Aromatic compounds Flashcards
Benzene formula
C6H6
C-C bonds in benzene are all equal =
1.4Å (single C-C 1.54Å, double C=C 1.33Å)
Resonance hybrid of the two Kekulé structures: the pi electrons are _________
delocalised
Aromatic compound:
be a cyclic compound containing some number of conjugated double bonds and having an unusually large resonance energy
Aromatic compounds meet the following criteria:
[4]
1] Structure must be cyclic, containing some number of conjugated pi bonds
2] Each atom in the ring must have an unhybridized p orbital. Usually sp2 or occasionally sp hybridised
3] Unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals (conjugated). Structure must be planar (or nearly planar) for effective overlap to occur
4] Delocalisation of the pi electrons over the ring must lower the electronic energy
Antiaromatic compound:
meets the first three criteria, but delocalisation of the pi electrons over the ring increases the electronic energy
Aromatic structures are _____ stable than their open-chain counterparts
more
What is a Nonaromatic compound (aliphatic)?
cyclic compound that does not have a continuous, overlapping ring of p orbitals. Its electronic energy is similar to that of its open-chain counterpart
Aromatic or antiaromatic: a cyclic compound must have
a continuous ring of overlapping p orbitals, usually in a planar conformation
Benzene (4N+2) = N=1. How many pi electrons can the benzene obtain?
6 pi e-
sp3 carbon is nonaromatic because
no unhybridized p orbital and no continuous ring of p orbitals
4N the system is
nonaromatic
sp3 present
nonaromatic
When carbon have -ive charge it is sp2 instead of sp3
antiaromatic
when a carbon is +ive NOT sp3 but sp2
aromatic
4 pi e- system with N=1 obtains
4N = 4 pi e-
Derivative of benzene:
substituents named just as they were attached to an alkane
Disubstituted benzenes:
prefixes ortho- (o), meta- (m), and para- (p).
Numbers can also be used
Three or more substituents
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
Benzene ring named as a substituent:
prefix- phenyl group.
Often abbreviated Ph in drawing a complex structure
Nomenclature of Benzene Derivatives
Benzyl group:
benzene ring + methylene (-CH2-) group (7 carbons)
Do not confuse with the phenyl group (six carbons)
aromatic hydrocarbons can be called
arenes
Aryl group (Ar):
Aromatic group after the removal of a H atom from an aromatic ring. The phenyl group, Ph, is the simplest aryl group.
what are the 6 steps of Aromatic Compounds Reactions?
- Electrophilic aromatic substitution
- Nucleophilic aromatic substitution
- Organometallic Couplings
- Addition reactions
- Side-chain reactions
- Oxidation of phenols to quinones
Electrophilic Aromatic Substitution
Benzene has clouds of pi electrons above and below its sigma bond framework
Attack a strong electrophile to give a carbocation
2 steps of electrophilic aromatic substitution
Step 1: Attack on the electrophile forms the sigma complex (a resonance-stabilised carbocation)
Step 2: Loss of a proton regains aromaticity and gives the substitution product
Overall reaction of e- substitution is a sub of:
electrophile (E+) for a proton (H+) on the aromatic ring
Electrophilic Aromatic Substitution (electrophiles)
a. Bromination of Benzene
b. Nitration of Benzene
c. Sulfonation of Benzene
d. The Friedel–Crafts Alkylation
e. The Friedel–Crafts Acylation
Activating, Ortho, Para-Directing Substituents
What is the role of methyl group?
electron-donating (ED): stabilises the sigma complex
Inductive stabilisation:
donate electron density through the sigma bond to the benzene ring
Resonance stabilisation:
[Alkoxy groups] oxygen atom is called resonance-donating or pi-donating because it donates electron density through a pi bond in one of the resonance structures
in 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.
Any substituent with a lone pair of electrons on the atom bonded to the ring can provide resonance stabilisation to a _____ complex
sigma
Is nitro a strong or weak deactivating group?
(Deactivating, Meta-Directing Substituents)
Strong - 1. Nitrobenzene: 100,000 times less reactive
2nd step;
Deactivating, Meta-Directing Substituents
- One product: meta-directors, deactivate the meta position less than the ortho/para positions, allowing meta substitution
Inductively withdraws electron density from the aromatic ring, which is less electron-rich, so
DEactivated
Deactivating substituents: groups with _____ _______ (or a partial positive charge) on the atom bonded to the aromatic ring
positive charge
Halogen Substituents
Halogens: deactivating groups but ortho, para-directors
1) Strongly electronegative: withdrawing electron density from a carbon atom through the sigma bond (inductive withdrawal)
2) Nonbonding electrons donate electron density through pi bonding
(resonance donation)
_________ reacts at ortho/para and the positive charge of the sigma complex is shared by the carbon atom bearing the halogen.
Eloctrophile
Directing effects of substituents
1) e- donating and 2) e-withdrawing
1) activating = pi donors and sigma donors
2) deactivating = halogens, carbonyls and others
Effects of Multiple Substituents
Activating groups are usually 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
Nucleophilic Aromatic Substitution
SN1 and SN2 mechamisms
SN2 mechanism: NO. The aromatic ring blocks approach of the nucleophile to the back of the carbon bearing the halogen
SN1 mechanism: NO. Strong nucleophiles are required and the reaction rate is proportional to the concentration of the nucleophile
two can be involved - depending reactants
Organometallic coupling: Heck reaction
Coupling of an aryl or vinyl halide with an alkene to give a new C-C bond at the less substituted end of the alkene, usually with trans stereochemistry
Organometallic coupling; The Suzuki Reaction (Suzuki Coupling)
Palladium-catalysed substitution that couples an aryl or vinyl halide with an alkyl, alkenyl, or aryl boronic acid or boronate ester
