Aromatic compounds Flashcards
Benzene
- Derived from the Kekule structure (C6H6)
- Spectroscopy shows benzene are all equal in length
- Resonance hybid of kekule structure
- Pi bond is delocalised over the ring 6 Sp2 carbon atoms forming pi overlap all 120
Definition
Aromatic compound
- Cyclic compound containing some number of conjugate double bonds and having an unusually large resonance energy
Aromatic compound criteria
- Structure must by cyclic containing some number of conjugate pi bonds
- Each atom in the ring must have unhybridized p orbital which is usually sp2 or occasionally sp hybrid
- Unhybridised p orbital must overlap form parallel orbital - must be planar
- Delocalisation of pi electron over the ring must lower the electronic energy
Antiaromatic compound
- Meets first 3 criteria but delocalisation of pi electrons over the ring increase electronic energy
Stability of aromatic compounds
- More stable than openchained counterpart
- Antiaromatic less stable than open counterpart
Nonaromatic compound
- Cyclic comound that doesnt have a continuous overlapping ring of p orbitals
- Electronic energy is similar to open-chain counterpart
Hijckel rule aromatic
- If number of pi-electrons in the cyclic system is (4n +2) the system is aromatic
- Commonly N = 0, 1 or 2 system is 2,6,10 pi electrons
Hijkel rule antiaromatic
- If number of pi-electrons in the cyclic system is (4n) the system is antiaromatic
- N = 1,2,3 and the number of aromatic systems aromatic 4 8 12
Non-aromatic
Cyclopentadiene
- sp3 hybrid C no unhybridized p orbital,
no continuous ring of p
orbitals - Dehydrogenate also makes it aromatic or anti
Heterocyclic compounds
With rings containing sp2 hybridised heteroatoms
Polynuclear aromatic hydrocarbons
Two or more fused benzene rings. Fused
rings share two carbon atoms and the bond between them
Naming benzene
- Give the lowest possible numbers to the
substituents. - Carbon atom bearing the functional group that defines the base name (as in phenol or benzoic acid) is assumed to be C1
Benzyl group
benzene ring + methylene
Aryl group
- Aromatic group after the removal of a H atom from an aromatic ring.
- The phenyl group, Ph, is the simplest aryl group
Aromatic Compounds Reactions
- Electrophilic aromatic substitution
- Nucleophilic aromatic substitution
- Organometallic Couplings
- Addition reactions
- Side-chain reactions
- Oxidation of phenols to quinones
Electrophilic Aromatic Substitution
- Clouds of pi electrons above and below its sigma bond framework
- Attack a strong electrophile to give a carbocation
- Loss of a proton regains aromaticity and gives the substitution product
Nitration of Benzene
Nitric acid hydroxy group is protonated and
leave as water, similar to the dehydration of
an alcohol
Methyl group electron-donating
- Stabilises the sigma complex effect is large when in ortho or para to the site of substitution CH3
is an Activating group
Inductive stabilisation
- Donate electron density through the
sigma bond to the benzene ring
Alkoxy Groups
Resonance stabilisation
- oxygen atom is called resonance-donating or pi-donating because it donates electron density through a pi bond in one of the resonance
structures
Amine Groups Resonance stabilisation
- Nitrogen’s lone pair electrons provide
resonance stabilisation to the sigma complex if attack takes place ortho or para to the position of the nitrogen atom.
Nitrobenzene
Deactivating Meta-Directing Substituents
- More deactivating groups therefore nitro makes the compound less reactive
Inductively withdraws electron density
Aromatic ring, which is less
electron-rich, so deactivated
Deactivating substituents definition
- Groups with positive charge on the atom bonded to the aromatic ring
Halogen Substituents
- Deactivating groups but ortho, para-directors
Strongly electronegative
- Withdrawing electron density from a carbon atom through the sigma bond
- Non-bonding electrons donate electron density trough pi bonding
Electron donating groups
- NH2
- OH
- OR
- NHCOCH3
- F
- Cl & halogens (O,P directing)
Electron-withdrawing
- Carbonyls
- NO2
- CN
Activating groups
Stronger directors than deactivating groups
- Powerful ortho, para-directors that stabilise the sigma complexes
through resonance. -OH, -OR, and -NR2 groups - Moderate ortho, para-directors, such as alkyl groups and halogens
- All meta-directors
Sn2 mechanism
- Aromatic ring blocks approach of nucleophile to the back of carbon bearing halogen
Sn1 mechanism
- Strong nucleophiles are required and the reaction rate is proportional to the concentration of the nucleophile
- 2 mechanisms may be involved
Heck reaction
- Coupling of an aryl or vinyl halide with alkene to give new C-C bond at the less substituted end of the alkene the less substituted end of the alkene, usually with trans stereochemistry
The Suzuki Reaction
- Palladium-catalysed substitution that couples an aryl or vinyl halide with an alkyl, alkenyl, or aryl boronic acid or boronate ester
- Aryl halide with an arylboronic acid, using palladium on carbon and water as the solvent
Williamson ether synthesis
- Reaction of phenol with NaOH to form stong nucleophile
- Then react with Halogen with alkyl group
