Aromaticity:- ¯¯¯¯¯¯¯¯¯¯¯ • Aromaticity is a property associated with the extra stability of certain types of pi systems due to the nature of the molecular orbitals. • Fundamentally it arises from having the maximum number of electrons in the pi bonding molecular orbitals. • The property of aromaticity is generally about a significant extra stability associated with a resonance delocalized structure. • This extra stability moderates the reactivity of these systems compared to alkenes and means aromatics can be considered as their own functional group. • The presence of aromatic systems can also influence the reactivity of functional groups directly attached to the aromatic system particularly in the benzylic position. Aromatic systems:- ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ • Based on the properties of compounds, there are FOUR criteria about the pi system that need to be met in order for the "special" aromatic stabilization to be observed: 1. Conjugated - there needs to one "pi" orbital from each atom in the cycle, so each atom must be either sp2 or sp hybridised. 2. Cyclic - Linear systems are not aromatic 3. Planar -so there is good overlap / interaction between the "pi" orbitals. 4. The Huckel Rule. - 4n+2 pi electrons (this is equivalent to an odd number of p-electrons pairs) in the cyclic conjugated system. However, there are some ring systems that do not obey Huckel’s rule but are aromatic. That is an exception. •• The most important and well known aromatic system is benzene Resonance Energy ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ • The resonance energy of a compound is a measure of the extra stability of the conjugated system compared to the corresponding number of isolated double bonds. • This can be calculated from experimental measurements. Other aromatic systems ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 1. Naphthalene 2. Anthracene 3. Phenanthrene 4. Pyridine - N analogous of Benzene Polyaromatic Hydrocarbons ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ • Larger systems of benzene rings fused together are known. 1. Naphthalene 2. Anthracene 3. Phenanthrene Heteroaromatics ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ • Aromatic compounds which contain heteroatoms (e.g. O, N, S) are part of the cyclic conjugated pi system are called heteroaromatics. • The involvement of the heteroatom in the cyclic system requires that it provides a p-orbital to be part of the conjugated p system. • This implies that the heteroatom be sp2 hybridized. 1. Furan 2. Thiophene 3. Pyrrole 4. Pyridine 5. Indole 6. Imidazole ••••••••••••••••••••••••••••••••• �
Posted on: Mon, 15 Jul 2013 12:30:01 +0000
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