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AROMATIC COMPOUNDS. SATHEESH KUMAR G. Huckel’s Rule: The 4n+2 π Electron Rule Planar monocyclic rings with a continuous system of p orbitals and 4n + 2 π electrons are aromatic (n = 0, 1, 2, 3 etc)
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AROMATIC COMPOUNDS SATHEESH KUMAR G
Huckel’s Rule: The 4n+2π Electron Rule Planar monocyclic rings with a continuous system of p orbitals and 4n + 2π electrons are aromatic (n = 0, 1, 2, 3 etc) • Aromatic compounds have substantial resonance stabilization Benzene is aromatic: it is planar, cyclic, has a p orbital at every carbon, and 6 π electrons (n=1)
There is a polygon-and-circle method for deriving the relative energies of orbitals of a system with a cyclic continuous array of p orbitals. • Benzene has 3 bonding and 3 antibonding orbitals All the bonding orbitals are full and there are no electrons in antibonding orbitals; benzene has a closed shell of delocalized electrons and is very stable
Aromatic, Antiaromatic, and Nonaromatic Compounds • A comparison of cyclic annulenes with their acyclic counterparts provides a measures of the stability conferred by aromaticity . • If the ring has lower π-electron energy than the open chain, then it is aromatic If the ring has the same π-electron energy as the open chain, then it is nonaromatic .
If the ring has higher π-electron energy than the open chain, then it is antiaromatic Benzene and cylcopentadientl anion are aromatic Cyclobutadiene is antiaromatic Cyclooctatetraene, if it were planar, would be antiaromatic
Benzenoid Aromatic Compounds -Polycyclic benzenoid aromatic compounds have two or more benzene rings fused together. • Nonbenzenoid Aromatic Compounds Nonbenzenoid aromatic compounds do not contain benzene rings Examples are cyclopentadienyl anion and the aromatic annulenes (except [6] annulene)
Heterocyclic Aromatic Compounds Heterocyclic compounds have an element other than carbon as a member of the ring.
The Criteria for Aromaticity—Hückel’s Rule Four structural criteria must be satisfied for a compound to be aromatic. [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. [2] A molecule must be planar.
[3] A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. [4] A molecule must satisfy Hückel’s rule, and contain a particular number of electrons. Hückel's rule: An aromatic compound must contain 4n+2π electrons [n (integer no.) = 0, 1, 2, and so…] Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is nonaromatic and especially unstable because it contains 4 electrons.
Note that Hückel’s rule refers to the number of electrons, not the number of atoms in a particular ring.
Examples of Aromatic Rings • Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2 electrons. • Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes. • To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene.
Thus, although five resonance structures can also be drawn for the cyclopentadienyl cation and radical, only the cyclopentadienyl anion has 6 electrons, a number that satisfies Hückel’s rule. Indicate which of the following are aromatic and non-aromatic?
