Chemistry > Aromatic Compounds > 1.0 The Structure of Benzene
Aromatic Compounds
1.0 The Structure of Benzene
1.1 A Resonance Picture of Benzene
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
2.0 Electrophilic Aromatic Substitution Reactions
3.0 Nitration
4.0 Sulphonation
5.0 Halogenation
6.0 Friedel-Crafts Alkylation
7.0 Friedel-Crafts Acylation
8.0 Orientation and Reactivity in Electrophilic Aromatic Substitution
8.1 Donation of electrons into a benzene ring by resonance
8.2 Withdrawal of electrons from a benzene ring by resonance
9.0 Ortho / Para Ratio
9.1 Directive influence of the groups during substitutions in benzene ring
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
10.0 Reactions of Alkyl Benzenes
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
8.2 Withdrawal of electrons from a benzene ring by resonance
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
Hückel found that whether a cyclic hydrocarbon is aromatic or not depends on its number of $\pi $ electrons. According to Hückel among, planar, monocylic fully conjugated polyenes only those possessing $(4n + 2)\pi $ electrons, where n is a whole number will have special stability; that is be aromatic. In other words planar monocyclic rings with 2, 6, 10, 14, …, delocalised $\pi $-electrons should be aromatic.
We can now understand why cyclobutadiene and cyclooctatetraene are not aromatic they are having a total of 4 and 8 $\pi $ electrons respectively, both having an unpaired electron in each of two non bonding orbitals. Molecules with unpaired electrons are not usually stable; they are typically highly reactive and unstable.
Because cyclooctatetraene does not gain stability by becoming planar, it assumes the tub shape shown below (cyclooctatetraene would actually lose stability by becoming planar).
Cyclobutadiene belongs to a class of compounds called antiaromatic.
Heterocyclic compounds containing nitrogen, oxygen, or sulfur are by far the most common. Aromaticity is also shown by some heterocyclic compounds (some examples are shown below).
