Lecture 20: Kinetics III Flashcards Preview

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Flashcards in Lecture 20: Kinetics III Deck (33)

What are the decomposition processes we want to prevent?

Isomerisation (including racemisation and epimerisation)


What do environmental elements do?

They provide the energy for decomposition reactions to occur


What are some examples of common environmental elements?

Moisture content
Uv light


What factor does temperature play on degradation?

The arrhenius expression gave us k, the rate constant as a function of activation energy and temperature.

As T increases, the degradation rate constant also increases


How can decomposition be prevented, specifically targeting temperature?

We can store temperature sensitive drug products in the cold


What is hydrolysis?

Involves the reaction of drugs with water.

It is the major cause of drug deterioration, especially in aqueous conditions


What are the three scenarios that may cause a drug to decompose via hydrolysis?

Catalytic: catalysis of drug by hydroxyl or hydronium ions (due to specific acid base catalysis)
Catalytic: the alteration of the buffer species (due to general acid base catalysis)
Uncatalytic: decomposition with water


What is the extent of hydrolysis dependent on?

The amount of moisture and pH of the drug solution


How is moisture introduced to solid dosage forms?

Through excipients such as starch.


What does the moisture content affecting stability depend on?

How strongly moisture is bound to the excipient, whether moisture can get in contact with the drug

Moisture can also facilitate microbial growth and cause more decomposition of the drug


How can hydrolysis be prevented.

Using buffers in formulation to prevent pH based hydrolysis

Adding dessicants to the package
Protecting the drug by formulation means e.g. Micelles, encapsulation etc.


What is oxidation?

Involves the reactions with oxygen


What is auto oxidation?

This is the reaction with atmospheric oxygen and causes the compound to go into a free radical phase with unpaired electrons


What is a free radical.

Highly unstable and reactive species, catalyses oxidative degradation reactions

They are like robbers deficient in energy and snatch energy from other cells


What factors affect the rate of auto oxidation?

Degree of unsaturation of organic compound, the higher the degree, the more prone they are to oxidation

The presence of free fatty acids, the free carboxylic group enhances the reactivity

Temperature: increased oxidation occurs with increased temperature

Physical state of compound: solids are oxidised at a lesser rate than liquids. As in the presence of water, acid or bases are catalytic to oxidation reactions


How can oxidation be preventd

Exclusion of oxygen with product, via air tight packaging, but this is hard to do especially after opening

Adding antioxidants which more readily oxidises than the drug

Adding a reducing agent e,g, sodium metabisulfite

Diluting with an inert solvent e.g. Carbon tetrachloride, ethyl ether


What are some examples of antioxidants that may be added to prevent oxidation?

Ascorbic acid vitamin C
Tocopherols vitamin E
Butylated hydroxytoluene BHT
Butylated hydroxyanisole BHA


What increases the oxidation rate of vitamin C?

Even traces of copper increases the rate by a factor of 10,000


Why is vitamin C frequently added to food and pharmaceutical productS.

It is a cheap antioxidant


Why are storage conditions for vitamin C a non metal, absent from light and in a cool temperature container?

Because ascorbic acid is easily oxidised to form dehydroascorbate. The conditions of that container prevent extensive oxidation


What is isomerisation?

The transformation of a molecule into another which has exactly the same structural formula but different stereochemical configurations


What is racemisation?

The conversion of chiral molecules from the levorotory form to the dextrorotory form

Levo is pharmacologically more active than dextro


What order of reaction does racemisation usually follow?

First order kinetics,


What does racemisation depend on?



What is photolysis?

When light causes degradation of the drug.

Light energy can provide the activation energy required to start a reaction. Molecules must be exposed to radiation of proper frequency and absorb sufficient energy for a reaction to happen.


What is the relationship between light and heat energy?

Photochemical reactions are independent of temperature. But once molecules have absorbed enough radiant energy collisions will occur and heat energy will be generated for further reactions to take place

Heat and light are linked but independent with regards to activation of reaction


What is photolysis independent of?



What kind of order does photolysis reactions follow?

Zero order reaction


What is epimerisation?

The conversion of chiral molecules from one non superposable, non mirror images to the other


How can photolysis be prevented?

By packaging drugs in medicine amber brown glass containers to protect the product


What are excipients?

Substances that are inactive themselves, but aid in the formulation of a suitable dosage form


How can excipients effect how the drug works?

They can introduce moisture or acid base catalyst by affecting drug stability
Interact chemically with drug and alter bioavailability
Bring about a physical change to the drug characteristics (change in MP, polymorphic form etc)

Perform drug e


How can we prevent drug decomposition by excipients?

Perform a drug excipient compatibility study at accelerated conditions