THE SCIENTIFIC METHOD USE IN CHEMISTRY BASICS

Science differs from other fields of study in the method that scientists use to acquire knowledge and the special significance of this knowledge. Scientific knowledge can be used to explain natural phenomena and, at times, to predict future events.

The ancient Greeks developed some powerful methods of acquiring knowledge, particularly in mathematics. The Greek approach was to start with certain basic assumptions, or premises. Then, by the method known as deduction, certain conclusions must logically follow.

For example, if and then Deduction alone is not enough for obtaining scientific knowledge, however. The Greek philosopher Aristotle assumed four fundamental substances: air, earth, water, and fire. All other materials, he believed, were formed by combinations of these four elements.

Chemists of several centuries ago (more commonly referred to as alchemists) tried, in vain, to apply the four-element idea to turn lead into gold. They failed for many reasons, one being that the four-element assumption is false.

The scientific method originated in the seventeenth century with such people as Galileo, Francis Bacon, Robert Boyle, and Isaac Newton. The key to the method is to make no initial assumptions, but rather to make careful observations of natural phenomena.

When enough observations have been made so that a pattern begins to emerge, a generalization or natural law can be formulated describing the phenomenon. Natural laws are concise statements, often in mathematical form, about natural phenomena.

The form of reasoning in which a general statement or natural law is inferred from a set of observations is called induction. For example, early in the sixteenth century, the Polish astronomer Nicolas Copernicus (1473 1543), through careful study of astronomical observations, concluded that Earth revolves around the sun in a circular orbit, although the general teaching of the time, not based on scientific study, was that the sun and other heavenly bodies revolved around Earth.

We can think of Copernicus s statement as a natural law. Another example of a natural law is the radioactive decay law, which dictates how long it takes for a radioactive substance to lose its radioactivity.

The success of a natural law depends on its ability to explain, or account for, observations and to predict new phenomena. Copernicus s work was a great success because he was able to predict future positions of the planets more accurately than his contemporaries.

We should not think of a natural law as an absolute truth, however. Future experiments may require us to modify the law. For example, Copernicus s ideas were refined a half-century later by Johannes Kepler, who showed that planets travel in elliptical, not circular, orbits. To verify a natural law, a scientist designs experiments that show whether the conclusions deduced from the natural law are supported by experimental results.

A hypothesis is a tentative explanation of a natural law. If a hypothesis survives testing by experiments, it is often referred to as a theory. In a broader sense, a theory is a model or way of looking at nature that can be used to explain natural laws and make further predictions about natural phenomena.

When differing or conflicting theories are proposed, the one that is most successful in its predictions is generally chosen. Also, the theory that involves the smallest number of assumptions the simplest theory is preferred. Over time, as new evidence accumulates, most scientific theories undergo modification,
and some are discarded.

The scientific method is the combination of observation, experimentation, and the formulation of laws, hypotheses, and theories. Scientists may develop a pattern of thinking about their field, known as a paradigm.

Some paradigms may be successful at first but then become less so. When that happens, a new paradigm may be needed or, as is sometimes said, a paradigm shift occurs. In a way, the method of inquiry that we call the scientific method is itself a paradigm, and some people feel that it, too, is in need of change.

In any case, merely following a set of procedures, rather like using a cookbook, will not guarantee scientific success. Another factor in scientific discovery is chance, or serendipity. Many discoveries have been made by accident.

For example, in 1839, the American inventor Charles Goodyear was searching for a treatment for natural rubber that would make it less brittle when cold and less tacky when warm. In the course of this work, he accidentally spilled a rubber sulfur mixture on a hot stove and found that the resulting product had exactly the properties he was seeking.

Other chance discoveries include X-rays, radioactivity, and penicillin. So scientists and inventors always need to be alert to unexpected observations. Perhaps no one was more aware of this than Louis Pasteur, who wrote, Chance favors the prepared mind.

FRIEDEL-CRAFTS REACTIONS BASIC INFORMATION

Several chemicals are manufactured by application of the Friedel-Crafts condensation reaction. Efficient operation of any such process depends on:

1. The preparation and handling of reactants
2. The design and construction of the apparatus
3. The control of the reaction so as to lead practically exclusively to the formation of the specific products desired
4. The storage of the catalyst (aluminum chloride)

Several of the starting reactants, such as acid anhydrides, acid chlorides, and alkyl halides, are susceptible to hydrolysis. The absorption of moisture by these chemicals results in the production of compounds that are less active, require more aluminum chloride for condensation, and generally lead to lower yields of desired product.

Furthermore, the ingress of moisture into storage containers for these active components usually results in corrosion problems.

Anhydrous aluminum chloride needs to be stored in iron drums under conditions that ensure the absence of moisture. When, however, moisture contacts the aluminum chloride, hydrogen chloride is formed, the quantity of hydrogen chloride thus formed depends on the amount of water and the degree of agitation of the halide.

If sufficient moisture is present, particularly in the free space in the container or reaction vessel or at the point of contact with the outside atmosphere, then hydrochloric acid is formed and leads to corrosion of the storage container.

In certain reactions, such as the isomerization of butane and the alkylation of isoparaffins, problems of handling hydrogen chloride and acidic sludge are encountered. The corrosive action of the aluminum chloride–hydrocarbon complex, particularly at 70 to 100oC, has long been recognized and various reactor liners have been found satisfactory.

The rate of reaction is a function of the efficiency of the contact between the reactants, i.e., stirring mechanism and mixing of the reactants. In fact, mixing efficiency has a vital influence on the yield and purity of the product. Insufficient or inefficient mixing may lead to uncondensed reactants or to excessive reaction on heated surfaces.