ChemActivity 24: Carboxylic Acids & Derivatives

Transcription

1 hemactivity 24: arboxylic Acids & Derivatives PAT A: ABXYLI AIDS, ESTES, AD AMIDES (ow does the acidic group affect the reaction of a carboxylic acid with a nucleophile?) Memization Task 24.1: ecognize (and ame) ommon arboxylic Acid Derivatives ' = alkyl (not ) = alkyl ' l Acid hlide Acid Anhydride arboxylic Acid Ester Amide itriles (- ) are often considered another carboxylic acid derivative because a nitrile can be converted easily into a carboxylic acid by addition of acid and water (see Synth. Transfmation 11.6.) aming rules f carboxylic acids and derivatives are outlined in omenclature Wksheet 4. Model 1: arbonyl eactivity Acid hlide Acid Anhydride Ester Amide 3 l l F each compound above we can draw a second-der resonance structure to show donation of electron density into the bond marked with =>. This reduces the reactivity of the carbonyl toward a nucleophile. ritical Thinking Questions 1. F each set in Model 1, draw curved arrows showing conversion of the top resonance structure into the second-der resonance structure shown below it. a. f the atoms shown donating an electron pair toward the carbonyl carbon, which one donates the least electron density? Explain your reasoning. (int: onsider electronegativity.) b. f the atoms shown donating an electron pair toward the carbonyl carbon, which one donates the most electron density? Explain your reasoning. (int: onsider electronegativity.) opyright oughton Mifflin arcourt ompany. All rights reserved.

2 hemactivity 24: arboxylic Acids & Derivatives 435 Memization Task 24.2: elative arbonyl eactivity (with a ucleophile) ' = alkyl (not ) = alkyl ' l ' Acid hlide Acid Anhydride Aldehyde Ketone Ester Amide most reactive carbonyl carbon least reactive carbonyl carbon ote that aldehydes and ketones are not carboxylic acid derivatives so it can be hard to make a direct comparison. Additionally, very large groups can reduce the reactivity of a carbonyl due to increased steric repulsion of the nucleophile. 2. (heck your wk.) Are your answers to TQ 1 consistent with Memization Task 24.2? Explain. Model 2: Acid-Base eactivity of a arboxylic Acid (A) uc uc uc ritical Thinking Questions 3. Model 2 shows three possible ways that a nucleophile that is also a base (represented as uc ) could react with acetic acid. Predict which one of these pathways is faved over the other two. a. Draw the product(s) of each set of curved arrows. (Include all imptant resonance structures.) b. Label each set of curved arrows with the appropriate reaction type. hoose from: Acid-Base, Elimination, Electrophilic Addition, ucleophilic Addition. 4. (heck your wk.) Two of the three reactions depicted in Model 2 are acid-base reactions. Each of these acid-base reactions results in a conjugate base with two imptant resonance structures. a. heck that you drew both imptant resonance structures f each conjugate base. b. ircle the conjugate base that is lower in potential energy, and explain your reasoning. (Label this reaction in Model 2 faved reaction pathway. ) 5. (heck your wk.) Is your answer in part b of TQ 4 consistent with Memization Task 24.3? opyright oughton Mifflin arcourt ompany. All rights reserved.

3 436 hemactivity 24: arboxylic Acids & Derivatives Memization Task 24.3: arboxylic Acids Donate an + (except in acidic conditions) A nucleophile that is also a base will remove the acidic from a carboxylic acid rather than bond to the carbonyl carbon. arboxylic Acid uc X arboxylate Anion uc nce this is removed, the anionic carboxylate will repel almost all nucleophiles. otable exceptions are hydride (as supplied by LiAl 4 see Synth. Transf. 24.1) and lithium reagents. Synthetic Transfmation 24.1: arboxylic Acid Derivatives eduction with ydride Y = (aldehyde) * Y = l Br (acid halide) * Y = (acid anhydride) * 1) LiAl 4 ab 4 Y Y = (carboxylic acid) 2) 3 Y = ' (ester) *will react with * ketone 1) LiAl 4 ab 4 ab 4 2) 3 2 primary alcohol secondary alcohol amide 2 1) LiAl 4 2) amine 6. List functional groups that require use of the me powerful LiAl 4 to accomplish a reduction. Model 3: ucleophilic Addition to a arboxylic Acid (at low p) In acidic conditions (low p) the bond of a carboxylic acid stays intact. This allows nucleophilic addition to the carbonyl carbon by an alcohol to fm an ester, as shown in Synth. Transf Synthetic Transfmation 24.2: Acid-atalyzed Ester Synthesis carboxylic acid ' alcohol acidic conditions ester ' 2 boiling off water drives reaction to the right ritical Thinking Questions 7. Devise a mechanism f Synthetic Transfmation int: The first step is protonation of the carbonyl oxygen by the acid catalyst. opyright oughton Mifflin arcourt ompany. All rights reserved.

4 hemactivity 24: arboxylic Acids & Derivatives onstruct an explanation f why the following amide-fming reaction analogous to the esterfming reaction shown in Synthetic Transfmation 24.2 does T produce the desired product. (int: onsider what species in solution is most likely to react with the acid catalyst.) carboxylic acid ' 2 amine () X amide ' 2 eaction does T wk Memization Task 24.4: Few ucleophiles eact Directly with the arbonyl of a A. Alcohols are among the only nucleophiles that can undergo nucleophilic addition to the carbonyl of a carboxylic acid. ther, me basic nucleophiles fail because of one of the following reasons (1) (Under basic conditions with basic nucleophile) The nucleophile removes the acidic from the carboxylic acid, rendering its carbonyl virtually unreactive. (2) (Under acidic conditions with acid) A nucleophile that is also a good base will be protonated by the acid catalyst and consequently become non-nucleophilic 9. (heck your wk.) Which reason in Memization Task 24.4 best explains the failure of the reaction at the top of the page? 10. nly three of the following reactions produce a significant yield of the product shown. a. ross out the reaction arrows of the other three reactions the ones expected to fail. b. Put a (1) (2) next to each failed reaction to indicate which reason in Memization Task 24.4 best explains why that reaction is doomed to failure. (Any species in is labeled.) LiAl S Et Et Et Et a 2 S 4 Et 2 If your group finishes early, begin wking on the extensive homewk Exercises f Part A. These contain a number of imptant synthetic transfmations that build on the wk you have just completed. opyright oughton Mifflin arcourt ompany. All rights reserved.

5 438 hemactivity 24: arboxylic Acids & Derivatives PAT B: AID ALIDES AD AID AYDIDES (Is there a way to make an amide from a carboxylic acid?) Model 4: Acid alides (Alkanoyl alides) In Part A of this activity we discovered that the group of a carboxylic acid makes it impossible to transfm it directly into an amide. This problem and many others can be solved by replacing the group with a halogen via Synthetic Transfmation Synthetic Transfmation 23.4: Preparation of Acid alides from arboxylic Acids carboxylic acid Sl 2 (thionyl chlide) X = l PBr 3 (phosphus tribromide) X = Br X acid halide ritical Thinking Questions 11. Based on Model 1 at the start of this hemactivity, which carbonyl is me reactive, that of a carboxylic acid that of an acid halide? 12. Use curved arrows to show the mechanism of Step B of the following synthesis. ecall that S 1 and S 2 reactions cannot take place at sp 2 -hybridized carbons. Sl2 Step A l 2 3 Step B 2 3 l 13. When hydride (e.g., ab 4 LiAl 4 ), lithium reagent (-Li), Grignard (-MgBr) is added to an acid halide, two molar equivalents of nucleophile are incpated into the product. Devise a mechanism f this reaction, assuming the nucleophile is hydride anion ( ). l ' l neutralize with dilute acid opyright oughton Mifflin arcourt ompany. All rights reserved.

6 hemactivity 24: arboxylic Acids & Derivatives 439 Model 5: Acid alides with Li 2 u eagents A lithium dialkyl cuprate (Li2u) reagent will Br ( 3 ) 2 uli T react with an aldehyde ketone, but will react with the activated carbonyl of an acid halide. ( 3 ) 2 uli ritical Thinking Questions 14. (E) Which is me reactive? the carbonyl of an aldehyde/ketone the carbonyl of an acid halide (circle one) l Li Li 15. (heck your wk.) Explain how infmation in Model 5 suppts your answer to the previous question. ab 4 MgBr 16. Draw the maj product of each reaction on this page uli Write no reaction if you do not expect a reaction. Assume nucleophile is present in (except where noted) uli and that if necessary, the product mixture is neutralized at the end of each reaction. * Br careful addition of 1 molar equivalent at low temperature MgBr * This reaction is hard to control and will result in some alcohol side product. opyright oughton Mifflin arcourt ompany. All rights reserved.

7 440 hemactivity 24: arboxylic Acids & Derivatives Mem. Task 24.5: Esters are less reactive than acid halides in reactions with nucleophiles Esters will react with LiAl 4, lithium reagents (-Li), and Grignard reagents (-MgBr), but not ab 4 lithium dialkyl cuprate reagents (Li 2 u). Memization Task 24.6: Acid halides and esters consume two equivalents of When LiAl 4, a lithium reagent (-Li), a Grignard (-MgBr) is added to an acid halide ester it is nearly impossible to stop the reaction at the aldehyde/ketone (see dotted box). Y = l Br (acid halide) Y = ' (ester) ' = alkyl " = alkyl " = alkyl hydride, lithium hydride, lithium Grignard reagent " Grignard reagent " 1st MLA EQUIVALET Y " 2nd MLA EQUIVALET acid halide ester hard to stop reaction here " " alcoxide F this reason, nucleophile is almost always used. This gives a high yield of the fully reduced alkoxide product instead of a mixture of alkoxide, aldehyde/ketone, and unreacted starting material. Unless otherwise noted, assume nucleophile is present in these reactions. To stop at the aldehyde a special reducing agent (such as DIBA) may be used. To stop at the ketone a lithium dialkyl cuprate reagent may be used. These reactions are summarized at the end of this activity. 17. Draw the maj product/s of each reaction at right Write no reaction if you do not expect a reaction. Assume nucleophile is present in and that if necessary, the product mixture is neutralized at the end of each reaction. 3 3 Li MgBr ab 4 example of a lactone 18. (heck your wk.) The products of the first reaction above are 1,1-diphenylethanol + methanol. The second reaction yields no products. owever, if LiAl 4 is used in place of ab 4 it produces ethanol + cyclohexanol. Show the mechanism of this second reaction with LiAl 4, assuming the nucleophile is hydride anion ( ). opyright oughton Mifflin arcourt ompany. All rights reserved.

8 hemactivity 24: arboxylic Acids & Derivatives (heck your wk.) A cyclic ester is called a lactone. The reaction of the lactone in the third reaction on the previous page yields 4,5-dimethylhexane-1,5-diol. Draw this product, check your wk above, and devise a mechanism f this reaction (assuming the nucleophile is a 3 anion). Model 6: Acid Anhydrides Think of an acid anhydride as a less-reactive replacement f an acid halide. Acid anhydrides are often used in place of acid halides when a slower, me controlled reaction is necessary. The name acid anhydride refers to the fact that you can generate a symmetrical acid anhydride by heating a pure sample of the cresponding carboxylic acid while removing the water that is produced ( anhydride means without water ). An easier way to generate an acid anhydride is to mix an acid halide with a carboxylate anion, as shown below. a carboxylate ion ' l acid chlide ' acid anhydride ritical Thinking Questions 20. (E) Accding to Models 1 and 6, which is me reactive, the carbonyl of an acid halide the carbonyl of an acid anhydride? (circle one). 21. Devise a mechanism f the reaction in Model 6. al 22. The ester synthesis at the beginning of this activity can be accomplished via an acid anhydride as shown below. Show the mechanism of Step in this synthesis. (Another product is fmed in this reaction, but not shown below. What is it?) 3 Sl 2 Step A 3 l a Step B 3 3 Step opyright oughton Mifflin arcourt ompany. All rights reserved.

9 442 hemactivity 24: arboxylic Acids & Derivatives Synthetic Transfmations : Acid alides and Anhydrides eactions " 2 " = alkyl " 2 (2 equiv.) pyridine Sl 2 PBr 3 ' Li ' ' pyridine X ' MgBr Li' 2 u ' 1) ' Li ) dilute acid ) ' Li ' MgBr 2) dilute acid ' MgBr 1) LiAl 4 ab 4 LiAl 4 ab 4 2) dilute acid Even with 1 equiv. of hydride, it is difficult to isolate the aldehyde (unless you use a special reducing agent such as DIBA). ' ' ritical Thinking Questions 23. Label the structures in the Synthetic Transfmations above accding to their functional group categy. hoose from alcohol (specify 1 o, 2 o, 3 o if indicated), amine, aldehyde, ketone, amide, ester, carboxylic acid, carboxylate, acid halide, acid anhydride. 24. The acid anhydride shown in Model 6 has an group on one end and an group on the other end. When = this it is called a symmetrical acid anhydride. (These are most common.) When and are different it is called an asymmetrical acid anhydride. 3 a. (E) Identify each acid anhydride below as symmetrical asymmetrical b. onstruct an explanation f why use of an asymmetrical acid anhydride usually gives rise to two different products. c. Draw both amide products that arise if the asymmetrical acid anhydride below is treated with ammonia ( 3 ) opyright oughton Mifflin arcourt ompany. All rights reserved.

10 hemactivity 24: arboxylic Acids & Derivatives 443 Exercises f Part A 1. Which compound in Model 1 has the most double-bond character in the bond marked in the top structure with an arrow =>? Explain your reasoning. 2. onstruct an explanation f why a carbon M spectrum of,-dimethylfmamide obtained at room temperature shows three peaks at 161 ppm, 36 ppm and 32 ppm Is your explanation in the previous question consistent with the finding that a carbon M spectrum of,-dimethylfmamide obtained at an elevated temperature shows only two peaks: one at 161 ppm and the other at 34 ppm? Explain. 4. Label the bond-breaking arrow in Model 2 with a +5 to indicate that it takes approximately +5 pk a units of energy to break this bond and liberate an. 5. Label the bond-breaking arrow in Model 2 with a +20 to indicate that it takes approximately +20 pk a units of energy to break this bond and liberate the on the carbon alpha (α) to the carbonyl. ecall that nmal bonds take up to 50 pk a units of energy to break. esonance stabilization makes α α bonds much easier to break. The resulting chemistry is the subject of the next three hemactivities. 6. Methoxide ion ( 3 ) is an excellent nucleophile, yet the following addition does not occur. 3 3 X 3 3 eaction Does ot ccur a. Draw the reaction and resulting products that are likely to fm instead of the one shown. b. onstruct an explanation f why the conjugate base of acetic acid is fmed preferentially over the nucleophilic addition product above 7. Synthetic Transfmation 24.2 is reversible. Describe a labaty procedure that would allow you to reverse this reaction and generate a carboxylic acid from an ester as shown in Synthetic Transfmation 24.3, below. Synthetic Transfmation 24.3: Acid-atalyzed Ester ydrolysis ester ' 2 acidic conditions carboxylic acid ' removal of alcohol drives reaction to right 8. Show the mechanism of Synthetic Transfmation opyright oughton Mifflin arcourt ompany. All rights reserved.

11 444 hemactivity 24: arboxylic Acids & Derivatives 9. An attempt to generate a carboxylic acid from an ester under basic conditions leads to a carboxylate, as shown in Synthetic Transfmation 24.4 below. (ote that the carboxylate can be easily protonated in a second step to give the desired carboxylic acid.) This reaction is called saponification f the Latin wd f soap (saponis) because this reaction is used in the making of soap from fats (which are long-chain esters). Show the mechanism of this transfmation. Synthetic Transfmation 24.4: Ester Saponification (Base-atalyzed A Synthesis) ester ' 2 K saponification carboxylate ' removal of alcohol drives reaction to right acidify carboxylic acid 10. Trans-esterification (the conversion of one ester into a different ester) can be accomplished using reaction conditions analogous to either Synth. Transf (acidic conditions) Synth. Transf (basic conditions). Show the mechanisms of both acid- and base-catalyzed trans-esterification. Synthetic Transfmation 24.5: Trans-Esterification ester A ' " (acid catalyzed) " a (base catalyzed) ester B " ' removal of alcohol drives reaction to right 11. ydrolysis of an amide is similar to trans-esterification. Show the mechanisms of both acid- and base-catalyzed amide hydrolysis. Synthetic Transfmation 24.6: Acid- Base-atalyzed Amide ydrolysis amide = alkyl 2 (acid catalyzed) 1) (base catalyzed) 2) neutralize with dilute acid carboxylic acid onversion of a carboxylic acid to an ester does T wk in basic conditions, and the product in the box below does not fm. Use curved arrows to show the mechanism and product of the side reaction that would result if this reaction were attempted in basic conditions. 3 3 a The reaction in the previous question does not wk in basic conditions. Explain how it is different from Synthetic Transfmations 24.4 through 24.6, which do wk in basic conditions. opyright oughton Mifflin arcourt ompany. All rights reserved.

12 hemactivity 24: arboxylic Acids & Derivatives Synthetic Transfmations 24.7 and 11.6 are both common ways of preparing a carboxylic acid. Use curved arrows to show the mechanism of each reaction. Synthetic Transfmation 24.7: 1 o Grignard Lithium eagent with arbon Dioxide MgBr Li 1) 2) neutralize with dilute acid Synthetic Transfmation 11.6: itrile ydrolysis group is called a nitrile group 2 (acid catalyzed) 1) (base catalyzed) 2) neutralize with dilute acid The bond of a nitrile is analogous to the = bond of a carbonyl compound. Given this, devise a mechanism f the reaction at right. (int: An imine is one of the intermediates.) Synth. Transf. 24.8: Grignard + itrile ' MgBr 2 ' (eview) Write appropriate reagents in the boxes over each reaction arrow. see Synth. Transf ' give reagents ' see Synth. Transf give reagents see Synth. Transf = any alkyl group with a benzyllic (not t-butyl) give reagents 17. Devise a one-step mechanism f the following reaction. (What type of mechanism is this?) Synthetic Transfmation 24.9: Fmation of an Ester from a 1 o Alkyl alide carboxylate ' 2 X 1 o alkyl halide ester 2 ' X 18. ead the assigned pages in your text, and do the assigned problems. opyright oughton Mifflin arcourt ompany. All rights reserved.

13 446 hemactivity 24: arboxylic Acids & Derivatives Exercises f Part B 19. ank the following compounds from 1 = best electrophile to 4 = wst electrophile, and explain your reasoning F each reaction, state which product is me likely, and explain your reasoning. 3 l l pyridine onstruct an explanation f why the last reaction in TQ 16 can be controlled enough to give a reasonable yield of ketone, while an analogous reaction using a smaller nucleophile (such as a methyl group [ 3 MgBr]) cannot be controlled. 22. onstruct an explanation f why two molar equivalents of amine are needed to obtain a high yield of amide from an acid halide acid anhydride. 23. Draw the maj product, and show the mechanism of each of the following reactions with an acid catalyst and then with an appropriate base catalyst. (Draw the structure of the best base catalyst f each reaction.) 3 acid base catalyst 3 acid base catalyst 3 2 acid base catalyst 3 2 acid base catalyst lactone (cyclic ester) lactam (cyclic amide) opyright oughton Mifflin arcourt ompany. All rights reserved.

14 hemactivity 24: arboxylic Acids & Derivatives Try to draw the most likely product of each of the following reactions, then check your wk by drawing the mechanism of the reaction. Tip: These types of questions can be very difficult to answer crectly without sketching the mechanism. 2 2 S KEt 3 2 a 2 a 25. omplete the synthesis of the asymmetrical anhydride, and draw all four ganic products that result (two esters and two carboxylic acids). 3 l butanoyl chlide Step A 2 3 Step B 26. Acid halides and acid anhydrides allow convenient synthesis of carboxylic acid derivatives because they each have a good leaving group attached to the carbonyl carbon. a. What is this leaving group in the case of an acid halide? b. What is this leaving group in the case of an acid anhydride? (int: The leaving group is not just a single atom.) c. If you use a nucleophile that is strong enough, you don't need a good leaving group. A Grignard reagent is an example of such a nucleophile. In the following synthesis of an alkoxide. Show the mechanism of the reaction, assuming the nucleophile is 3 2. (emember that are good leaving groups LY in strongly basic conditions, such as in a Grignard Lithium reaction.) 3 2 MgBr epeat part c of the previous question using LiAl 4 in place of ethyl magnesium bromide. ote that ganometallic reagents (lithium reagents, Grignard reagents, LiAl 4, etc.) often have a mechanisms of action that involve the metal atom and are me complicated than if the nucleophile were truly a carbanion a hydride with a simple counterion. F our purposes the crect product and a reasonable answer can be found by assuming these reagents wk as if they were a carbanion a hydride with a simple counterion. opyright oughton Mifflin arcourt ompany. All rights reserved.

15 448 hemactivity 24: arboxylic Acids & Derivatives 28. Starting from any carboxylic acid(s), devise a synthesis of the 1 st three acid anhydrides in TQ 24a. 29. The * on the molecules below represents a chiral group. Assume that the carboxylic acid starting material is very costly because of this chiral group. onstruct an explanation f why, in this case, it would be a po choice to use an acid anhydride to transfm this acid into a derivative (e.g., the dimethyl amide shown). int: Draw the other ganic product produced in Step. * Sl 2 Step A * l * a Step B * * 3 3 Step * onstruct an explanation f why the following asymmetrical anhydride gives a much higher yield of ethyl acetate than ethyl 2,2-dimethylpropionate Et Et a 31. Design an efficient synthesis of each of the following target molecules. All carbon atoms must come from the starting material(s) given. Target Ph 2 2 Ph Starting material 3 2 l ead the assigned pages in your text, and do the assigned problems. The Big Picture In principle, carboxylic acids, esters, and amides can be interconverted by manipulating Le háelier s principle (as long as you avoid the complications caused by deprotonation of a carboxylic acid). ot all of these transfmations are presented as official Synthetic Transfmations because some are not often used in synthesis. F example, you can change an ester into an amide by treating it with amine and catalyst, and you can even do the opposite with sufficient time and heat. owever, in practice it often makes sense in terms of time and overall yield to accomplish most carboxylic-acid-derivative syntheses by first making an activated carbonyl compound, either an acid halide acid anhydride. opyright oughton Mifflin arcourt ompany. All rights reserved.

16 hemactivity 24: arboxylic Acids & Derivatives 449 ommon Points of onfusion It seems so simple, but many students fget that carboxylic acids are acidic! This gives rise to the following problems and resolutions 3 ucleophilic addition to a carboxylic acid carbonyl group is impossible when the nucleophile is also a good base. (uc acts as a base and removes the acidic + instead.) carboxylic acid YES 3 methoxide ion (good nucleophile & strong base) 3 3 To solve this problem we must use a nucleophile that is a very weak base. F example, use methanol ( 3 ) instead of methoxide ion ( 3 ). owever, since methanol is a weak nucleophile, we must activate the carbonyl with an acid catalyst to make the carbonyl carbon a better electrophile. emember that hydroysis of an ester amide in basic conditions leads to a carboxylate (not a carboxylic acid). Students often fget that use of LiAl 4, lithium, Grignard reagent with an acid halide, acid anhydride ester will result in incpation of two molar equivalents of nucleophile in the product; whereas lithium dialkyl cuprates will deliver only one equivalent (except to esters). This is a consequence of the fact that lithium dialkyl cuprates do T react with aldehydes, ketones, esters. eduction of an ester ( 2 ) with a hydride reagent (e.g., LiAl 4 ) leads to an alcohol (), not an ether ( ) as some students assume. The source of this misconception appears to be the fact that reduction of a carboxylic acid amide with this same reagent appears to imply that the reagent turns a = into a 2. The mechanism of the ester reduction reveals that the group is lost as a leaving group. (This is one of the rare instances of functioning as a leaving group). The carbonyl oxygen remains and becomes the alcohol oxygen of the product. See TQ 17. Synthetic transfmations on test and quizzes often do not specify how much of a particular reagent is added. In such situations you are to assume that the appropriate amount is added. That is, if the reagent is a catalyst that is required only in trace amounts, then assume there is a trace amount (even if it doesn t say trace amount next to the reagent). onversely, if reagent is necessary to effect a reasonable reaction it may not say next to the reagent, but you are to assume reagent is present. The exception to this rule is when 1 equivalent of a reagent gives a very different product than multiple equivalents. In such instances (such as the addition of 1 molar equivalent of a bulky Grignard to an ester at low temperature) the number of molar equivalents of reagent will likely be specified. ing opening of lactones (cyclic ester) and lactams (cyclic amides) really belongs in the previous activity (but there were already too many long mechanisms there, so they are presented here). The key err that students make in answering questions (especially multiple-choice questions) is to assume that the ring oxygen of a lactone remains attached to the carbonyl carbon in a resulting ester carboxylic acid. This misconception is often tested using an 18 -labeled oxygen as in Exercise 24. The best way to answer such questions is to quickly sketch the mechanism of the reaction. opyright oughton Mifflin arcourt ompany. All rights reserved.