Aromatic compounds

Question 1

Benzene

Answer 1

• 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

Question 2

Definition

Aromatic compound

Answer 2

• Cyclic compound containing some number of conjugate double bonds and having an unusually large resonance energy

Question 3

Aromatic compound criteria

Answer 3

• 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

Question 4

Antiaromatic compound

Answer 4

• Meets first 3 criteria but delocalisation of pi electrons over the ring increase electronic energy

Question 5

Stability of aromatic compounds

Answer 5

• More stable than openchained counterpart
• Antiaromatic less stable than open counterpart

Question 6

Nonaromatic compound

Answer 6

• Cyclic comound that doesnt have a continuous overlapping ring of p orbitals
• Electronic energy is similar to open-chain counterpart

Question 7

Hijckel rule aromatic

Answer 7

• 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

Question 8

Hijkel rule antiaromatic

Answer 8

• 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

Question 9

Non-aromatic

Cyclopentadiene

Answer 9

• sp3 hybrid C no unhybridized p orbital,
  no continuous ring of p
  orbitals
• Dehydrogenate also makes it aromatic or anti

Question 10

Heterocyclic compounds

Answer 10

With rings containing sp2 hybridised heteroatoms

Question 11

Polynuclear aromatic hydrocarbons

Answer 11

Two or more fused benzene rings. Fused
rings share two carbon atoms and the bond between them

Question 12

Naming benzene

Answer 12

• 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

Question 13

Benzyl group

Answer 13

benzene ring + methylene

Question 14

Aryl group

Answer 14

• Aromatic group after the removal of a H atom from an aromatic ring.
• The phenyl group, Ph, is the simplest aryl group

Question 15

Aromatic Compounds Reactions

Answer 15

• Electrophilic aromatic substitution
• Nucleophilic aromatic substitution
• Organometallic Couplings
• Addition reactions
• Side-chain reactions
• Oxidation of phenols to quinones

Question 16

Electrophilic Aromatic Substitution

Answer 16

• 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

Question 17

Nitration of Benzene

Answer 17

Nitric acid hydroxy group is protonated and
leave as water, similar to the dehydration of
an alcohol

Question 18

Methyl group electron-donating

Answer 18

• Stabilises the sigma complex effect is large when in ortho or para to the site of substitution CH3
  is an Activating group

Question 19

Inductive stabilisation

Answer 19

• Donate electron density through the
  sigma bond to the benzene ring

Question 20

Alkoxy Groups

Resonance stabilisation

Answer 20

• oxygen atom is called resonance-donating or pi-donating because it donates electron density through a pi bond in one of the resonance
  structures

Question 21

Amine Groups Resonance stabilisation

Answer 21

• 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.

Question 22

Nitrobenzene

Deactivating Meta-Directing Substituents

Answer 22

• More deactivating groups therefore nitro makes the compound less reactive

Question 23

Inductively withdraws electron density

Answer 23

Aromatic ring, which is less
electron-rich, so deactivated

Question 24

Deactivating substituents definition

Answer 24

• Groups with positive charge on the atom bonded to the aromatic ring

Question 25

Halogen Substituents

Answer 25

• Deactivating groups but ortho, para-directors

Question 26

Strongly electronegative

Answer 26

• Withdrawing electron density from a carbon atom through the sigma bond
• Non-bonding electrons donate electron density trough pi bonding

Question 27

Electron donating groups

Answer 27

• NH2
• OH
• OR
• NHCOCH3
• F
• Cl & halogens (O,P directing)

Question 28

Electron-withdrawing

Answer 28

• Carbonyls
• NO2
• CN

Question 29

Activating groups

Answer 29

Stronger directors than deactivating groups

Answer 30

• 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

Question 31

Sn2 mechanism

Answer 31

• Aromatic ring blocks approach of nucleophile to the back of carbon bearing halogen

Question 32

Sn1 mechanism

Answer 32

• Strong nucleophiles are required and the reaction rate is proportional to the concentration of the nucleophile
• 2 mechanisms may be involved

Question 33

Heck reaction

Answer 33

• 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

Question 34

The Suzuki Reaction

Answer 34

• 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

Question 35

Williamson ether synthesis

Answer 35

• Reaction of phenol with NaOH to form stong nucleophile
• Then react with Halogen with alkyl group