JNTU Pharm. D I Year PHARMACEUTICAL ORGANIC CHEMISTRY Previous Question Paper (January/February - 2021 - R08)

Physical properties of organic compounds include physical state, solubility, melting point, boiling point, density, and refractive index. These properties are primarily determined by the strength of intermolecular forces and the molecular structure.

Intermolecular forces are the attractive forces between molecules that dictate phase and boiling/melting points. These include: 1. London dispersion forces, which are weak temporary dipoles present in all molecules; 2. Dipole-dipole interactions, occurring between polar molecules; and 3. Hydrogen bonding, a strong interaction occurring when hydrogen is covalently bonded to highly electronegative atoms like N, O, or F.

Polarity of bonds arises from the difference in electronegativity between bonded atoms. When electrons are shared unequally, the bond develops a dipole moment, represented as δ+ and δ-. The overall polarity of an organic molecule depends on the vector sum of these individual bond dipoles, which influences solubility in polar versus non-polar solvents.

The free radical chain reaction of alkanes, specifically halogenation, proceeds through three distinct stages:

1. Initiation: Homolytic cleavage of the halogen molecule (X₂) by heat or UV light produces two highly reactive free radicals.
   X-X → 2X•
2. Propagation: The halogen radical abstracts a hydrogen atom from the alkane to form an alkyl radical, which then reacts with another halogen molecule to form the alkyl halide and regenerate a halogen radical.
   R-H + X• → R• + HX
   R• + X₂ → R-X + X•
3. Termination: The chain reaction ends when two radicals collide and combine to form a stable molecule, consuming the reactive species.
   R• + R• → R-R; R• + X• → R-X; X• + X• → X₂

The stability of free radicals follows the order: tertiary (3°) > secondary (2°) > primary (1°) > methyl radical. This stability is due to hyperconjugation and the inductive effect of alkyl groups, which help disperse the unpaired electron density, lowering the activation energy required for the formation of more substituted radicals.