16 Part l:Brushing Up on Important Organic Chemistry I Concepts You just can't get away from those carbonyls,so you get another taste of these reactions,many of them named reactions,in Chapter 15.You may be able to avoid biomolecules if your course doesn't cover them,but if it does, Chapter 16 is there for you. Finally,what's a good organic course without multistep and retrosynth nesis along with road aps ope that our tips can ease your pain at t nis point dmaps are in the chemistry stu t just hang end that you stry,an I you enjo c.A there'always quantitative and physical
16 Part I: Brushing Up on Important Organic Chemistry I Concepts You just can’t get away from those carbonyls, so you get another taste of these reactions, many of them named reactions, in Chapter 15. You may be able to avoid biomolecules if your course doesn’t cover them, but if it does, Chapter 16 is there for you. Finally, what’s a good organic course without multistep and retrosynthesis along with roadmaps? We hope that our tips can ease your pain at this point. Roadmaps are the bane of most organic chemistry students, but just hang in there. There is life after organic chemistry, and you may just find in the end that you actually enjoyed organic. And for those of you who missed the chemical calculations, there’s always quantitative analysis and physical chemistry. 05_178157-ch01.indd 16 5/28/10 9:45 AM
Chapter2 Remembering How We Do It: Mechanisms In This Chapter Analyzing arrows Breaking down basic moves Contemplating combining basic moves Mastering free-radical mechanisms isms are the key toorganic chemistry.Understanding the s to control the od the me In this chapter you review the basics of mechanisms and their conventions and look at some of the more comm ways that electrons shift during a reaction.You also see how these individual ster ques in free radical mechanisms. Duck-Here Come the Arrows hem conveys heaQeaeoulbi d arrow,and d single rrow The resonance arrow.a single line with arrow heads at both ends(see Figure 2-1).separates different resonance structures.The actual structure is a weighted average of all resonance forms.More resonance forms usually indicate a more stable structure.One or more of the resonance structures may be useful in predicting what will happen during a reaction(a mechanism)
Chapter 2 Remembering How We Do It: Mechanisms In This Chapter ▶ Analyzing arrows ▶ Breaking down basic moves ▶ Contemplating combining basic moves ▶ Mastering free-radical mechanisms Mechanisms are the key to organic chemistry. Understanding the mechanism allows organic chemists to control the reaction and to avoid unwanted side reactions. Understanding the mechanism many times allows chemists to increase the yield of product. In this chapter you review the basics of mechanisms and their conventions and look at some of the more common ways that electrons shift during a reaction. You also see how these individual steps can fit together in the overall reaction mechanism and apply some of these techniques in freeradical mechanisms. Duck — Here Come the Arrows Many types of arrows are used in organic chemistry, and each of them conveys information about the particular reaction. These arrows include the resonance arrow, equilibrium arrow, reaction arrow, double-headed arrow, and singleheaded arrow. The resonance arrow, a single line with arrow heads at both ends (see Figure 2-1), separates different resonance structures. The actual structure is a weighted average of all resonance forms. More resonance forms usually indicate a more stable structure. One or more of the resonance structures may be useful in predicting what will happen during a reaction (a mechanism). 06_178157-ch02.indd 17 5/28/10 9:45 AM
18 Part l:Brushing Up on Important Organic Chemistry I Concepts Figure2-1: The resonance arrow The equilibrium arrow,which has two lines pointing in opposite directions (see Figure 2-2),separates materials that are in equilibrium.Materials on each side of the arrow are present.Unlike a resonance arrow,the materials actually exist and aren't a hybrid.If one of the arrows is longer than the other is,it indicates that one side of the equilibrium predominates over the other. igure 2-2- ply separa the reactants fom t use rrow usu ally tes that equilibrium arrow) Figure 2-3: The reaction arrow. double and singleheaded curved arrows indicate the vement of the movement of one electron.The electrons always move in the direction indicated by the arrow.The head (point)of the arrow is where the electron is going,and the tail is the electron's source
18 Part I: Brushing Up on Important Organic Chemistry I Concepts Figure 2-1: The resonance arrow. The equilibrium arrow, which has two lines pointing in opposite directions (see Figure 2-2), separates materials that are in equilibrium. Materials on each side of the arrow are present. Unlike a resonance arrow, the materials actually exist and aren’t a hybrid. If one of the arrows is longer than the other is, it indicates that one side of the equilibrium predominates over the other. Figure 2-2: The equilibrium arrows. A reaction arrow, a single arrow pointing in one direction (see Figure 2-3), simply separates the reactants from the products. The use of this arrow usually indicates that the reaction proceeds in only one direction (unlike the equilibrium arrow). Figure 2-3: The reaction arrow. Double- and single-headed curved arrows indicate the movement of electrons. Double-headed curved arrows (shown in Figure 2-4a) show the movement of two electrons, whereas single-headed curved arrows (Figure 2-4b) indicate the movement of one electron. The electrons always move in the direction indicated by the arrow. The head (point) of the arrow is where the electron is going, and the tail is the electron’s source. 06_178157-ch02.indd 18 5/28/10 9:45 AM
Chapter 2:Remembering How We Do It:Mechanisms 19 Figure2-4 the move ment of electrons. Coming Around to Curved Arrows Mechanisms,like res (Resonance ating th more important differences: apply In resonance,the electrons don't actually move,whereas in mechanisms there is an actual movement of electrons. In resonance,you should never,ever break a single bond;however, many mechanisms involve the breaking of a single bond.Nonetheless you should never,ever exceed an octet of electrons for any atom in the second Deriod. A mechar sm provides a means toward understanding why a reaction rred.When you understand why aon occurred.you'rein act e the and the echar s how the ectron mov cleophnle electron ndica the ns fron (electron acceptor) donor)to t To be successful in organic chemistry,you must know the mechanism for the reaction you're studying
Chapter 2: Remembering How We Do It: Mechanisms 19 Figure 2-4: Curved arrows indicating the movement of electrons. a b Coming Around to Curved Arrows Mechanisms, like resonance structures, utilize curved arrows. (Resonance structures are ways of illustrating the various resonance forms that contribute to the resonance hybrid. If you need more review, refer to Organic Chemistry I For Dummies.) Many of the same rules apply to both; however, there are some important differences: ✓ In resonance, the electrons don’t actually move, whereas in mechanisms there is an actual movement of electrons. ✓ In resonance, you should never, ever break a single bond; however, many mechanisms involve the breaking of a single bond. Nonetheless, you should never, ever exceed an octet of electrons for any atom in the second period. A mechanism provides a means toward understanding why a reaction occurred. When you understand why a reaction occurred, you’re much closer to understanding organic chemistry. Reactions involve the breaking and the forming of bonds. The mechanism shows how the electrons move (flow) to break old bonds and to form new bonds. Curved arrows indicate the flow of the electrons from the nucleophile (electron donor) to the electrophile (electron acceptor). To be successful in organic chemistry, you must know the mechanism for the reaction you’re studying. 06_178157-ch02.indd 19 5/28/10 9:45 AM
20 Part I:Brushing Up on Important Organic Chemistry I Concepts Mechanisms in this book are,in general,advanced examples of mechanisms appearing in Organic Chemistry I For Dummies.A college organic chemistry course presents very few completely new mechanisms.Perfecting a few mech anisms goes a long way toward understanding all reaction mechanisms, and,therefore,all organic reactions.Although many students feel that memorization is important,unders g the mec hanism is what's neces- sary to compreher chemistry.If you simply memor rize mech nisms you'll hopel ight plac ws:The tail of the ads to h ight place (Sin right?Don't forget that electrons occupy orbitals.Other than radicals.the electrons in the orbitals are either bonding pairs or lone pairs.This means that the tail of the curved arrow must be at a lone pair or a bonding pair.(A radical may have the tail of the curved half arrow originating at the unpaired electron.)The head of the curved arrow indicates where a lone pair is going or where a bond will form. Getting Ready for Some Basic Moves The tail of a curved arrow has two possible positions,and the head of a curved ar whas tw eitione TThie combinations are po eans that,in theory,four sible.The combinations are Bond→lone pair Bond→bond Lone pair→bond Lone pair→lone pair The last combination doesn't work,at least not in a single step,because it tends to force an atom to exceed an octet.This leaves only three important types The basic idea behind th reactions is the same:An electron-rich atom a nucle ctrophile)
20 Part I: Brushing Up on Important Organic Chemistry I Concepts Mechanisms in this book are, in general, advanced examples of mechanisms appearing in Organic Chemistry I For Dummies. A college organic chemistry course presents very few completely new mechanisms. Perfecting a few mechanisms goes a long way toward understanding all reaction mechanisms, and, therefore, all organic reactions. Although many students feel that memorization is important, understanding the mechanism is what’s necessary to comprehend organic chemistry. If you simply memorize mechanisms, you’ll become hopelessly confused by even minor changes; however, if you understand a mechanism thoroughly, you can accommodate any changes. Keep two things in mind when drawing curved arrows: The tail of the arrow needs to be in the right place, and the head of the arrow needs to be in the right place. (Simple, right?) Don’t forget that electrons occupy orbitals. Other than radicals, the electrons in the orbitals are either bonding pairs or lone pairs. This means that the tail of the curved arrow must be at a lone pair or a bonding pair. (A radical may have the tail of the curved half arrow originating at the unpaired electron.) The head of the curved arrow indicates where a lone pair is going or where a bond will form. Getting Ready for Some Basic Moves The tail of a curved arrow has two possible positions, and the head of a curved arrow has two possible positions. This means that, in theory, four combinations are possible. These combinations are ✓ Bond → lone pair ✓ Bond → bond ✓ Lone pair → bond ✓ Lone pair → lone pair The last combination doesn’t work, at least not in a single step, because it tends to force an atom to exceed an octet. This leaves only three important types. The basic idea behind these reactions is the same: An electron-rich atom with a lone pair (a nucleophile) donates that lone pair to an electron-poor atom (an electrophile). 06_178157-ch02.indd 20 5/28/10 9:45 AM