The International Union of Pure and Applied Chemistry’s (IUPAC) definition of a chemical reaction is “a process that results in the interconversion of chemical species.” A chemical species is “an ensemble of the chemical elements which is identifiable as a separately distinguishable entity.” The transformation of reacting species into a new chemical product can be likened conceptually to events that occur in everyday life—it involves the interaction of objects (molecules, atoms, ions) with each other, and the outcome of the process is governed by their relative sizes, positions, and energies. A key has to be inserted correctly into a lock for it to be opened, and an egg’s shell can be broken only by the application of sufficient force. The relation of these ideas will become clearer as the fundamental processes governing a chemical reaction are examined.
A discussion of chemical reactions in the context of time is highly appropriate: They have occurred since the dawn of time, they are both controlled by time, and, given the huge range of timescales on which reactions can take place, for example, the rusting of an iron nail compared to dynamite exploding, they exert an influence over the course of time. The history of chemical reactions in time is discussed in a manner complementary to the discovery of the chemical elements, although the two are very closely linked. This entry gives detailed consideration to the timescale on which chemical reactions occur: the very fast; those that are easier for human beings to contemplate; and the very slow. This highlights the technical ingenuity of scientists who have developed many sophisticated techniques to study ultrafast reactions.
History of Chemical Reactions
The world as we know it, indeed the entire universe, is composed of the chemical elements. The 94 known chemical elements (there are more in the periodic table, but they do not occur naturally) were formed shortly after the start of time as we know it—the big bang—and have been combining with each other ever since. Shortly after the big bang, extreme temperatures are believed to have favored the fusion of the separate neutrons and protons present, forming helium and deuterium nuclei, in a process termed big bang nucleosynthesis. These positively charged nuclei subsequently combined with the negatively charged primordial particles present, electrons, to produce the first atoms, of which most were hydrogen. The extreme conditions generated by the production of the first stars are believed to have created heavier elements through the processes of stellar and supernova nucleosynthesis, and cosmic ray spallation. The first chemical reactions would have now occurred, with simple chemical entities based on light atoms such as carbon, nitrogen, and oxygen produced in abundance.
The formation of the earth (c. 4.6 billion years ago) was followed by the Hadean eon, where the earth began to take shape. Changes in the chemical composition of the atmosphere, afforded by volcanic eruptions, the cooling of the earth’s surface, and asteroid bombardment, have been proposed to have been instrumental in the development of life. The generation of small organic molecules, known to be monomeric constituents of living organisms from the most basic chemical species, is propounded as one of the most widely accepted theories of the origin of life. The combination of a-amino acid molecules to form polypeptide chains; the similar production of nucleotides, phosphates, and sugars could, theoretically, react together in such a way as to give the double-helix of life, DNA. The classic Miller-Urey experiment exposed a mixture of simple gases and water vapor to an electrical discharge (to mimic lightning), and the ensuing chemical reactions were indeed found to produce simple biomolecules that are essential to life. An early earth atmosphere rich in ammonia, methane, water, carbon dioxide, and nitrogen could have been transformed chemically to give a so-called primordial soup.
The suggested transformation of simple biomolecules into the polymeric constituents of life, and their organization to give a living cell, remains a controversial theory. Such an event would mark the cornerstone of evolution and of the history of the earth. The first simple single-celled organisms were all likely to have been heterotrophic, requiring relatively complex organic substrates, for example, simple sugars, to be sustained and to reproduce. It has been proposed that such organisms diverged as a process of evolution to produce autotrophic species, which were able to use an alternative source of energy and were subsequently able to transform the most basic chemical precursors into much more complex ones. The most important example of autotrophic chemical reactions is photosynthesis: the conversion of water and carbon dioxide driven by sunlight into carbohydrates. Photosynthesis is one of the most important natural chemical reactions known to humankind, and the challenge of reproducing it satisfactorily within the laboratory has still not been met.
Profound environmental changes occurred as a result of the production of oxygen. Initially, oxygen dissolved in the oceans and combined with oxophilic elements to produce numerous oxides such as those of the common metals, iron in particular. Over time, its concentration in the atmosphere gradually increased, with far-reaching consequences for the diversity of the living organisms that, until then, existed in anaerobic conditions. Concomitantly, the conversion of oxygen (O2) to ozone (O3) in the upper atmosphere by ultraviolet light formed a protective layer around the earth that actually reduced the amount of damaging ultraviolet radiation reaching the earth’s surface. A popular notion is that this could have safeguarded the development of simple organisms that were then given the chance to evolve in an increasingly aerobic environment. Moreover, those organisms that did not evolve to be able to assimilate oxygen were largely wiped out by the increasingly toxic environment. Today, the earth is finely balanced by cycles of respiration (effectively the production of energy by oxidation of relatively complex carbon-rich organic molecules producing carbon dioxide and water) and the reverse of this process, photosynthesis. The energy produced by respiration provides the driving force for the conversion of very unreactive nitrogen gas into soluble nitrogen compounds. The combination of photosynthesis, respiration, and the fixation of nitrogen forms the basis of life itself.
The discovery and exploitation of fossil fuels as an energy source by humankind was almost certainly the next most significant way chemical reactions would alter the environment of the planet. Reserves of coal, gas, and oil were formed from decaying organic matter by very slow chemical reactions, but comparatively recently given the estimated age of the planet. Combustion of fossil fuels for heating, driving industrial processes, and travel has been held responsible for the rising levels of carbon dioxide in the atmosphere and is claimed to be a major contributor to global warming.
The commercialization of chemical reactions occurred as an inevitable part of the Industrial Revolution. New raw materials such as sulfuric acid (the production of which is thought to be the first chemical process to be performed on an industrial scale), soda (sodium carbonate), and caustic (sodium hydroxide) were required to support many thriving industries including glassmaking, soap production, and cotton bleaching in the 19th century. It is appropriate to mention that many of these chemicals now being produced on a grand scale had their origins in the alchemical era. The accidental discovery of mauvine (an intensely colored purple dye) during an attempted synthesis of the antimalarial quinine by William Perkin in 1856 was the catalyst for the generation of the modern- day chemical industry. Existing chemical companies such as Imperial Chemical Industries (ICI) and Bayer began to diversify into photography, food, pharmaceuticals, and explosives. The improvements in scientific understanding during this period drove the desire to better understand chemical reactions and the structures of molecules; both academic and industrial chemical research have since flourished. Our current level of understanding means we are able to design and control chemical reactions to provide us with food, health, fashion, and entertainment.
The Timescale and Study
of Chemical Reactions
The rate at which a chemical reaction proceeds is studied by reaction kinetics. The timescale on which a chemical reaction can occur ranges from the very slow, taking days or years to complete, to the ultrafast, of the order 10-15 seconds (one quadrillionth of a second). The majority of chemical reactions require a millisecond (one thousandth of a second) or less. For a chemical reaction to take place, the reacting molecules must collide with one another in a favorable orientation to overcome the energetic barrier to reaction: the activation energy to form the activated complex (or transition state). This represents the highest energy point on the reaction pathway (rather like the top of a mountain); it will then rapidly collapse, with the products of the reaction tumbling downhill, to either the unchanged starting species or the new product(s).
A catalyst lowers the activation energy in forming the activated complex, meaning more of the reacting entities will have the energy required to react on collision. The presence of a catalyst therefore increases the overall rate of reaction. Enzymes are nature’s catalysts, and numerous manmade materials are employed in industrial processes in an attempt to compromise between making the speed of a reaction reasonable without making the process economically unfeasible.
In a kinetic study, it is conceptually useful to imagine the whole reaction process being photographed from start to finish. Each “snapshot” represents a different stage in the reaction pathway, and, depending on the rate of reaction, it may be possible to view the discrete stages of the reaction.
The scientific investigation of reaction kinetics has always presented a challenge to chemists. Classical investigations (which predate 1900) of reaction kinetics could be performed only on very slow reactions, as the progress of the reaction had to be followed by some physically variable property that was perceptible to human beings, such as a color change or the evolution of a gas. The change in such a property was monitored over time, producing a relationship between the progression of the reaction and the time elapsed, which could then be used to determine the reaction rate. The shortest observable reactions were of the order of seconds. The development of rapid mixing techniques early in the 20th century allowed so-called stopped flow methods to probe reactions of the order of milliseconds. A significant problem encountered using this technique was that, for very fast reactions, efficient mixing takes an infinitely long time compared to the speed of the reaction itself. Therefore, the reactions would have already taken place before the reacting species had mixed properly, giving inaccurate results. Recent improvements in instrumentation have meant that reactions on the microsecond timescale can now be studied.
Scientists recognized early on that new techniques would have to be developed for the study of faster reactions. Manfred Eigen, Ronald Norrish, and George Porter were co-recipients of the 1967 Nobel Prize in Chemistry for their development of flash photolysis in 1949: a pump-probe technique that could interrogate reactions of the order of microseconds. A chemical sample was irradiated using a very short burst of intense light (from a very powerful flashlight) to produce a reactive (and most often short lived) intermediate, the subsequent decay of which by, for example, chemical decomposition or the reemission of light, was monitored spectroscopically.
With the advent of lasers in the 1960s, the shutter speeds of our notional “camera” could be increased substantially, and the pico- (one millionth of one millionth of a second) followed by the femtosecond regimes were now accessible.
Chemical reactions are also concerned with reaction dynamics: changes to the molecular architecture, such as the perturbations in bond lengths and angles that occur as a reaction is taking place. As understanding of chemical reactions improved over the 20th century, scientists became much more ambitious and began to explore the possibility of probing extremely fast reactions, or, more specifically, the direct observation of the shortest lived species known in chemistry: the activated complex. The reorganization of electrons in chemical bonds and the nuclei of the constituent atoms over atomic dimensions (a vibration of a chemical bond) is of the order of femtoseconds (10-15 seconds). Thus, to achieve resolution of the activated complex, the shutter speed of our “camera” would have to be around 10 to 100 femtoseconds. Ahmed Zewail pioneered the experimental techniques for the exploration of chemical reactions into the femtosecond regime (with the main technological requirement being sufficiently short exciting laser pulses) for which he was awarded the Nobel Prize for Chemistry in 1999. Femtochemistry has now diversified and been rebranded in many branches of science. Femtobiology has been applied to fundamental biochemical processes such as vision, protein dynamics, photosynthesis, and proton and electron transfer. The understanding of early femtosecond events in a protein environment is important for understanding the function of these systems.
Additional spectroscopic techniques have been developed during the 21st century, not with the aim of studying kinetics of chemical reactions, but to exploit the different timescales of molecular processes that do not necessarily lead to chemical reactions but are more suited to probe reaction dynamics. The Heisenberg uncertainty principle stipulates that, in measuring the position and momentum of an object, the more accurately one property is determined, it follows that the other becomes less accurately determined. Therefore, as a molecule of a particular energy is less well defined, its lifetime will be increased.
Since different molecular architectures have different energies, improvements in spectroscopic techniques that have allowed the changes in energy between molecular states to be very accurately measured can be used to determine how molecular properties such as the distance between two nuclei change over the course of a chemical reaction. A large difference in energy between two different molecules allows them to be distinguished even if they interchange very quickly. High-frequency techniques such as ultraviolet visible (10-15 s) and infrared spectroscopy (10-14 s) are easily able to distinguish between molecular states that are blurred to electron- (10-4-10-8 s) and nuclear-spin resonance (10-1-10-9 s) probes.
There is much speculation as to how the earth and even the universe will come to an end, if indeed they will. Humankind’s negative influence upon the environment as a result of chemical reactions is only too apparent. The depletion of ozone in the stratosphere caused by the photochemical degradation of CFCs (chlorofluorocarbons); the emission of greenhouse gases; and the poisoning of terrestrial land and water systems promises a bleak future for the earth. It is possible that an increasingly toxic and humid environment will make life on the planet unsustainable.
Looking even farther into the future, in a climax of destruction, all objects created by chemical reactions could disintegrate under the influence of the continually expanding universe into radiation and elementary particles that will move away from each other—an awesome contradiction to the formation of the first elements and their ensuing chemical reactions at the dawn of time.
See also Aging; Attosecond and Nanosecond; Big Bang Theory; Chemistry; Decay, Organic; DNA; Dying and Death; Evolution, Chemical; Global Warming; Life, Origin of; Oparin, A. I.; Photosynthesis; Thanatochemistry
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