JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II PHYSICAL PHARMACEUTICS-II Previous Question Paper (July - 2021 - R17)

A second-order reaction is a chemical reaction where the rate of reaction depends on the concentration of two reactants, or the square of the concentration of a single reactant. The rate is proportional to the product of the concentrations of the reacting species.

For a reaction where two reactants A and B react to form products with initial concentrations 'a' and 'b', and 'x' is the concentration reacted at time 't', the rate equation is:

When the initial concentrations are equal (a = b), the equation becomes:

Rearranging and integrating:

Integrating both sides from 0 to x and 0 to t:

The final integrated rate equation is:

Where 'k' is the second-order rate constant, 'a' is the initial concentration, and 'x' is the amount reacted at time 't'.

Chemical degradation of pharmaceutical products is significantly influenced by various chemical factors that affect the stability of the active pharmaceutical ingredient (API):

• Hydrolysis: This is the most common degradation pathway, involving the cleavage of chemical bonds (like esters, amides, or lactams) by water. It is highly sensitive to pH, where specific acid or base catalysis can accelerate the process.
• Oxidation: Many drugs are susceptible to atmospheric oxygen, leading to the formation of free radicals. This process is often catalyzed by trace metal ions (e.g., Fe, Cu), light, and high temperatures.
• Photolysis: Exposure to light, particularly UV radiation, can provide sufficient energy to break covalent bonds, causing discoloration or loss of potency. Photosensitive drugs must be stored in amber-colored containers.
• pH Effects: The stability of a drug is often pH-dependent. Most drugs exhibit a pH of maximum stability; deviation from this range can exponentially increase the rate of degradation via acid-base catalysis.
• Solvent Effects: The dielectric constant of the solvent influences the rate of ionic reactions. Changing the solvent composition can alter the transition state energy, thereby affecting the reaction rate.
• Ionic Strength: The presence of electrolytes in solution can influence the reaction rate of charged species according to the primary salt effect, where increased ionic strength can stabilize or destabilize the transition state.