化學(xué)專業(yè)英語之有機(jī)化學(xué)中的氧化與消解反應(yīng)

1、化學(xué)專業(yè)英語之有機(jī)化學(xué)中的氧化與消解反應(yīng)OXIDATION AND REDUCTION IN ORGANIC CHEMISTRYThe conversion of alcohols into carbonyl compounds is an important reaction of primary and secondary alcohols, and is one of many examples in organic chemistry of oxidation. How do we know when an organic compound has been oxidized? In

2、the last section, we recognized that conversion of an alcohol to a ketone is an oxidation because it is brought about by the reduction of Cr (VI ). But there are other oxidations in which the oxidizing agent is less obvious. Our goal in this section is to be able to recognize an oxidation or reducti

3、on merely by examining the transformation of the organic compound itself. The procedure for doing this involves three steps;Step1. Assign an oxidation level to each carbon atom in reactant and product. (It is only necessary to assign an oxidation level to carbons that undergo some chemical change du

4、ring the transformation; other carbons may be ignored. ) The oxidation level of a particular carbon is assigned by considering the relative electronegativities of the groups bound to the carbon, as follows.(a) For every bond to an element less electronegative than carbon (including hydrogen ),and fo

5、r every negative charge on the carbon, assign a -1.(b) For every bond to another carbon atom, and for every unpaired electron on the carbon, assign a zero,(c) For every bond to an element more electronegative than carbon, and for every positive charge on the carbon, assign a +1.(d) Add the numbers a

6、ssigned in (a),(b),and (c)to obtain the oxidation level for the carbon atom under consideration.Let us apply this first step to the transformation of isopropyl alcohol to acetone.Since the carbon atoms of the two methyl groups do not change, we do not need to assign oxidation levels to these carbons

7、. Notice in the treatment of acetone that the C = O double bond is counted as two bonds; +l for each bond gives a total of +2 for the double bond.Step 2. The oxidation number NOI for each compound is computed by adding the oxidation levels of all carbons. In the structures above, only one carbon has

8、 changed its oxidation level, so the Nox values of the reactant and product are simply equal to the respective oxidation levels of this carbon. Therefore, the oxidation level of the reactant is 0 and that of the product is +2. In other reactions involving more than one carbon atom, No, is computed b

9、y summing the oxidation levels of all carbon atoms that undergo a chemical change.Step 3. Compute the difference.Nox (product) Nox (reactant) If this difference is positive, the transformation is an oxidation. If this difference is negative, the transformation is a reduction. If the difference is ze

10、ro, neither an oxidation nor a reduction has taken place. For the reaction of Eq. 171 this difference is +2 0 = +2. This transformation is thus an oxidation.Although the oxidation-number formalism is very useful, we should not lose sight of the following two general characteristics of organic oxidat

11、ions and reductions. These two points can enable us to spot an oxidation or reduction at a glance.1. In most oxidations of organic compounds, either hydrogen in a CH bond or carbon in a CC bond is replaced by a more electronegative element, such as halogen or oxygen. The converse is true for reducti

12、ons.2. The oxidation state of a molecule is determined from the oxidation states of its individual carbon atoms.The oxidation number concept can be simply related to a definition of oxidation that isoften used in inorganic chemistry. According to this definition, oxidation is the loss ofelectrons an

13、d reduction is the gain of electrons. To see how this definition applies toorganic compounds, let us consider as an example the oxidation of ethanol to acetic acid:We can write this oxidation as a balanced half-reaction using H2O to balance missing oxygens, protons to balance missing hydrogens, and

14、dummy electrons to balance charges. According to this half-reaction, four electrons are lost from the ethanol molecule v/hen acetic acid is formed. (Since this is only a half-reaction, a corresponding number of electrons must be gained by the species that brings about the oxidation.) It can be said

15、that the oxidation of ethanol to acetic acid is a four-electron oxidation. This type of terminology, which is frequently used in biochemistry, comes from the half-reaction formalism.If we compute the oxidation numbers of ethanol and acetic acid, we can see that the .change in oxidation number for Eq

16、. 172 is +4 (verify this statement). This example illustrates the following point: the change in oxidation number is equal to the number of electrons lost. If the change in oxidation number is negative, the reaction is a reduction, and the number corresponds to electrons gained.Oxidations and reduct

17、ions, like acid-base reactions, always occur in pairs. Therefore, whenever something is oxidized. something else is reduced. When an organic compound is oxidized, the reagent that brings about the transformation is called an oxidizing agent. Likewise, when an organic compound is reduced, the reagent

18、 that effects the transformation is called a reducing agent. For example, suppose that chromate ion (CrO42-) is used to bring about the oxidation of ethanol to acetic acid in Eq. 17 2 in this reaction, chromate ion is reduced to Cr3+.Three electrons are gained in the reduction of chromate to Cr3+. S

19、ince four electrons are lost in the oxidation (Eq. 17 3 ) , stoichiometry requires that for every three ethanol molecules oxidized to acetic acid (twelve electrons lost), four CrO42- are reduced (twelve electrons gained).By considering the change in oxidation number for a transformation, we can tell

20、 whether an oxidizing or reducing agent is required to bring about the reaction. For example, the following transformation is neither an oxidation nor a reduction (verify this statement):Although one carbon is oxidized, another is reduced. Even though we might know nothing else about the reaction, i

21、t is clear that an oxidizing or reducing agent alone would not effect this transformation. (In fact, the reaction is the pinacol rearrangement, which is brought about by mineral acid.)The oxidation-number concept can be used to organize organic compounds into functional groups with the same oxidation level. Compounds within a given box are generally interconnected by reagents that are neithe