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Chapt 10

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chapt 10
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  365 CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS N ot all the properties of alkenes are revealed by focusing exclusively on the func-tional group behavior of the double bond. Adouble bond can affect the proper-ties of a second functional unit to which it is directly attached. It can be a sub-stituent, for example, on a positively charged carbon in an allylic carbocation, or on acarbon that bears an unpaired electron in an allylic free radical, or it can be a substituenton a second double bond in a conjugated diene. Conjugare is a Latin verb meaning “to link or yoke together,” and allylic carbocations,allylic free radicals, and conjugated dienes are all examples of conjugated systems. Inthis chapter we’ll see how conjugation permits two functional units within a moleculeto display a kind of reactivity that is qualitatively different from that of either unit alone. 10.1THE ALLYL GROUP The group CH 2 œ CHCH 2 ± is known as allyl*, which is both a common name and apermissible IUPAC name. It is most often encountered in functionally substituted deriv-atives, and the following compounds containing this group are much better known bytheir functional class IUPAC names than by their substitutive ones: CCC  Allylic carbocation CCC Allylic free radical CCCC Conjugated diene *“Allyl”isderivedfromthebotanicalnameforgarlic(  Allium sativum ).Itwasfoundin1892thatthemajorcomponentobtainedbydistillinggarlicoilisCH 2 œ CHCH 2 SSCH 2 CH œ CH 2 ,andtheword“allyl”wascoinedfortheCH 2 œ CHCH 2 ± grouponthebasisofthisorigin. Back Forward Main Menu TOC Study Guide TOCStudent OLCMHHE Website  The term “ allylic ”  refers to a C œ C ± C unit. Its sp 3 -hybridized carbon is calledthe allylic carbon, and an allylic substituent is one that is attached to an allylic car-bon. Conversely, the sp 2 -hybridized carbons of a carbon – carbon double bond are called vinylic carbons, and substituents attached to either one of them are referred to as vinylicsubstituents. “ Allylic ”  is often used as a general term for molecules that have a functional group atan allylic position. Thus, the following compounds represent an allylic alcohol and an allylic chloride, respectively. 10.2ALLYLIC CARBOCATIONS Allylic carbocations are carbocations in which the positive charge is on an allylic car-bon. Allyl cation is the simplest allylic carbocation.  Representative allylic carbocations Asubstantial body of evidence indicates that allylic carbocations are more stablethan simple alkyl cations. For example, the rate of solvolysis of a chloride that is bothtertiary and allylic is much faster than that of a typical tertiary alkyl chloride.The fi rst-order rate constant for ethanolysis of the allylic chloride 3-chloro-3-methyl-1-butene is over 100 times greater than that of tert- butyl chloride at the same temperature. 3-Chloro-3-methyl-1-buteneMore reactive: k  (rel) 123 CH 2 CH 3 CH 3 CHCCl tert  -Butyl chlorideLess reactive: k  (rel) 1.0 CH 3 CH 3 CH 3 CClCHCH 2 CH 2  Allyl cation CHCHCH 3 CH 3 CH  1-Methyl-2-butenylcation  2-Cyclopentenylcation3-Methyl-2-buten-1-ol(an allylic alcohol) HOCH 2 CHCH 3 CH 3 C 3-Chloro-3-methyl-1-butene(an allylic chloride) CH 2 CH 3 CH 3 CHCClHHCH 3 HCC VinylichydrogensAllylic hydrogensVinylic hydrogenAllyl alcohol(2-propen-1-ol) CH 2 œ CHCH 2 OH Allyl chloride(3-chloro-1-propene) CH 2 œ CHCH 2 Cl Allyl bromide(3-bromo-1-propene) CH 2 œ CHCH 2 Br 366 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Back Forward Main Menu TOC Study Guide TOCStudent OLCMHHE Website  Both compounds react by an S N 1 mechanism, and their relative rates re fl ect their acti-vation energies for carbocation formation. Since the allylic chloride is more reactive, wereason that it ionizes more rapidly because it forms a more stable carbocation. Struc-turally, the two carbocations differ in that the allylic carbocation has a vinyl substituenton its positively charged carbon in place of one of the methyl groups of tert- butyl cation.Avinyl group stabilizes a carbocation more than does a methyl group. Why?Avinyl group is an extremely effective electron-releasing substituent. Aresonanceinteraction of the type shown permits the  electrons of the double bond to be delocal-ized and disperses the positive charge.It ’ s important to recognize that the positive charge is shared by the two end carbons in theC œ C ± C  unit; the center carbon does not bear a positive charge in either of the reso-nance structures that we just wrote. Keep that fact in mind as you answer Problem 10.1. PROBLEM 10.1 Write a second resonance structure for each of the followingcarbocations:(a)(b)(c) SAMPLE SOLUTION (a) When writing resonance forms of carbocations, elec-trons are moved in pairs from sites of high electron density toward the positivelycharged carbon. Electron delocalization in allylic carbocations can be indicated using a dashed lineto show the sharing of a pair of  electrons by the three carbons. The structural formulais completed by placing a positive charge above the dashed line or by adding partial pos-itive charges to the carbons at the end of the allylic system.In the case of the parent cation CH 2 œ CH ± CH 2  both the terminal carbons areequivalently substituted, and so each bears exactly half of a unit positive charge.  CH 3 CCCHHHCH 3  CH 3 CCCHHHCH 3 or Two dashed-line representations of 1,1-dimethylallyl cation CH 3 CHCH 2  CHCH 3 CH  CH 2 CHC(CH 3 ) 2  CH 3 CH 2 CCH 2  CH 3 CHCHCH 2  CHCH 3 CH 3 CCH 2  CHCH 3 CH 3 CCH 2  1,1-Dimethylallyl cation(more stable) CHCH 3 CH 3 CCH 2  tert  -Butyl cation(less stable) CH 3 CH 3 CH 3 C  10.2 Allylic Carbocations  367 A rule of thumb is that aC œ C substituent stabilizes acarbocation about as well astwo alkyl groups. Althoughallyl cation (CH 2 œ CHCH 2  ) isa primary carbocation, it isabout as stable as a typicalsecondary carbocation suchas isopropyl cation,(CH 3 ) 2 CH  . Back Forward Main Menu TOC Study Guide TOCStudent OLCMHHE Website  This same sharing of positive charge between the fi rst and third carbons inCH 2 œ CH ± CH 2  is shown by the use of colors in an electrostatic potential map (Fig-ure 10.1).An orbital overlap description of electron delocalization in 1,1-dimethylallyl cation CH 2 œ CH ±  C(CH 3 ) 2 is given in Figure 10.2. Figure 10.2 a shows the  bond and thevacant  p orbital as independent units. Figure 10.2 b shows how the units can overlap togive an extended  orbital that encompasses all three carbons. This permits the two  electrons to be delocalized over three carbons and disperses the positive charge.Since the positive charge in an allylic carbocation is shared by two carbons, thereare two potential sites for attack by a nucleophile. Thus, hydrolysis of 3-chloro-3-methyl-1-butene gives a mixture of two allylic alcohols: 3-Chloro-3-methyl-1-butene (CH 3 ) 2 CCHClCH 2 2-Methyl-3-buten-2-ol(85%) (CH 3 ) 2 CCHOHCH 2   3-Methyl-2-buten-1-ol(15%) (CH 3 ) 2 CCHCH 2 OH H 2 ONa 2 CO 3 Allyl cation HCCCHHHH 12  12  368 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems 2  p ( a )( b ) π π FIGURE 10.1 An elec-trostatic potential map forallyl cation. The middle car-bon (red region) has the leastpositive charge of the threecarbons; the end carbons(blue regions) have the mostpositive charge. FIGURE 10.2 Electron delocalization in an allylic carbocation. ( a ) The  orbital of the doublebond, and the vacant 2  p orbital of the positively charged carbon. ( b ) Overlap of the  orbital andthe 2  p orbital gives an extended  orbital that encompasses all three carbons. The two electronsin the  bond are delocalized over two carbons in ( a ) and over three carbons in ( b ) . Back Forward Main Menu TOC Study Guide TOCStudent OLCMHHE Website
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