement is called a 1,2-hydride ion shift. A hydride ion is H−.
Aromatic Compunds
Electrophilic Substitution
Friedel Crafts Acylation
- Similar to the Friedel-Crafts alkylation reaction except that the substance that reacts with benzene is an acyl halide, instead of an alkyl halide, R — X
Aromatic Compunds
Electrophilic Cubstitution
Friedel Crafts Acylation
Mechanism
- The reaction of acetyl chloride with aluminum chloride forms an electrophile.
- The electrophile attracts the π electron system of the benzene ring to form a nonaromatic carbocation.
- The positive charge on the carbocation that is formed is delocalized throughout the molecule.
- The loss of a proton from the atom to which the acetyl group has bonded restores the aromaticity.
- The proton reacts with the AlCl4− to regenerate the AlCl3 catalyst and form the product HCl.
Aromatic Compounds
Directing Group Influence
Halogen Atoms
- Halogen atoms show both activating and deactivating characteristics.
- Because they have three pairs of unshared electrons, halogen atoms can supply electrons toward the ring.
- However, because of their high electronegativities, halogen atoms also tend to remove electrons from the benzene ring.
- These conflicting properties make halogens a weak ortho-para director and also a ring deactivator.
- This means that the presence of a halogen atom on a benzene causes an incoming electrophile to attach at an ortho or para position.
- However, these positions are not very electron rich, so the reaction proceeds poorly under ordinary electrophilic aromatic substitution conditions, leading to poor yields of the disubstituted product.
Aromatic Compounds
Directing Group Influence
-NH2
Activator
Strong
Aromatic Compounds
Directing Group Influence
-NHR
Activator
Strong
Aromatic Compounds
Directing Group Influence
-NR2
Activator
Strong
Aromatic Compounds
Directing Group Influence
-OH
Activator
Strong
