Text B. Chemical Reactions ( Part II )
Chemical reactions are often classified according to some prominent feature of the process under study. These categories are not mutually exclusive, however, for each may combine aspects of two or more categories. A polymerization reaction, for example, may also be a chain reaction.
In simple synthesis reaction two reactants, which may be elements or compounds, form a compound as the product. In a decomposition reaction a compound breaks up into at least two simpler parts. A polymerization reaction is a type of synthesis in which the product is formed from hundreds or thousands of simpler reagent species. A chain reaction is a series of reactions in which the product of each step is a reagent for the next. A reaction in which a compound loses one or more atoms and subsequently gains one or more other atoms is a substitution reaction. One in which a compound loses one or more atoms is an elimination reaction. One in which a compound gains one or more atoms is an addition reaction.
Oxidation – reduction reactions, a large and important class of chemical processes, are characterized by the transfer of oxygen or hydrogen atoms, or electrons, from one substance to another. A substance that loses electrons is said to be oxidized; one that gains electrons is said to be reduced. Of all the other types of reactions, perhaps the most important is the acid – base reaction.
There are several other useful principles of classification that can be noted. When two atoms (or groups of atoms) combine and each supplies an electron to their covalent bond, the process is called colligation, and the reverse reaction is called homolysis. If, on the other hand, two atoms (or groups of atoms) combine to form a covalent bond and both electrons come from only one of the reactants, the process is called coordination, and the reverse reaction is called heterolysis.
Reagents in a heterolytic reaction may be classified as nucleophilic or electrophilic depending on whether the reactant carries the electron pair (and seeks a nucleus to combine with) or lacks the electron pair (and seeks to combine with a unit having a pair of electrons).
Among the major types of reaction mechanisms are nucleophilic substitutions at either saturated or unsaturated carbon centres, electrophilic substitutions, addition reactions, and elimination reactions.
In general, modern theoretical studies centre either on reaction mechanisms (variously termed kinetics, kinetics and mechanisms, or dynamics of chemical reactions) or on the physical structure of matter (called structural chemistry). The empirical study of chemical reactions usually employs techniques for separating the products of a reaction. Subsequently the products must be identified, and finally in many cases the masses or volumes of the products and any remaining reactants are measured.
|
|
|
III. Make up a scheme of classification of chemical reactions according to their mechanism.
IV. Read the descriptions of the following chemical reactions and name the type of every reaction.
a) This reaction, the opposite of oxidation, has very wide applications in the production of pharmaceuticals. Examples of reducing agents are aluminum amalgam, hydriodic acid, hydrogen and a catalyst, hypophosphoros acid, lithium aluminum hydride, metal and acid, sodium and alcohol, stannous chloride, and zinc dust and water. Reduction is employed in the synthesis of racemic menthol, an anti-itching agent. It is produced by the catalytic hydrogenation of the aromatic analogue, thymol, or of the saturated ketone, menthone, or of the unsaturated ketone, pulegone.
b) This chemical reaction can only take place with unsaturated compounds, but the addition products vary greatly in nature. The mode of addition, where there are two possibilities, is determined by the structure of the unsaturated compound and the mechanism of addition.
The anesthetic drug halothane is manufactured from trichloroethylene by an addition reaction with hydrogen chloride. The product is treated with hydrogen fluoride; reaction with bromine then gives halothane. The manufacture of the anesthetic drug trichloroethylene involves, as a first step, the addition of chlorine to acetylene, followed by dehydrohalogenation.
c) Medicinal nitro compounds are invariably aromatic carbocyclic or heterocyclic derivatives. These are usually nitrated with a mixture of concentrated sulfuric acid and concentrated or fuming nitric acid (mixed acid). Other nitrating agents include oxides of nitrogen and acetyl nitrate, CH3CO2NO2..
Nitrofurantoin is prescribed as a urinary antiseptic and provides an example of nitration. Furfural is reacted with acetyl nitrate, and the resulting triacetate is treated with pyridine to give 5-nitro-2-furaldehyde diacetate. This is condensed (with elimination of acetic acid) with 1-aminohydantoin to yield nitrofurantoin.
|
|
|
LESSON 10
Грамматика: словообразование, правила чтения, вопросительные слова, вопросительные предложения, типы вопросов в английском языке, заменители модальных глаголов.
Main Medical Forms
Запомните : - tion ( ion) , - ison, - ture ( ure), - ment, - ance ( ence) – суффиксы
существительного ( повторение)
I. Прочтите слова:decoction, mixture, tincture, ointment, medication, preparation, solution, application.
II. Образуйте существительные от следующих глаголов:
to mix, to prepare, to apply, to develop, to prescribe, to solve, to infuse, to define.
III. Переведите предложения на русский язык , обращая внимание на образование вопросительной формы:
1. What is a solution?
2. Can you show me those vitamins, please?
3. Are you a chemist?
4. Do you work at a pharmaceutical company?
5. What is called a suppository?
6. A tablet is the most common form of medication, isn’t it?
7. Is powder a dry or a wet homogeneous mixture?
8. Who works at a pharmacy?
9. What is usually sold at a chemist’s?
Дата добавления: 2018-02-28; просмотров: 529; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!