Looking for revision notes that are specific to the exam board you are studying? If so, click the links below to view our condensed, easy-to-understand revision notes for each exam board, practice exam question booklets, mindmap visual aids, interactive quizzes, PowerPoint presentations and a library of past papers directly from the exam boards.

Role of chloroalkanes in Ozone Layer depletion

Summary

  • Chloroalkanes are organic compounds containing carbon and chlorine atoms. 
  • These organic compounds pose serious threat to our lovely planet.
  • They are responsible for ozone depletion.
  • The chemical reaction for ozone depletion is catalyzed by UV radiations.
  • Comprehensive efforts are in place now to combat emission of these chloroalkanes

Montreal protocol has been a great effort to cut down usage of these compounds.

1. What are chloroalkanes?

Chloroalkanes are group of alkanes containing chloro group attached to the carbon atom. They may be mono, di, tri or tetrachloro substituted compounds. In simple words, we call them alkyl halides. Their simple chemical structure can be shown as 

These organic compounds undergo wide range of chemical reactions with other reagents. For example, alkyl halides can follow nucleophilic substitution reactions to make series of other organic compounds. Another good example is Grignard reagent employed to make carbon-carbon bond and increase the chain length.

Haloalkanes in general and chloroalkanes in particular gained tremendous attention during the last three decades. They are held responsible for the depletion of ozone layer in our atmosphere. This article highlights some of the features of chloroalkanes and their role in ozone depletion.

2. The atmosphere of our earth

Our Earth has various layers of gases that surround the planet earth. They are actually mixture of gases known as air. Collectively these layers are called the atmosphere of earth. These different layers are illustrated below

The first layer of troposphere contains bulk of mass of our atmosphere. The second is  stratosphere where ozone depletion takes place. This is the mainly important layer where most of the light induced chemical reactions take place. The occurrence of these chemical reactions and presence of ozone is the important point of discussion for global warming, climate change and ultraviolet layer dangers. 

3 Ozone and ozone layer

Ozone is a triatomic molecule of oxygen with the formula of O3. It is the most important component of stratosphere where it is responsible to absorb harmful ultraviolet (UV) radiations of sun. Thus ozone plays a vital role of protecting our planet from dangerous radiations coming from sun. In atmosphere, it is made by dioxygen (O2) by the action of UV light and electrical discharges within the atmosphere. Its formation is an endothermic reaction. Ozone is a powerful oxidizing agent and may react with various inorganic and organic substances. Beside its role in atmospheric science, ozone can be used as a chemical reagent in synthesis, potable water purification, as a disinfectant in sewage treatment, and for the bleaching of natural fibers. It is much more powerful oxidizing agent than ordinary oxygen (O2). 

Ozone came to attention first time in 1840s. Later on its role in atmospheric science came to light. Measurement of ozone in atmosphere started in early 1920s. Its concentration in atmosphere is reported by using Dobson units or point measurements are reported in parts per billion (ppb). Stratosphere or ozone layer lives around 50km above the earth surface and though it is called ozone layer, the concentration of ozone is significantly low. Mostly oxygen is present which under UV radiations is converted to ozone. 

3.1.1 Importance of ozone layer & its depletion

Ozone has quite strange and confusing role in atmosphere. In the lower layer of atmosphere (troposphere), ozone is actually a pollutant and may cause smog formation posing significant danger to humans and other living animals. If present in concentration above 0.1ppm level, it may cause serious respiratory problems to humans. However, as this ozone is pollutant at ground level, same ozone is beneficial at upper levels of stratosphere where it is responsible to capture major part of the sun’s ultraviolet radiation and prevent these radiations from reaching the earth surface. Hence it plays exceptionally fundamental part in our life on earth. Without a protective ozone layer in the atmosphere, our life, animals and plants could not exist upon earth.

With all its importance for us and our lovely planet earth, ozone layer has been under threat. In 1970s, the first study highlighted the decreased concentration of ozone in stratosphere. This decrease in amount of ozone in atmosphere is called ozone depletion. During that time, halogenated compounds particularly chloromethane, fluoromethane and other chlorofluoro hydrocarbons were described as responsible for this ozone depletion. In the early twentieth century, these chlorinated (or halogenated) alkanes were used for refrigeration and air conditioning systems. These compounds are known to be volatile and chemically inert and due to these characteristics, these chlorofluoro carbon compunds (CFCs) had been widely used.

4. Role of chloroalkanes in ozone depletion

Chloroalkanes are discharged into atmosphere because of their continuous use in organic synthesis and other industrial processes. They are hydrocarbons containing carbon, hydrogen, chlorine (or fluorine or bromine for other halogenated hydrocarbons). The alkyl chain mainly comes from paraffin and small molecules are made using methane, ethane or propane. Some of the commonly known chloroalkanes are

The industrial usage of these halogenated compounds has been found as a source of chlorine into the atmosphere. Interestingly, very little chlorine exists naturally in the atmosphere. However, the industrial usage of these chlorinated alkanes can introduce chlorine into the atmosphere. The ultraviolet radiation at this altitude breaks down these compounds, thus generating highly reactive chlorine atoms. These chlorine atoms may react with ozone through a variety of catalytic reactions. For example, a chlorine atom may react with an ozone molecule (O3), taking an oxygen atom to form chlorine monoxide (ClO) and leaving an oxygen molecule (O2). The ClO can then react with a second molecule of ozone, releasing the chlorine atom and yielding two molecules of oxygen. This reaction can be presented as 

The reactive chlorine atom formed in step (B) is regenerated and thus available to enter into another cycle. The overall impact is a decrease in the amount of ozone. The reaction scheme above is a simple illustration of complex and complicated chemical reactions taking place in atmosphere. Researchers have discovered more complicated mechanisms that lead to ozone destruction. 

A single chlorine atom is able to react with thousands of ozone molecules. This fact and the amount of chlorine continuously being released into atmosphere yearly demonstrate the danger to ozone layer and our planet. Eventually, this chlorine has the potential to destroy large amounts of ozone. This has indeed been observed, especially over Antarctica. As a consequence, levels of harmful ultraviolet radiation have increased.

5. Impact on humans

Ozone depletion has posed serious concerns to the human race and other living organisms (both animals & plants). Increased UV radiation may influence human health, both positively (including production of vitamin D) and negatively (including sunburn, skin cancer, and cataracts). We all know that increased exposure to harmful UV radiations may increase chances of skin cancer. The mechanism involves degradation of DNA. 

Melanoma is another form of skin cancer. It is far less in frequency, however, more dangerous. The correlation of melanoma to UV radiation is difficult to establish, however, the studies have reported that both UVA and UVB radiation are involved in accelerating this problem. 

Increased UV radiations are also hammering animal lives. Recently British scientists have reported sun damage to whales on pacific costal area of USA. They blame ozone depletion for this sun damage. Similarly crops and plants are under threat to increased UV exposure. Economical crops require microorganism to survive and offer production. However some microorganisms e.g. bacteria are very sensitive to UV radiations. 

In short, ozone depletion is posing serious threat to the whole eco system of our planet. Rapid & uncontrolled industrialization is to blame for this. Humans are producing significant volumes of dangerous chloroalkanes and causing massive damage to our planet.

6. Where do we stand now?

The seriousness of ozone depletion has made headlines across the globe. Public awareness has been raised and pressure has been on politicians and policy makers to design a strategy for this problem. Montreal protocol has been a significant achievement in this regard. The agreement was signed in 1987. It was agreed to phase out halogenated alkanes to cut down chlorine emission. Results demonstrate that this has been a successful agreement. Since then ozone depletion in atmosphere has decreased. Moreover, in 2019, NASA announced the "ozone hole" was the smallest ever since it was first discovered in 1982. We hope, ozone layer repair and recover with the decreased use of chloroalkanes. 

Reference

  1. Kirschner KM. Ozone, in "Ullmann's encyclopedia of industrial chemistry." (1988).
  2. Chlorofluorocarbons and Ozone Depletion, A National Historic Chemical Landmark: American Chemical Society page: Accessed on 10-11-19. https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/cfcs-ozone.html
  3. The Ozone Hole, accessed online: 10-11-19. http://www.theozonehole.com/index.htm 
  4. Cary, A, F. Organic Chemistry, 3rd edition, (1996). 
  5. Volhardt K, P, C. Organic Chemistry, (1987).
  6. Ozone Hole Above Antarctica Shrinks to Smallest Size on Record: Accessed online on 10-11-19 https://www.wsj.com/articles/ozone-hole-above-antarctica-shrinks-to-smallest-size-on-record-11571847944