The Scientific Method

Experimentation. Demonstrating the correctness of hypotheses and theories is at the heart of the scientific method. This is done by carrying out carefully designed experiments that will either support or disprove the theory or hypothesis.

Characteristics of the scientific process include the following:

1) The scientific method is a systematic approach to the discovery of new information.

2) Formulation of a question. Humankind’s fundamental curiosity motivates questions of why and how things work.

3) Summarizing information. A scientific law is nothing more than the summary of a large quantity of information. For example, the law of conservation of matter states that matter cannot be created or destroyed, only converted from one form to another. This statement represents a massive body of chemical information gathered from experiments.

4) How do we learn about the properties of matter, the way it behaves in nature, and how it can be modified to make useful products? Chemists do this by using the scientific method to study the way in which matter changes under carefully controlled conditions. The scientific method is not a “cookbook recipe” that, if followed faithfully, will yield new discoveries; rather, it is an organized approach to solving scientific problems. Every scientist brings his or her own curiosity, creativity, and imagination to scientific study. But scientific inquiry still involves some of the “cookbook approach.”

5) Pattern recognition. If a scientist finds a cause-and-effect relationship, it may be the basis of a generalized explanation of substances and their behavior.

Developing theories. When scientists observe a phenomenon, they want to explain it. The process of explaining observed behavior begins with a hypothesis. A hypothesis is simply an attempt to explain an observation, or series of observations, in a commonsense way. If many experiments support a hypothesis, it may attain the status of a theory. A theory is a hypothesis supported by extensive testing (experimentation) that explains scientific facts and can predict new facts.

6) Observation. The description of, for example, the color, taste, or odor of a substance is a result of observation. The measurement of the temperature of a liquid or the size or mass of a solid results from observation.

Tell about your scientific research.

Listening

1. XCulture clips: London life

Welcome to London Life. This time, we find out about London's science museum. It is one of London's most popular attractions. During the programme, we look at some useful science vocabulary. Write down at least 5 questions that

you think or hope will be answered by the programme.As you listen, try to answer the following questions:

1: What kinds of things are children most interested in?

2: What is William's favorite thing at the museum?

2.What museums are there in your city/town? Have you ever visited any?

3.Have you ever visited science museum of the “KPI”? Are there any in your university? Imagine that you are a guide at such museum, tell about the most interesting museum piece.

Writing

1. Imagine that you are Margarita. This is a part of the letter that your friend has sent you. Read it, then look at Business English section and write him/her a letter giving your advice. Use the following useful expressions and plan.

Dear Margarita,

I’ve just found out that I’ve failed to pass organic chemistry. I haven’t told my parents yet because I’m too scared. The most difficult for me is the topic “Bonding and structure”. I’ll have to retake the exams, but I know I’ll fail again! What can I do? Please help me!

Useful expressions:

Start with: I just got your letter and I think I can help you. / I was sorry to hear about your problem.

Giving advice: If I were you…, You should…, Why don’t you…, It would be a good idea to…, The best thing you can do is…, I strongly advise you to…

Finish with: I hope this helps you./ Let me know what happens./ Hope this advice is of some help to you./ Things will get better soon.

Plan

Introduction

Dear (the persons first name)

Paragraph 1: express sympathy

Main body

Paragraph 2: give your advice (tell parents…)

Paragraph 3: give your advice how to master the topic

Conclusion

Paragraph 4: end the letter offering some encouragement

Good luck

Margarita

UNIT 2

PRINCIPLES, NOMENCLATURE & SYMBOLS

Reading

Section A

1. Scan the text, find and read the definitions of the following terms:

Stereochemistry, stereoisomer, enantiomer, chiral, achiral, stereogenic center.

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The Greek word stereos means “solid,” and stereochemistry refers to chemistry in three dimensions. The foundations of organic stereochemistry were laid by Jacobusvan’t Hoff and Joseph Achille Le Bel in 1874. Independently of each other, van’t Hoff and Le Bel proposed that the four bonds to carbon were directed toward the corners of a tetrahedron. One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms in space is different. Isomers that have the same constitution but differ in the spatial arrangement of their atoms are called stereoisomers. We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns in alkenes and in cycloalkanes.

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Everything has a mirror image, but not all things are superposable on their mirror images. Mirror-image superposability characterizes many objects we use every day. Cups and saucers, forks and spoons, chairs and beds are all identical with their mirror images. Many other objects though—and this is the more interesting case—are not. Your left hand and your right hand, for example, are mirror images of each other but can’t be made to coincide point for point, palm to palm, knuckle to knuckle, in three dimensions. In 1894, William van’t Hoff was the recipient of the first Nobel Prize in chemistry in 1901 for his work in chemical dynamics and osmotic pressure—two topics far removed from stereochemistry.

Our major objectives in this chapter are to develop a feeling for molecules as three-dimensional objects and to become familiar with stereochemical principles, terms, and notation. A full understanding of organic and biological chemistry requires an awareness of the spatial requirements for interactions between molecules; this chapter provides the basis for that understanding.

Thomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if it is not superposable on its mirror image. Applying Thomson’s term to chemistry, we say that a molecule is chiral if its two mirror-image forms are not superposable in three dimensions. The work “chiral” is derived from the Greek word “cheir”, meaning “hand”, and it essentially appropriate to speak of the “handedness” of molecules. The opposite of chiral isachiral. Amolecule that is superposable on its mirror image is achiral.

In organic chemistry, chirality most often occurs in molecules that contain a carbon that is attached to four different groups. An example is bromochlorofluoromethane (BrClFCH).

The two mirror images of bromochlorofluoromethane cannot be superposed on each other. Since the two mirror images of bromochlorofluoromethane are not superposable, BrClFCH is chiral. The two mirror images of bromochlorofluoromethane have the same constitution. That is, the atoms are connected in the same order. But they differ in the arrangement of their atoms in space; they are stereoisomers. Stereoisomers that are related as an object and its nonsuperposable mirror image are classified as enantiomers. The word “enantiomer” describes a particular relationship between two objects. One cannot look at a single molecule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, “Is that person a cousin?” Furthermore, just as an object has one, and only one, mirror image, a chiral molecule can have one, and only one, enantiomer.

The two enantiomers of bromochlorofluoromethane are similarly oriented, that the difference between them corresponds to an interchange of the positions of bromine and chlorine. It will generally be true for species of the type C(w, x, y, z), where w, x, y, and z are different atoms or groups, that an exchange of two of them converts a structure to its enantiomer, but an exchange of three returns the original structure, albeit in a different orientation.

Consider next a molecule such as chlorodifluoromethane (ClF2CH), in which two of the atoms attached to carbon are the same. Since mirror-image representations of chlorodifluoromethane are superposable on each other, ClF2CH is achiral.

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Molecules of the general type

are chiral when w, x, y, and z are different substituents. A tetrahedral carbon atom that bears four different substituents is variously referred to as a chiral center, a chiral carbon atom, an asymmetric center, or an asymmetric carbon atom. A more modern term is stereogenic center, and that is the term that we’ll use. (Stereocenter is synonymouswith stereogenic center.) Noting the presence of one (but not more than one) stereogenic center in a molecule is a simple, rapid way to determine that it is chiral. For example, C-2 is a stereogenic center in 2-butanol; it bears a hydrogen atom and methyl, ethyl, and hydroxyl groups as its four different substituents. By way of contrast, none of the carbon atoms bear four different groups in the achiral alcohol 2-propanol.

Molecules with stereogenic centers are very common, both as naturally occurring substances and as the products of chemical synthesis. (Carbons that are part of a double bond or a triple bond can’t be stereogenic centers.)

A carbon atom in a ring can be a stereogenic center if it bears two different substituents and the path traced around the ring from that carbon in one direction is different from that traced in the other. The carbon atom that bears the methyl group in 1,2-epoxypropane, for example, is a stereogenic center. The sequence of groups is O±CH2 as one proceeds clockwise around the ring from that atom, but is CH2±O in the anti-clockwise direction. Similarly, C-4 is a stereogenic center in limonene.

Even isotopes qualify as different substituents at a stereogenic center. The stereochemistry of biological oxidation of a derivative of ethane that is chiral because of deuterium (D = ²H) and tritium (T= ³H) atoms at carbon, has been studied and shown to proceed as follows:

The stereochemical relationship between the reactant and the product, revealed by the isotopic labeling, shows that oxygen becomes bonded to carbon on the same side from which H is lost.

One final, very important point is about stereogenic centers. Everything we have said in this section concerns molecules that have one and only one stereogenic center; molecules with more than one stereogenic center may or may not be chiral.

2. What was said in the text about:

· the origin of the word “stereochemistry”;

· the foundation of organic stereochemistry;

· enantiomers;

· chirality;

· stereogenic center.

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