AROMATICITY

• Aromaticity is a property associated with the extra stability of certain types

of systems due to the nature of their molecular orbital.

• Aromaticity is a property in which the stabilization of the

molecule is strong due to the ability of the electrons in the

p-orbitals to delocalize and act as a framework to create a

planar molecule.

• The most common Aromatic compound is BENZENE.

CRITERIA FOR AROMATICITY

1. Conjugated: There needs to one "p" orbital from each atom in the cycle, so each atom must be either sp2 or sp hybridised or Each atom in the cycle must have p orbital, forming p orbital loop.

2. Cyclic: Pi bonds must lie within cyclic structure.

3. Planar: : All p-orbitals in the loop must overlap (planarity)

4. HUCKLE RULE

For  monocyclic planar compounds in which each atom has a p-orbital (as in benzene) Hückel showed that:

compounds with (4n + 2) p electrons, where n = 0, 1, 2, 3 etc, would have closed shells of delocalised p electrons and such compounds show exceptional stability (high resonance energy “aromatic”).i.e. planar monocycles with 2, 6, 10, 14 and so on, delocalised p electrons should be “aromatic”.i.e. p electrons are delocalised over the entire ring and the compound is thereby stabilised by the delocalisation.

HYPERCONJUGATION

Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give an extended molecular orbital that increases the stability of the system.

Tautomerism

1. Tautomers are isomers (constitutional isomers) of organic compounds that readily interconvert by a chemical reaction called tautomerization.

2. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond.

3. The concept of tautomerization is called Tautomerism. 4. Tautomerism is a special case of structural isomerism and can play an

important role in non-canonical base pairing in DNA and especially RNA molecules.

EXAMPLES OF TAUTOMERISM

HYDROGEN BONDING

1. A hydrogen bond is a weak type of force that forms a special type of dipole-dipole attraction which occurs when a hydrogen atom bonded to a strongly electronegative atom exists in the vicinity of another electronegative atom with a lone pair of electrons.

2. These bonds are generally stronger than ordinary dipole-dipole and dispersion forces, but weaker than true covalent and ionic bonds.

DONORS AND ACCEPTORS

1. In order for a hydrogen bond to occur there must be both a hydrogen donor and an acceptor present.

2. The donor in a hydrogen bond is the atom to which the hydrogen atom participating in the hydrogen bond is covalently bonded, and is usually a strongly electronegative atom such as N,O, or F.

3. The hydrogen acceptor is the neighboring electronegative ion or molecule, and must posses a lone electron pair in order to form a hydrogen bond.

WHY DOES A HYDROGEN BOND OCCUR?

1. The hydrogen donor is strongly electronegative, it pulls the covalently bonded electron pair closer to its nucleus, and away from the hydrogen atom.

2. The hydrogen atom is then left with a partial positive charge, creating a dipole-dipole attraction between the hydrogen atom bonded to the donor, and the lone electron pair on the accepton.

3. This results in a hydrogen bond.

TYPES OF HYDROGEN BONDS

1. Intramolecular hydrogen bond: Occur within one single molecule

• This occurs when two functional groups of a molecule can form hydrogen bonds with each other.

• In order for this to happen, both a hydrogen donor an acceptor must be present within one molecule, and in close proximity of each other in the molecule.

• Example, intramolecular hydrogen bonding occurs in ethylene glycol (C2H4(OH)2) between its two hydroxyl groups.

2. Intermolecular hydrogen bonds: Occur between separate molecules in a substance.

• They can occur between any number of like or unlike molecules as long as hydrogen donors and acceptors are present an in positions in which they can interact.

• For example, intermolecular hydrogen bonds can occur between NH3 molecules alone, between H2O molecules alone, or between NH3 and H2O molecules.

CHARGE TRANSFER COMPLEX

• It is an association of two or more molecules, or of different parts of one very large molecule, in which a fraction of electronic charge is transferred between the molecular entities.

• The resulting electrostatic attraction provides a stabilizing force for the molecular complex.

• The source molecule from which the charge is transferred is called the electron donor and the receiving species is called the electron acceptor.

• The nature of the attraction is not a stable chemical bond, and is thus much weaker than covalent forces.

• The attraction is created by an electronic transition into an excited electronic state, and is best characterized as a weak electron resonance.

• The excitation energy of this resonance occurs very frequently in the visible region of the electro-magnetic spectrum, which produces the intense color characteristic for these complexes.

• These optical absorption bands are often referred to as charge-transfer bands (CT bands). Optical spectroscopy is a powerful technique to characterize charge-transfer bands.

• Example- Blue charge-transfer band exhibited by iodine when combined with starch.

Inclusion Compound (Inclusion complex)

• A complex in which one component (the host) forms a cavity or, in the case of a crystal, a crystal lattice containing spaces in the shape of long tunnels or channels in which molecular entities of a second chemical species (the guest) are located.

• There is no covalent bonding between guest and host, the attraction being generally due to van der Waals forces.

• If the spaces in the host lattice are enclosed on all sides so that the guest species is trapped as in a cage, such compounds are known as clathrates or cage compounds’.