Essential Organic Chemistry. Paula Y. Bruice Second Edition

Transcription

1 Essential rganic hemistry Paula Y. Bruice Second Edition

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3 Delocalized Electrons and Their Effect on Stability, eactivity, and pk a esonance contributors are shown with a double-headed arrow between them. The double-headed arrow does not mean that the structures are in equilibrium with one another. ather, it indicates that the actual structure lies somewhere between the structures of the s. esonance structures are merely a convenient way to show the p electrons; they do not depict any real electron distribution. The following analogy illustrates the difference between s and the. Imagine that you are trying to describe to a friend what a rhinoceros looks like. You might tell your friend that a rhinoceros looks like a cross between a unicorn and a dragon. Like s, the unicorn and the dragon do not really exist. Furthermore, like s, they are not in equilibrium: a rhinoceros does not change back and forth between the two forms, looking like a unicorn one instant and a dragon the next. The unicorn and the dragon are simply ways to represent what the actual structure the rhinoceros looks like. esonance contributors, like unicorns and dragons, are imaginary, not real. nly the, like the rhinoceros, is real. Electron delocalization is shown by double-headed arrows ( ). Equilibrium is shown by two arrows pointing in opposite directions ( ). unicorn dragon rhinoceros Electron delocalization is most effective if all the atoms sharing the delocalized electrons lie in or close to the same plane, so that their p orbitals can maximally overlap. For example, cyclooctatetraene is not planar; its sp 2 carbons have bond angles of 120, whereas a planar eight-member ring would have bond angles of 135. Because the ring is not planar, a p orbital can overlap with one adjacent p orbital, but it can have little or no overlap with the other adjacent p orbital. Therefore, the eight p electrons are not delocalized over the entire cyclooctatetraene ring, and its carbon carbon bonds do not all have the same length. 3-D Molecule: yclooctatetraene no overlap cyclooctatetraene 183

4 Delocalized Electrons and Their Effect on Stability, eactivity, and pk a 4 W T DAW ESAE TIBUTS We have seen that an organic compound with delocalized electrons is generally represented as a structure with localized electrons to let us know how many p electrons are present in the molecule. For example, nitroethane is represented as having a nitrogen oxygen double bond and a nitrogen oxygen single bond. 3 2 owever, the two nitrogen oxygen bonds in nitroethane are actually identical; they each have the same bond length. A more accurate description of the molecule s structure is obtained by drawing the two s. Both s show the compound with a nitrogen oxygen double bond and a nitrogen oxygen single bond, but they indicate that the electrons are delocalized by depicting the double bond in one contributor as a single bond in the other. nitroethane Delocalized electrons result from a p orbital overlapping the p orbitals of more than one adjacent atom. The, in contrast, shows that the p orbital of nitrogen overlaps the p orbital of each oxygen. In other words, it shows that the two p electrons are shared by three atoms. The also shows that the two nitrogen oxygen bonds are identical and that the negative charge is shared equally by both oxygen atoms. Thus, we need to visualize and mentally average both s to appreciate what the actual molecule the looks like. 3 2 d d ules for Drawing esonance ontributors To draw a set of s, we first draw a Lewis structure for the molecule it becomes our first and then we move the electrons, following the rules listed below, to generate the next. To draw s, move only P electrons or lone-pair electrons toward an sp 2 carbon. 1. nly electrons move. Atoms never move. 2. nly p electrons (electrons in p bonds) and nonbonding electrons can move; s electrons never move. 3. The total number of electrons in the molecule does not change. Therefore, each of the s for a particular compound must have the same net charge. If one has a net charge of zero, all the others must also have net charges of zero. (A net charge of zero does not necessarily mean that there is no charge on any of the atoms: a molecule with a positive charge on one atom and a negative charge on another atom has a net charge of zero.) otice, as you study the following s and practice drawing them yourself, that the electrons ( p electrons or lone pairs) are always moved toward an sp 2 carbon. emember that an sp 2 carbon is either a positively charged carbon or a 184

5 Delocalized Electrons and Their Effect on Stability, eactivity, and pk a double-bonded carbon. Electrons cannot be moved toward an sp 3 carbon because an sp 3 carbon has a complete octet so it cannot accommodate any more electrons. The following carbocation has delocalized electrons. To draw its, we move the p electrons toward an sp 2 carbon. A curved arrow can help you decide how to draw the second contributor. emember that the tail of the curved arrow shows where the electrons start from and the head of the arrow shows where the electrons are going. The shows that the positive charge is shared by two carbons. Tutorial: Drawing s an sp 2 carbon s 3 d d 3 3 Let s compare this carbocation with a similar compound in which all the electrons are localized. The p electrons in the compound shown below cannot move because the carbon they would move toward is an sp 3 carbon; sp 3 carbons cannot accept electrons. Tutorial: Localized and delocalized electrons an sp 3 carbon cannot accept electrons localized electrons In the next example, p electrons again move toward a positive charge. The resonance hybrid shows that the positive charge is shared by three carbons. an sp 2 carbon s d d d 3 The for the next compound is obtained by moving lone-pair electrons toward an sp 2 carbon. The sp 2 carbon can accommodate the new electrons by breaking a p bond. The lone-pair electrons in the example, on the right below, are not delocalized because they would have to move toward an sp 3 carbon. 2 lone-pair electrons 2 an sp 2 carbon s d d an sp 3 carbon cannot accept electrons 185

6 Delocalized Electrons and Their Effect on Stability, eactivity, and pk a For additional practice drawing resonance contributors, see Special Topics II in the Study Guide and Solutions Manual. The following s are obtained by moving p electrons toward an sp 2 carbon. otice that the electrons move toward (not away from) the most electronegative atom (the oxygen). 3 2 d The only time you move electrons away from the most electronegative atom in order to arrive at a is when that is the only way electrons can be moved. In other words, movement of electrons away from the most electronegative atom is better than no movement at all, because electron delocalization makes a molecule more stable (as you will see in Section 5). d « adicals can also have delocalized electrons. The s are obtained by moving single electrons toward sp 2 carbons s d d 3 2 PEPTIDE BDS Every third bond in a protein is a peptide bond. A can be drawn for a peptide bond by moving the lone pair on nitrogen toward the sp 2 carbon. peptide bond Because of the partial double bond character of the peptide bond, the carbon and nitrogen atoms and the two atoms bonded to each are held rigidly in a plane, as represented below by the blue and green boxes. This planarity affects the way proteins fold, so it has important implications for the three-dimensional shape of these biological molecules. a segment of a protein 186

7 Delocalized Electrons and Their Effect on Stability, eactivity, and pk a PBLEM 1 Which of the following compounds have delocalized electrons? a. e. 2 b. c. 2 2 f. g. 3 2 «d h PBLEM 2 For the compounds in Problem 1 that have delocalized electrons, draw their resonance contributors. 5 TE PEDITED STABILITIES F ESAE TIBUTS All s do not necessarily contribute equally to the. The degree to which each contributes depends on its predicted stability. Because s are not real, their stabilities cannot be measured. Therefore, the stabilities of s have to be predicted based on molecular features found in real molecules. The greater the predicted stability of the, the more it contributes to the structure of the resonance hybrid; and the more it contributes to the structure of the, the more similar the contributor is to the real molecule. The examples that follow illustrate these points. The two s for a carboxylic acid are shown below, labeled A and B. Structure B has two features that make it less stable than structure A: one of its oxygen atoms has a positive charge not a comfortable situation for an electronegative atom and the structure has separated charges. A molecule with separated charges has a positive charge and a negative charge that can be neutralized by the movement of electrons. esonance contributors with separated charges are relatively unstable (relatively high in energy) because energy is required to keep the opposite charges separated. Structure A, therefore, is predicted to be more stable than structure B. onsequently, A makes a greater contribution to the, so the resonance hybrid looks more like A than like B. The greater the predicted stability of the, the more it contributes to the structure of the. A B a carboxylic acid The two s for a carboxylate ion are shown next. D a carboxylate ion separated charges 187