More About 20 Oxidation–Reduction Reactions OOC n important group of organic reactions consists of those that O A involve the transfer of electrons C from one molecule to another. Organic chemists H OH use these reactions—called oxidation–reduction reactions or redox reactions—to synthesize a large O variety of compounds. Redox reactions are also important C in biological systems because many of these reactions produce HH energy. You have seen a number of oxidation and reduction reactions in other chapters, but discussing them as a group will give you the opportunity to CH3OH compare them. In an oxidation–reduction reaction, one compound loses electrons and one com- pound gains electrons. The compound that loses electrons is oxidized, and the one that gains electrons is reduced. One way to remember the difference between oxidation and reduction is with the phrase “LEO the lion says GER”: Loss of Electrons is Oxi- dation; Gain of Electrons is Reduction. The following is an example of an oxidation–reduction reaction involving inorganic reagents: Cu+ + Fe3+ ¡ Cu2+ + Fe2+ In this reaction,Cu+ loses an electron, so Cu+ is oxidized. Fe3+ gains an electron, so Fe3+ is reduced. The reaction demonstrates two important points about oxidation– reduction reactions. First, oxidation is always coupled with reduction. In other words, a compound cannot gain electrons (be reduced) unless another compound in the reaction simultaneously loses electrons (is oxidized). Second, the compound that is oxidized (Cu+) is called the reducing agent because it loses the electrons that are used to reduce the other compound (Fe3+). Similarly, the compound that is reduced (Fe3+) is called the oxidizing agent because it gains the electrons given up by the other compound (Cu+) when it is oxidized. It is easy to tell whether an organic compound has been oxidized or reduced simply by looking at the change in the structure of the compound. We will be looking primarily 841 842 C H A P T E R 2 0 More About Oxidation–Reduction Reactions at reactions where oxidation or reduction has taken place on carbon: If the reaction in- creases the number of C ¬ H bonds or decreases the number of C ¬ O, C ¬ N, or C ¬ X bonds (where X denotes a halogen), the compound has been reduced. If the reaction decreases the number of C ¬ H bonds or increases the number of C ¬ O, C ¬ N, or C ¬ X bonds, the compound has been oxidized. Notice that the oxidation state of a carbon atom equals the total number of its C ¬ O, C ¬ N, and C ¬ X bonds. Tutorial: oxidation reactions Changes in oxidation state OXIDATION STATE 01234 number of C Z bonds O O (Z = O, N, or halogen) CH4 CH3OH HCH HCOH O CO O O O CH3OCH3 CH3CCH3 CH3COCH3 CH3OCOCH3 NCH3 O O CH3CCH3 CH3CNH2 CH3OCNHCH3 OCH3 O O CH3CCH3(H) CH3CCl ClCCl OCH3 reduction reactions Let’s now take a look at some examples of oxidation–reduction reactions that take place on carbon. You have seen these reactions in previous chapters. Notice that in each of the following reactions, the product has more C ¬ H bonds than the reactant has: The alkene, aldehyde, and ketone, therefore, are being reduced (Sections 4.11, 18.5, and 15.15). Hydrogen, sodium borohydride, and hydrazine are the reducing agents. H2 RCH CHRPt RCH2CH2R Reduction at carbon increases the an alkene number of C ¬ H bonds or decreases the number of C ¬ O, C ¬ N, or C ¬ X bonds. O 1. NaBH4 Oxidation at carbon decreases the C + RCH2OH 2. H3O number of C ¬ H bonds or increases RH the number of C ¬ O, C ¬ N, an aldehyde or C ¬ X bonds. O H2NNH2 C − RCH2R RRHO , ∆ a ketone In the next group of reactions, the number of C ¬ Br bonds increases in the first reac- tion. In the second and third reactions, the number of C ¬ H bonds decreases and the number of C ¬ O bonds increases. This means that the alkene, the aldehyde, and the alcohol are being oxidized. Bromine and chromic acid (H2CrO4) are the oxidizing agents. Notice that the increase in the number of C ¬ O bonds in the third reaction results from a carbon–oxygen single bond becoming a carbon–oxygen double bond. Introduction 843 Br Br Br RCH CHR2 RCHCHR an alkene O O H CrO C 2 4 C RH R OH an aldehyde OH O H CrO RCHR 2 4 C an alcohol RR If water is added to an alkene, the product has one more C ¬ H bond than the reac- tant, but it also has one more C ¬ O bond. In this reaction, one carbon is reduced and another is oxidized. The two processes cancel each other as far as the overall molecule is concerned, so the overall reaction is neither an oxidation nor a reduction. H+ RCH CHR RCH2CHR H2O OH Oxidation–reduction reactions that take place on nitrogen or sulfur show similar structural changes. The number of N ¬ H or S ¬ H bonds increases in reduction reac- tions, and the number of N ¬ O or S ¬ O bonds increases in oxidation reactions. In the following reactions, nitrobenzene and the disulfide are being reduced (Sections 16.2 and 23.7), and the thiol is being oxidized to a sulfonic acid: NO2 NH2 H2 Pd/C nitrobenzene HCl CH3CH2SSCH2CH3 Zn 2 CH3CH2SH a disulfide a thiol HNO3 CH3CH2SH CH3CH2SO3H a thiol a sulfonic acid Many oxidizing reagents and many reducing reagents are available to organic chemists. This chapter highlights only a small fraction of the available reagents. The ones selected are some of the more common reagents that illustrate the types of trans- formations caused by oxidation and reduction. PROBLEM 1N Indicate whether each of the following reactions is an oxidation reaction, a reduction reaction, or neither: O O H a. C 2 C partially CH CH3 Cl deactivated 3 H Pd 844 C H A P T E R 2 0 More About Oxidation–Reduction Reactions Br HBr b. RCH CHR RCH2CHR Br c. Br2 h O H2CrO4 d. CH3CH2OH C CH3 OH H e. CH C N2 CHCH NH 3 Pt 3 2 2 HO− f. CH3CH2CH2Br CH3CH2CH2OH 20.1 Reduction Reactions An organic compound is reduced when hydrogen (H2) is added to it. A molecule of H2 can be thought of as being composed of (1) two hydrogen atoms, (2) two electrons and two protons, or (3) a hydride ion and a proton. In the sections that follow, you will see that these three ways to describe H2 correspond to the three mechanisms by which H2 is added to an organic compound. components of H:H H H − H+ − H+ H − H+ two hydrogen atoms two electrons and two protons a hydride ion and a proton Reduction by Addition of Two Hydrogen Atoms Movie: You have already seen that hydrogen can be added to carbon–carbon double and triple Catalytic hydrogenations bonds in the presence of a metal catalyst (Sections 4.11 and 6.8). These reactions, called catalytic hydrogenations, are reduction reactions because there are more C ¬ H bonds in the products than in the reactants. Alkenes and alkynes are both re- duced to alkanes. Pt, Pd, or Ni CH3CH2CH CH2 + H2 CH3CH2CH2CH3 1-butene butane Pt, Pd, or Ni CH3CH2CH2C CH+ 2 H2 CH3CH2CH2CH2CH3 1-pentyne pentane In a catalytic hydrogenation, the H ¬ H bond breaks homolytically (Section 4.11). This means that the reduction reaction involves the addition of two hydrogen atoms to the organic molecule. We have seen that the catalytic hydrogenation of an alkyne can be stopped at a cis alkene if a partially deactivated catalyst is used (Section 6.8). Lindlar CH3 CH3 catalyst CH3C CCH3 + H2 CC 2-butyne H H cis-2-butene Only the alkene substituent is reduced in the following reaction. The very stable benzene ring can be reduced only under special conditions. Section 20.1 Reduction Reactions 845 H2 CH CH CH CH 3-D Molecules: 2 Pd/C 2 3 Styrene; Ethyl benzene Catalytic hydrogenation can also be used to reduce carbon–nitrogen double and triple bonds. The reaction products are amines. Pd/C CH3CH2CH NCH3 + H2 CH3CH2CH2NHCH3 methylpropylamine Pd/C CH3CH2CH2CN+ 2 H2 CH3CH2CH2CH2NH2 butylamine The carbonyl group of ketones and aldehydes can be reduced by catalytic hydro- genation, with Raney nickel as the metal catalyst. (Raney nickel is finely dispersed nickel with adsorbed hydrogen, so an external source of H2 is not needed.) Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols. Murray Raney (1885–1966) was born in Kentucky. He received a B.A. O from the University of Kentucky in H2 1909, and in 1951 the university C CH CH CH CH OH Raney Ni 3 2 2 2 awarded him an honorary Doctor CH CH CH H a primary alcohol 3 2 2 of Science. He worked at the Gilman an aldehyde Paint and Varnish Co. in O OH Chattanooga, Tennessee, were he H patented several chemical and C 2 CH CH CHCH Raney Ni 3 2 3 metallurgical processes. In 1963, CH3CH2 CH3 a secondary alcohol the company was sold and renamed a ketone W. R. Grace & Co., Raney Catalyst Division. The reduction of an acyl chloride can be stopped at an aldehyde if a partially deac- tivated catalyst is used. This reaction is known as the Rosenmund reduction. The cat- alyst for the Rosenmund reduction is similar to the partially deactivated palladium catalyst used to stop the reduction of an alkyne at a cis alkene (Section 6.8). Karl W. Rosenmund (1884–1964) was born in Berlin.