Tuesday, September 26, 2006

Ozone Depletion (Alison)

It has been known for a long time that the depletion of the ozone layer is accelerated by the use of chlorofluorocarbons, or CFC's. This reaction is cyclical, as it leaves a chlorine radical in the 6th step of halogenation to destroy more ozone molecules. This can be seen in the reaction below:

The reaction begins when UV light strikes a CFC, creating a carbon radical and a chlorine radical. The chlorine radical then collides with an ozone molecule, removes an oxygen atom, and forms chlorine monoxide and an ordinary oxygen molecule. A free oxygen atom then collides with chlorine monoxide, thus creating an oxygen molecule and freeing the chlorine radical to destroy more ozone molecules.

Beyond the obvious environmental effects of this, there are many health effects which have also been documented. The article I read focused on these health effects, mainly skin cancer, cataracts, and immune deficiencies.

Click here for CiteULike reference.

Click here for full-text of article.

Biotinylation (Elizabeth)

Biotinylation is a reaction that is utilized in numerous biochemical experiments and processes. The value of biotinylation lies in the fact that biotin, a B-complex vitamin, forms strong bonds with streptavidin. This biotin-streptavidin complex can then be linked to numerous other biomolecules that function in various applications. Biotin is also readily available (it can be purchased from many different sources) to be used in these applications. The image shown here depicts the biotinylation of spermine, a polyamine that functions in many metabolic processes in eukaryotic cells. Jeon, Kim, Shin et al. utilized this reaction in research that they discussed their paper Differential incorporation of biotinylated polyamines by transglutaminase2 . The biotinylated spermine was used in this experiment to learn more about the intracellular actions of an enzyme, transglutaminase 2. It is believed that this enzyme may cause cell adhesions and apoptosis (cell death). Spermine served as a substrate for transglutaminase 2 so that researchers could better understand how this enzyme functions.

Researchers performed the pictured reaction by combining N, N –di-TFA spermine 2, biotin, 1,3-dicyclohexylcarbodiimide, and 1-hydroxybenzotriazole in anhydrous dimethylformamide (DMF). These substances were combined in a flask and stirred for twenty-four hours at room temperature.

One clinical application of biotinylation is seen in a new wound dressing that was recently created by a German laboratory. This bandage has insulin-like growth factor-1 (IGF-1) attached to the dressing material via biotinylation. The IGF-1 is bound to a biotin-steptavidin-biotin complex, which is connected to amines on the polyurethane gauze. IGF-1 is a hormone, similar in structure to insulin, that stimulates cell growth and reproduction. It is therefore beneficial to use this unique dressing on patients with severe wounds or ulcers, as it will promote healing and epithelial tissue regeneration.

Friday, June 09, 2006

Variation of the Claisen Condensation

Researchers at Kwansei Gakuin University developed a better way of synthesizing β-keto esters from esters and acid chlorides as well as from esters and carboxylic acids. It is based on the Ti-self-Claisen condensation performed in a previous study. The new method is a Ti-crossed-Claisen condensation with a high yield of the desired product and a low yield of the undesired self-condensed reaction. In the previous study, the catalyst used was TiCl4-Bu3N. In this study, a co-catalyst, N-methylimidazole, was used to prevent self-condensing in the ester/acid chloride reaction. During the reaction, the acid chloride condenses with the N-methylimidazole to form an electrophilic acylammonium intermediate. An example of the reaction is shown below:

In the reactions between esters and carboxylic acids, the researchers developed a new procedure using mixed anhydrides to react with the ester and create an intermediate. The mixed anhyrides are formed in situ between sodium carboxylates and Cl3CCOCl. An example is shown below:

In order to show that these new methods are easier and work better than previous procedures, the researchers synthesized cis-Jasmone and (R)-Muscone. The resulting yields were 46% and 53% respectively. This reaction is a useful new method particularly because products of Ti-crossed-Claisen condensation reactions are useful reagents for organic syntheses, especially those that involve asymmetric transformations.

More information is available here.

Sunday, May 07, 2006

Wittig Reaction (Thomas Dursch)

In the near future, perfection of Organic Light Emitting Diode (OLED) technology will create an alternative to LCD screens, revolutionizing the display market. Problems associated with the emission of blue light in OLEDs currently inhibit their mass production. According to Wikipedia.com, organic compounds that emit blue light have a lifespan of only 1,000 hours, whereas, the red and green emitting compounds have a lifespan of roughly 500,000 hours. Improvements upon the longevity of blue emitters will open a new door to display technology.

According to the article “Reliability and Degradation of Small Molecule-Based
Organic Light-Emitting Devices” in the IEEE Journal of Quantum Electronics, the cause of this limited lifespan is an undesirable reaction between oxygen molecules and the organic emitting layer. Due to the permeability of the glass substrate, oxygen molecules are able to diffuse into the organic emitting layer. During the operation of an OLED, singlet oxygen is formed due to the energy transfer from the excited organic molecules to oxygen molecules. The singlet oxygen molecules, known as radicals, then attack the structure of the organic compound eventually destroying the organic layer.

Currently, my freshman design group is working on a solution to lengthen the lifespan of the blue emitting compound. Since it is very difficult to find a blue emitting compound that does not react with oxygen at all, we decided to find a compound that reacts with oxygen, but extremely slowly. Our group is currently investigating a class of compounds known as the triarylamines, specifically, a triphenylamine-based conjugated polymer known as MPa (chosen due to MPa’s high thermal stability, low tg, and high quantum yield of nearly 64 %). In order to create MPa we must use the Wittig Reaction. According to Organic-chemistry.org, the Wittig Reaction allows the preparation of an alkene by the reaction of an aldehyde or ketone with the ylide generated from a phosphonium salt. MPa is created by a reaction of an aldehyde group found on PFT with benzyltriphenylphosphonium bromide and THF under N2. For a detailed description of the reaction please refer to “Macromolecules” by the Tokyo Institute of Technology.

Friday, May 05, 2006

Esterification Reaction of Phenylacetic acids

Esterification reactions are condensation reactions between alcohols and carboxylic acids in the presence of an acid catalyst to form esters. According to a Wikipedia article, esterification is commonly used in substances such as lotions or perfumes, as they tend to give off various scents and flavors. Phenylacetic acids are found in plants, as a type of hormone known as auxin, which is generally used in the plants’ fruits. Phenylacetic acids tend to possess a honey-like odor in smaller concentrations and may be used in perfumes. Esterification reactions of carboxylic acids create esters that are excellent solvents and can then be used in soaps and lotions (“Intro to General, Organic, and Biological Chemistry,” Solomon).
In an experimental setting, an esterification reaction, as shown above, was tested using a catalyst, cerium (IV) ammonium nitrate (CAN), and simple primary and secondary alcohols in order to increase the efficiency of basic esterification reactions. A link to this experiment can be found here.
The synthesis of aspirin, also known as acetylsalicylic acid, also uses an esterification reaction. For more information on the uses of aspirin, please see the class blog.

Monday, May 01, 2006

Aspirin - Acetylsalicylic Acid (Thomas Dursch)

Since the time of Hippocrates, there have been written records of pain relief treatments, including powder obtained from the insides of willow trees. According to From a Miracle Drug written by Sophie Jourdier for the Royal Society of Chemistry, “It was not long before the active ingredient in willow bark was isolated; in 1828, Johann Buchner, professor of pharmaceuticals at the University of Munich, isolated a tiny amount of bitter tasting yellow crystals, which he called salicin”.

According to Wikipedia, an online encyclopedia, salicin undergoes a “Kolbe-Schmitt reaction and the final product is an aromatic hydroxyl acid known as salicylic acid”. Minimally, what happens in a Kolbe-Schmitt reaction is a sodium phenolate is heated with carbon dioxide, then the final product is treated with sulfuric acid, eventually producing salicylic acid. Today, scientists recognize salicylic acid as being harmful when ingested. Instead of using salicylic acid, the commonly used pain reliever is acetylsalicylic acid, now known as aspirin. The newly discovered acetylsalicylic acid is what is known as an “acetyl ester” formed through a process known as esterification. According to Wikipedia, for this esterification, “a carboxylic acid reacts with the phenol group of salicylic acid to produce the acetyl ester”. Simply, an ester can be thought of as a product of a condensation reaction of an acid usually an organic acid and an alcohol or phenol.

Aspirin helps cure headaches, reduce fevers, and reduce swelling to minor injuries, and since the time of its discovery, has risen to one of the leading over-the-counter drugs. According to Paul May at the University of Bristol School of Chemistry, “each year, more than 40 million pounds of aspirin is produced in the United States alone, a rate that translates to about 300 tablets per year for every man, woman and child”.

To further supplement this information and for a complete reaction/experiment, please refer to the following link.

Saturday, April 29, 2006

Rigid-Rod Polyquinolines – the Creation of New Polymers (Zakiya)

Poly(2,2’-(p,p’-diphenylacetylene)-6,6’-bis(4-phenylquinoline) or PBAPQ is one of the group of polyquinolines used to create multilayer organic (thin film, polymer based) solar cells.

While now available commercially, this polymer was originally synthesized manually using the time intensive process described here and is an example of an enol/imine condensation.

For more information on how these polymers are used in organic solar cells click here.