Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 15. Ultraviolet Spectroscopy.
Editorial CommentsOzone, a gas composed of three atoms of oxygen arranged as a bent structure, is found mostly in the earth's stratosphere and thus referred to as the ozone layer. Ozone absorbs ultraviolet radiation. A form known as UV-B can directly cause damage to organisms by inducing gene mutations and causing photochemical changes in the DNA of humans. One of the specific mutations on DNA is the formation of pyrimidine bonds on the strands of the double helix of DNA. Chronic exposure to UV-B can cause the development of cataracts, depression of the human immune system, skin cancer, and "snow blindness" (the effect of light rays reflecting off of snow and burning the eyes causing temporary blindness) These symptoms can occur regardless of skin color, skin type, or use of sunscreen.
Ozone depletion is an unusual process. It will not take place unless certain conditions are met. The basic explanation of ozone depletion is that chlorofluorocarbons (CFCs), a compound commonly used as refrigerants, solvents, and foam blowing agents, are released into the atmosphere by factories and homes. Sunlight breaks down the CFCs in the stratosphere, and the broken down products destroy the ozone. In the past, CFC's were used because they were chemically inert and stable. These same qualities now make it difficult to destroy them once they are in the atmosphere. Photochemical destruction had been found as the only natural way to rid the atmosphere of CFC's.
A 1% drop in ozone produces a 4-6% increase in cancer. An estimation of a 10% depletion in ozone will cause 2 million new cases of cataracts per year globally. (World Health Organization) This issue affects everyone on earth. The seriousness of the health risks could be enough alone to cause concern and create a need for political action. The rate at which the ozone is depleting is a foreshadowing of future environmental and health problems.Pertinent Text References
Question 1: Why is the majority of ozone depletion occurring over the Arctic? Does this create a larger problem than if the depletion was occurring in another region?
A. The polar winter leads to the formation of the polar vortex which isolates the air within it. Cold temperatures form inside the vortex: cold enough for the formation of Polar Stratospheric Clouds (PSCs). As the vortex air is isolated, the cold temperatures and the PScs persist. Once the PSCs form, heterogeneous reactions take place and convert the inactive chlorine and bromine reservoirs to more active forms of chlorine and bromine. No ozone loss occurs until sunlight returns to the air inside the polar vortex and allows the production of active chlorine and initiates the catalytic ozone destruction cycles.
It would be more dangerous if the ozone depletion was occuring in another region because ultraviolet radiation would have more direct contact with the inhabitants of the region. However, researchers are still worried that the depletion in the Arctic will have an affect on the northern continents.
Question 2: What is the chemical process for the decomposition of O3 (ozone)?
A. (In a circular diagram) O3 plus hv goes to O2 +O which is then converted back into O3 with the loss of heat. (3/2 O2 +34 kcal goes to O3)
Question 3: Show the products from the process know as ozonolysis, which produces ketones and aldehydes. (Refer to Chapter 8 in Organic Chemistry, by L.G. Wade, Jr.)
A. Please refer to p. 364 of the text by Wade, 4/e.
Question 4: What is the bond order for each oxygen-oxygen bond in ozone? Illustrate the resonance structures and bond angles.
A. The single bond has a bond order of 1. The double bond has a bond order of 2. The overall bond order is 3/2. The bond angle is 116.8 degrees. The structure is bent and the resonance forms consist of the swapping of the double and single bonds [(-)O-(+)O=0 and O=O(+)-O(-)].
Question 5: Chlorine atoms are thought to lead to the destruction of the earth's ozone layer by the following sequences of reactions:
Cl + O3 -> ClO +O2 ClO + O -> Cl + O2where the oxygen atoms in the second step come from the decomposition of ozone by sunlight.
O3(g) <-> O(g) + O2(g)What is the net equation on summing these three equations? What name is given to species such as ClO?
A. 2 O3 (g) goes to 3 O2 (g). Ozone decomposes to oxygen molecules, resulting in a net loss of ozone in the stratosphere. Cl is a catalyst, and ClO is an intermediate.
Question 6: What daily activities of students contribute to ozone depletion? What control do we have in the prevention of further depletion?