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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twenty seven (Hydrocarbons and their classes)

Lesson twenty seven
ProfVietanhNgày: Thứ sáu, 2010-05-14, 11:27 PM | Tin nhắn # 1
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Hydrocarbons and their classes

Hydrocarbons are compounds that contain only carbon and hydrogen and are divided into two main classes: aliphatic hydrocarbons and aromatic hydrocarbons. This classification dates from the nineteenth century, when organic chemistry was almost exclusively devoted to the study of materials from natural sources, and terms were coined that reflected a substance’s origin. Two sources were fats and oils, and the word aliphatic was derived from the Greek word aleiphar (“fat”). Aromatic hydrocarbons, irrespective of their own odor, were typically obtained by chemical treatment of pleasant-smelling plant extracts. Aliphatic hydrocarbons include three major groups: alkanes, alkenes, and alkynes.

Alkanes are hydrocarbons in which all the bonds are single bonds, alkenes contain a carbon–carbon double bond, and alkynes contain a carbon–carbon triple bond. Examples of the three classes of aliphatic hydrocarbons are the two-carbon compounds ethane, ethylene, and acetylene.

Another name for aromatic hydrocarbons is arenes. Arenes have properties that are much different from alkanes, alkenes, and alkynes. The most important aromatic hydrocarbon is benzene.

Many of the principles of organic chemistry can be developed by examining the series of hydrocarbons in the order: alkanes, alkenes, alkynes, and arenes.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ sáu, 2010-05-14, 11:31 PM | Tin nhắn # 2
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REACTIVE SITES IN HYDROCARBONS

A functional group is the structural unit responsible for a given molecule’s reactivity under a particular set of conditions. It can be as small as a single hydrogen atom, or it can encompass several atoms. The functional group of an alkane is any one of its hydrogen substituents. For example:

CH3CH3 + Cl2 -> CH3CH2Cl + HCl

One of the hydrogen atoms of ethane is replaced by chlorine. This replacement of hydrogen by chlorine is a characteristic reaction of all alkanes and can be represented for the general case by the equation:

R-H + Cl2 -> R-Cl + HCl

In the general equation the functional group (-H) is shown explicitly while the remainder of the alkane molecule is abbreviated as R. This is a commonly used notation which helps focus our attention on the functional group transformation without being distracted by the parts of the molecule that remain unaffected. A hydrogen atom in one alkane is very much like the hydrogen of any other alkane in its reactivity toward chlorine. Our ability to write general equations such as the one shown illustrates why the functional group approach is so useful in organic chemistry. A hydrogen atom is a functional unit in alkenes and alkynes as well as in alkanes. These hydrocarbons, however, contain a second functional group as well. The carbon–carbon double bond is a functional group in alkenes, and the carbon–carbon triple bond is a functional group in alkynes. A hydrogen atom is a functional group in arenes, and we represent arenes as ArH to reflect this. What will become apparent when we discuss the reactions of arenes, however, is that their chemistry is much richer than that of alkanes, and it is therefore more appropriate to consider the ring in its entirety as the functional group.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ sáu, 2010-05-14, 11:50 PM | Tin nhắn # 3
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As a class, alkanes are not particularly reactive compounds, and the H in RH is not a particularly reactive functional group. Indeed, when a group other than hydrogen is present on an alkane framework, that group is almost always the functional group. Table 1 lists examples of some compounds of this type.


Some of the most important families of organic compounds, those that contain the carbonyl group (C=O), deserve separate mention and are listed in Table 2 Carbonylcontaining compounds rank among the most abundant and biologically significant classes of naturally occurring substances.



Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twenty seven (Hydrocarbons and their classes)
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