JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD Pharm. D I Year PHARMACEUTICAL ORGANIC CHEMISTRY Previous Question Paper (October - 2020 - R08)

Bayer's Strain Theory proposes that the stability of cycloalkanes depends on the deviation of their bond angles from the ideal tetrahedral angle of 109.5°. According to the theory, when the bond angle of a cycloalkane deviates from 109.5°, an internal strain known as angle strain is produced, which makes the molecule unstable and reactive. The theory calculates the strain as half the difference between the ideal angle and the actual bond angle: Strain = 1/2(109.5° - α).

Orbital pictures of angle strain illustrate that in small rings like cyclopropane, the carbon-carbon bonds cannot be formed by direct head-on overlap of sp3 orbitals. Instead, the orbitals overlap at an angle, resulting in bent bonds or banana bonds. This non-axial overlap leads to inefficient orbital overlap, high electron density outside the internuclear axis, and significant torsional and angle strain, accounting for the high reactivity of cyclopropane compared to larger, more flexible rings.

Organic reactions are primarily classified based on the nature of the transformation and the mechanism involved:

• Substitution Reactions: An atom or group of atoms in a molecule is replaced by another atom or group. These are classified as nucleophilic (SN1, SN2), electrophilic, or free-radical substitution.
• Addition Reactions: Two or more molecules combine to form a single larger molecule, typically occurring in unsaturated compounds like alkenes or alkynes. These include electrophilic addition, nucleophilic addition, and free-radical addition.
• Elimination Reactions: Two atoms or groups are removed from a molecule, often resulting in the formation of a double or triple bond. These are typically classified as unimolecular (E1) or bimolecular (E2) eliminations.
• Rearrangement Reactions: The migration of an atom or group within the same molecule to form a structural isomer, often involving carbocation intermediates.
• Pericyclic Reactions: Concerted reactions that proceed through a cyclic transition state without the formation of ionic or radical intermediates, such as Diels-Alder reactions.