Copy of Organic Chemistry Jonh Mc Murry8

Copy of Organic Chemistry Jonh Mc Murry8 - 14D CHAPTERS An...

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Unformatted text preview: 14D CHAPTERS An Overview of Organic Reactions 5.3 Radical Reactions Radical reactions are not as common as polar reactions but are nevertheless important in some industrial processes and in numerous biological pathways. Let’s see briefly how they occur. A radical is highly reactive because it contains an atom with an odd num- ber of electrons (usually seven) in its valence shell, rather than a stable, noble- gas octet. A radical can achieve valence-shell octet in several wavs. For example, the radical might abstract an atom and one bonding electron from another reactant, leaving behind a new radical. The net result is a radical sub- stitution reaction: Unpairecl Unpaired electron electron Rad + r. : a 4- Had : A + 3 a Reactant Substitution Product radical product radical Alternatively, a reactant radical might add to a double bond, taking one electron from the double bond and yielding anew radical. The net result is a radical addi- tion reaction: Unpaired Unpaired eleCtrOn _ electron ’ ~. , Rad ' \. r \ . . ‘/ \ //// Rarl- + /C *‘C\ —’ 7C *C\" Reactant Alkene Addition product radical radical As an example of an industrially useful radical reaction, look at the chlori- nation ot methane to yield chloromethane. This substitution reaction is the first step in the preparation of the solvents dichloromethane (CHZCIZJ and chloro~ form (CHC13). H H H—c—le + til—Ci H—(lI—Cl + 7—-cr rl rlr Methane Chlorine Chloromethane Like many radical reactions in the laboratory, methane chlorination requires three kinds of steps: initiation, propagation, and rer'riiinarirw, Initiation irradiation with ultraviolet light begins the reaction by brealc ing the relatively weak CliCJ bond of a small number of Gig molecules to give a few reactive chlmine radicals. \ lirjirl =€i5¢l= - 2 “ii- 53 Radical Reactions 141 Propagation Once produced, a reactive chlorine radical collides with a rnethai re molecule in a propagation step, abstracting a hydrogen atom to give HCl and a methyl radical (- CH3). This inethyJ radical reacts further with C12 in a second propagation step to give the product chloromethane plus anew chlorine radical, which cycles back and repeats the first propagation step. Thus, once the sequence has been initiated, it becomes a self-sustaining cycle of repeating steps ta] and (b), making the overall process a chain reaction. .. r---.,/—\ .. Ia‘ -r=i. _l_ LJ-r‘u, .—. Li'r"l' 4L .r‘u, \u; Ipi \ ll'LaIlj |l‘\_,l' T \/||3 .. . .. ‘-f'.“‘\_,/‘\\ .. .. lb) :Ci=Cl: + -CH3 —- :Cl- + :CI'CH3 .‘.I .. .. .. lerrninntir'in Occasionally, two radicals might collide and combine to form a stable product. When that happens, the reaction cycle is broken and the chain is ended. Such termination steps occur infrequently, however, because the concentration of radicals in the reaction at any given moment is very small. Thus, the likelihood that two radicals will collide is also small. .~\ 71 =éii- i sir fl “5%! = it]: ' :C'l ~CI-I3 —' '- CH3 I Possible termination steps /\/‘\ H3C' + ‘CH3 — H3C3CH3 As a biological example of a radical reaction, let’s look at the biosynthesis of pmstriglmrdins, a large class or molecules round in virtually all body tissues and flu— ids. A number of pharmaceuticals are based on or derived from prostaglandins, including medicines that induce labor during childbirth, reduce intraocular pres- sure in glaucoma, control bronchial asthma, and help treat congenital heart defects. l’rostaglandin biosynthesis is initiated by abstraction of a hydrogen atom from arachidonie acid by an iron—oxygen radical, thereby generating a new, carbon radical in a substitution reaction Don’t be intimidated by the size of the molecules; focus only on the Changes occurring in each step. (To help you do that, the unchanged part of the molecule is "ghosted," with only the reactive part clearly visible.) fFe; O \ \ A 1’; I “13’ H / i Oxygen tfg/,/ radical + Radical ‘ ' H substitution <7? ‘\ _ Arachidonic acid Carbon 142 CHAPTERS An Overview of Organic Reactions Following the initial abstraction of a hydrogen atom, the carbon radical then reacts with 02 to give an oxygen radical, which reacts with a C:C bond within the same molecule in an addition reaction. Several further transforma- tions ultimately yield prostaglandin HZ. Carbon Oxygen radical radical / flv/ W3 _R_a_dir:al _ addition Prostaglandin H2 [PGHZl l-l bH Problem 5.2 Radical chlorination of alkanes is not generally useful because mixtures of products often result when more than one kind of C—H bond is present in the substrate. Draw and name all inonochloro substitution products C6HHC1 you might obtain by reac- tion of 24nethylpentane with C12. Problem 5.3 Using a curved fishhook arrow, propose a mechanism for formation of the CYClOpElL tane ring ofprostaglandin 1-12. What kind of reaction is occurring? 5.4 Polar Reactions Polar reactions occur because of the electrical attraction between positive and neg- ative centers on functional groups in molecules. To see how these reactions take place, let’s first recall the discussion of polar covalent bonds in Section 2.1 and then look more deeply into the effects of bond polarity on organic molecules. Most organic compounds are electrically neutral; they have no net charge, either positive or negative, We saw in Section 2.1, however, that certain bonds within a molecule, particularly the bonds in functional groups, are polar. Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is due to the difference in electronegativity of the bonded atoms. Elements such as oxygen, nitrogen, fluorine, and chlorine are more electro- negative than carbon, so a carbon atom bonded to one of these atoms has a pan tial positive charge (8+ ). Conversely, metals arc- less electronegative than 5.4 PolarReactions 143 carbon, so a carbon atom bonded to a metal has a partial negative charge (57}. Electrostatic potential maps of chloromethane and inethyllithium illustrate these charge distributions, showing that the carbon atom in chloromethane is electronpoor (blue) while the carbon in methyllithium is electron-rich (red). “CI l “ii I {an ! we; _‘ J viii/“Ni I—I’7"\H H H Chloromethane Methvllithium The polarity patterns of some common functional groups are shown in Table 5.]. Carbon is always positively polarized except when bonded to a metal. Table 5.1 Polarity Patterns in Some Common Functional Groups \m n- \h-j— m- Alcohol *{JiOI-l Carbonyl CID / / of O \\ / i . """f/ Alkene Cr C Carboxylic acrd —t,\_ i, / \ 0H 6, m.-.// Alkyl halide it Carboxylic acid —t;\‘ / chloride C" C).— \ ' v-- ii 49 Amine —C—NH2 Aldehyde WK / H iii ,0 \w a» .-r// a. :17 Ether —t_‘.—O*Ci Ester I- / \ \i - O—C hi \. .. .11. .i [/0 Thiol *C "SH Ketone —t". / \ C Nitrile —i:jéi\i Grignard '*—__C—e'\.-lc;28r reagent / \i. H Alkyllithium ficv .- 144 CHAPTERS An Overview of Organic Reactions This discussion or bond polarity is oversimplified in that we‘ve consid- ered only bonds that are inherently polar due to differences in electronega- tivity. Polar bonds can also result from the interaction of functional groups with acids or bases. Take an alcohol such as methanol, for example. in neu- tral methanol, the carbon atom is somewhat electron—poor because the electronegative oxygen attracts the electrons in the C70 bond. On proton— ation of the methanol oxygen by an acid, however, a full positive charge on oxygen attracts the electrons in the C—0 bond much more strongly and makes the carbon much more electron-poor. We’ll see numerous examples throughout this book of reactions that are catalyzed by acids because of the resultant increase in bond polarity. :A c H\ a He f. H Ms W _‘ . -. \D/ a". ll f Ill. r7— H ' \H iii/“\H H H Methanol—weakly Protonated methanol— electron-poor carbon strongly electron-poor carbon Yet a further consideration is the polariznbilir‘y (as opposed to polarity) of atoms in a molecule. As the electric field around a given atom changes because of changing interactions with solvent or other polar molecules nearby, the electron distribution around that atom also changes. The measure of this response to an external electrical influence is called the polarizabiiity of the atom. Larger atoms with more, loosely held electrons are more polariz- able, and smaller atoms with fewer, tightly held electrons are less polarizable. Thus, sulfur is more polarizable than oxygen, and iodine is more polarizable than chlorine. The effect of this higher polarizability fOr sulfur and iodine is that carbon—sulfur and carbon—iodine bonds, although nonpolar according to eiectronegativity values (Figure 2.2), nevertheless usually react as if they were polar. What does functional-group polarity mean with respect to chemical reac- tivity? Because unlike charges attract, the fundamental characteristic ofall polar organic reactions is that electron-rich sites react with electron-poor sites. Bonds are made when an electron-rich atom shares a pair of electrons with an electron- poor atom, and bonds are broken when one atom leaves with both electrons from the former bond. As we saw in Section 2.11, chemists indicate the movement of an electron pair during a polar reaction by using a Curved, full-headed arrow. A curved arrow shows where electrons move when reactant bonds are broken and product bonds are formed. it means that an electron pair moves fimn the atom 5.4 PolarReactions 145 (or bond) at the tail of the arrow to the atom at the head of the arrow during the reaction. This curved arrow shows that /e|ecirons move from :8- to A‘L. / \\ + 1LT —'— J‘s—:8 Electrophile Nucleophrle \ The elecjmnsihat moved 4_.M.____ ____r ._.__,h__ _;,,._\ from :8 to A and up here \CICLLIU l'pUUIJ \CIELLIU l'llLIl} ' in this new covalent bond. in referring to the electron—rich and electron-poor species involved in polar reactions, chemists use the words rmcleophile and elracrl‘oplrlle. A nucleophile is a substance that is “nucleus-loving.” (Remember that a nucleus is positively charged.) A nucleophile has a negatively polarized, electron-rich atom and can form a bond by donating a pair of electrons to a positively polarized, electron- poor atom. Nucleophiles may be either neutral or negatively charged; ammonia, water, hydroxide ion, and chloride ion are examples. An electrophile, by con- trast, is "electron—loving.” An electrophile has a positively polarized, electron- poor atom and can form a bond by accepting a pair of electrons from a nucleophile. Electrophiles can be either neutral or positively charged. Acids (H+ donors], alkyl halides, and carbonyl compounds are examples (Figure 5.] ). H _- SF"T‘.t:l truclr-rrig.\.hilec—. - ' (electron-rich) H;.,O+ EITEH371‘3‘r , Figure 5.1 Some nucleophiies and electrophiles. Electrostatic potential maps identify the nucleophilie (red; negative) and electrophilic (blue; positive) atoms. 1f the definitions of nucleophiles and electrophiles sound similar to those given in Section 2.11 for Lewis acids and Lewis bases. that’s because there is 146 CHAPTER 5 An Overview of Organic Reactions “taxi-:9 EX=AMP_L_E 54 Thomson Click Organic Interactive to identify and characterize nucleophiles and eiectrophiles in organic reactions. Strategy Solution Problem 5.4 Problem 5.5 indeed a correlation. Lewis bases are electron donors and behave as nucleo- philes, whereas Lewis acids are electron acceptors and behave as electrophiles. Thus, much of organic chemistry is explainable in terms of aciclebase reactions. The main difference is that the words acid and base are used broadly, while nucleophile and electrophile are used primarily when bonds to carbon are involved. Identifying Electrophiles and Nucleophiles Which of the following species is likely to behave as a nucleophile and which as an electrophile? (a) NOZ+ (b) CN‘ (c) CH3NH2 (d) (C,H3)3S+ Nucleophiles have an electron-rich site, either because they are negatively charng or because they have a functional group containing an atom that has a lone pair of electrons. Electrophiles have an electrorrpoor site, either because they are positively charged or because they have a functional group containing an atom that is posi- tively polarized. (a) N02+ (nitroniuni ion) is likely to be an electrophile because it is positively charged. (b) :CEN‘ (cyanide ion) is likely to be a nucleophile because it is negatively charged. ((2) CH3NH2 (methylamine) is likely to be a nucleophile because it has a lone pair of electrons on the nitrogen atom. (d) [Cl-titty [trimethylsulfonium ion) is likely to be an electrophile because it is positively charged. Which of the following species is likely to be a nucleophile and which an elec- trophile? la) CH3C| lb) CH357 (Ci CH3 (El) CH3CH An electrostatic potential map of boron trifluoride is shown. ls B133 likely to be a nucleophile or an electrophile? Draw a Lewis structure for 8133, and explain your answer. BF3 5.5 Thomson Click Organic Processes to view an animation of the addition of HBr to an alkene. 5.5 An Example of a Polar Reaction: Addition of HBrto Ethylene 147 An Example of a Polar Reaction: Addition of HE: to Ethylene Let’s look at a typical polar process—the addition reaction of an alkene, such as ethylene, with hydrogen bromide. When ethylene is treated with HBr at room temperature, bromoethane is produced. Overall, the reaction can be for-mu- lated as + . —} H H H e= \ / | l C;C + II M fee H—C--C --H / \ l l H H H H Ethylene iii-5:1. Bromoethane lnucleophilel The reaction is an example of a polar reaction type known as an elr’ctropiiilic addi» tion reaction and can be understood using the general ideas discussed in the pre- vious section. Let’s begin by looking at the two reactants. What do we know about ethylene? We know from Section 1.8 that a carbon—carbon double bond results from orbital overlap of two spZ-hybridized carbon atoms. The rr part of the double bond results from spZ—sp2 overlap, and the 7 part results from p—p overlap. What kind of chemical reactivity might we expect of a (T r C bond? We know that alkanes, such as ethane, are relatively inert because all valence electrons are tied up in strong, nonpolar C7C and Cil-l bonds. Furthermore, the bonding electrons in all-\‘anes are relatively inaccessible to approaching reactants because they are sheltered in 0 bonds between nuclei. The electronic situation in (ilkEl‘lt’S is quite different, however. For one thing, double bonds have a greater electron density than single bondskfour electrons in a double bond versus only two in a single bond. Furthermore, the electrons in the 17 bond are accessible to approaching reactants because they are located above and below the plane of the double bond rather than being sheltered between the nuclei (Figure 5.2). As a result, the double bond is nucleophilic and the chemistry of alltenes is domi- nated by reactions with eiectrophiles. What about the second reactant, HBr? As a strong acid, HBr is a powerful proton [H+) donor and electrophile. Thus, the reaction between HBr and ethylene is a typical electrophileenucleophiIe combination, characteristic of all polar reactions. 148 CHAPTER 5 An Overview of Organic Reactions Figure 5.2 A comparison of carbon—carbon single and double bonds. A double bond is both more accessible to approaching reactants than a single bond and more electron, rich (more nucleophilic). An electrostatic potential map of ethylene indicates that the double bond is the region of highest negative charge (redl. Problem 5.6 F ué'jr‘i'iDn-Calijlflil ;;‘ bond; Carbon—carbon a- bond: weaker; more accessible electrons stronger; less accessible bonding electrons We’ll see more details about alkene electrophilic addition reactions Shortly, but for the present we can imagine the reaction as taking place in two steps by the pathway shown in Figure 5.3. The reaction begins when the alkene donates a pair of electrons frOm its C:C bond to HBr to form a new c—H bond plus Br‘, as indicated by the path of the curved arrows in the first step of Figure 5.3. One curved arrow begins at the middle of the double bond [the source of the elec- tron pair) and points to the hydrogen atom in HBr (the atom to which a bond will form). This arrow indicates that a new Cal-l bond forms using electrons from the former C:C bond. A second curved arrow begins in the middle of the H—Br bond and points to the Br, indicating that the l-l—Br bond breaks and the electrons remain with the Br atom, giving Br“. When one of the alkene carbon a toms bonds to the incoming hydrogen, the other carbon atom, having lost its share of the double-bond electrons, now has only six valence electrons and is left with a positive charge. This positively charged speciesva carbon-cation. or carbocation—is itself an electrophile that can accept an electron pair from nucleophilic Br“ anion in a second step, torrn ing a CiBr bond and yielding the observed addition product. Once again, a curved arrow in Figure 5.3 shows the electron—pair mavement from Br‘ to the positively charged carbon. The electrophilic addition of HBr to ethylene is only one example or' a polar process; there are many others that we’ll study in detail in later chapters But regardless of the details of individual reactions, all polar reactions take place between an electron-poor site and an electron-rich site and involve the dona- tion of an electron pair from a nucleophile to an electrophile. What product would you expect from reaction of cyclohexene with HBr? With [it]? | + HBr ? Figure 5.3 MECHANISM: The electrophilic addition reaction of ethylene and HBr. The reaction takes place in two steps, both of which involve electrophile— nocleophile interactions Problem 5.7 5.5 Rule 1 Test your knowledge of Key Ideas by using resources in ThomsonNOW or by answering end-of-chapter problems marked with 576 Using Curved Arrows in Polar Reaction Mechanisms 0 A hydrogen atom on the electrophile HBr is attacked by 77 electrons from the nucleophilic double bond, forming a new Cal-i bond. This . leaves the other carbon atom with a + charge 0| and a vacant p orbital. Simultaneously. two ‘ electrons from the H—Br bond move onto .. ‘[ bromine, giving bromide anioni 33f: '\_ I Carbocation 6 Bromideion donates an electron pairto the I positively charged carbon atom, forming a 9 C-Br bond and yielding the neutral addition l product. Br ll \ ,C— C\ H / \‘H - H H _ Bromoethane H3C CH3 \ , + HBI’ i) CHgiCliiBr H3C CH3 Z-Methylpropene 2-Bromo-2-methyipropane Using Curved Arrows in Polar Reaction Mechanisms / “‘ E “"“E Electrons usually flow \ 5 ‘ f R \ i from one of these 1L“ ,—-N1 "at," nucleophiles. / / / 14B Reaction of HBr with 2-methylpropene yields 2-bromo—Z-methylpropane. What is the structu re of the carbocation formed during the reaction? Show the mechanism of the reaction. It takes practice to use curved arrows properly in reaction mechanisms, but there are a few rules and a few common patterns you should look for that will help you become more proficient. Electrons move from a nucleophilic source (Nu: or Nu:_) to an electrophilic sink (E or 13+). The nucleophiiic source must have an electron pair available, usually either in alone pair or in a multiple bond. For example: 150 CHAPTER 5 An Overview of Organic Reactions The electrophilic sink must be able to accept an electron pair, usually because it has either a positively charged atom or a positively polarized atom in a func- tional group. For exam ple: Nu: NH: Nu; NU: Electrons usually flow \ t \ , ., ,:,_ ,-x..; ‘x, \ A... D, to one of these 7""{'i :35." T Halogen ":—~l-I “Hit: 0 i / / i _? _.\ m— / i J: electrophiles. ,2O_ :4 Rule 2 The nucleophiie can be either negatively charged or neutral. If the nucleo- phile is negatively charged, the atom that gives away an electron pair becomes neutral. For example: Negatively charged Neutral \i\ \ \‘n’fflimr \, .. .. , CH3—O: + ii—ligii: % 04370: + :E;.,: .. ,. , .. If the nucleophile is neutral, the atom that gives away an electron pair acquires a positive charge. For example: Neutral Positively charged H H _ H t" \ r / x. i ‘3 \ l CTC + ll :: ——> +C—C—I—l + / \ / | H H H H Rule 3 The electrophile can be either positively charged or neutral. If the electro phile is positively charged, the atom bearing that charge becomes neutral after accepting an electron pair. For example: Positively charged Neutral H H ‘\ H - , / v , \ | C=C -I 1' <= ——> +C—C—H + it / \ / j - H H a H H ‘--i If the electrophile is neutral, the atom that ultimately accepts the electron pair acquires a negative charge. For this to happen, however, the negative charge must be stabilized by being on an electronegative atom such as oxygen, nitro gen, or a halogen. For example: Neutral Negatively charged H l-l , i H --! \ i / _" \ l CmC + H , li'I 7—— +C—C—H + in / \ ‘- / i H H H H ...
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