CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS ot all the properties of alkenes are revealed by focusing exclusively on the fund tional group behavior of the double bond. a double 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 a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene Allylic carbocation Allylic free radical Conjugated diene onyjugare 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. In this chapter we'll see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone. 10.1 THE ALLYL GROUP The group CH2-CHCH2-is known as allyl, which is both a common name and a permissible IUPAC name. It is most often encountered in functionally substituted deriv atives, and the following compounds containing this group are much better ki nown their functional class IUPAC names than by their substitutive ones "Allyl"is derived from the botanical name for garlic(Allium sativum). It was found in 1892 tha the major component obtained by distilling garlic oil is CH2=CHCH2SSCH2 CH=CH2, and the word allyl"was coined for the CH2=CHCH2-group on the basis of this origi 365 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
365 CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Not all the properties of alkenes are revealed by focusing exclusively on the functional group behavior of the double bond. A double bond can affect the properties of a second functional unit to which it is directly attached. It can be a substituent, for example, on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on 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. In this chapter we’ll see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone. 10.1 THE ALLYL GROUP The group CH2œCHCH2± is known as allyl*, which is both a common name and a permissible IUPAC name. It is most often encountered in functionally substituted derivatives, and the following compounds containing this group are much better known by their functional class IUPAC names than by their substitutive ones: C C C Allylic carbocation C C C Allylic free radical C C C C Conjugated diene *“Allyl” is derived from the botanical name for garlic (Allium sativum). It was found in 1892 that the major component obtained by distilling garlic oil is CH2œCHCH2SSCH2CHœCH2, and the word “allyl” was coined for the CH2œCHCH2± group on the basis of this origin. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TEN Conjugation in Alkadienes and Allylic Systems CH,=CHCH,OH CH,-CHCH,CI CH,=CHCH,Br (3-chloro-1-propene) (3-bromo-1-propene) The term"allylic"refers to a C=C-C unit. Its sp-hybridized carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic car bon.Conversely, the sp2-hybridized carbons of a carbon-carbon double bond are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents CH Allylic hydrogen hydrogens Allylic"is often used as a general term for molecules that have a functional group at allylic position. Thus, the following compounds represent an allylic alcohol and an llylic chloride, respectively CH3 HOCHCHE CH=CHCCI CH 3-Methyl-2-buten-1-ol 3-Chloro-3-methyl-1-butene an allylic alcohol an allylic chloride) 10.2 ALLYLIC 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 CH,=CHCH CH CH=CHCHCH Allyl cation 1-Methyl-2-butenyl 2-Cyclopentenyl A substantial body of evidence indicates that allylic carbocations are more stable than simple alkyl cations. For example, the rate of solvolysis of a chloride that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride CH CH2-CHCCI H3 3-Chloro-3-methyl-l-butene tert-Butyl chloride More reactive: k(rel) 123 Less reactive: k(rel) 1.0 The first-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 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
The term “allylic” refers to a CœC±C unit. Its sp3 -hybridized carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic carbon. Conversely, the sp2 -hybridized carbons of a carbon–carbon double bond are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents. “Allylic” is often used as a general term for molecules that have a functional group at an allylic position. Thus, the following compounds represent an allylic alcohol and an allylic chloride, respectively. 10.2 ALLYLIC CARBOCATIONS Allylic carbocations are carbocations in which the positive charge is on an allylic carbon. Allyl cation is the simplest allylic carbocation. Representative allylic carbocations A substantial body of evidence indicates that allylic carbocations are more stable than simple alkyl cations. For example, the rate of solvolysis of a chloride that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride. The first-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-butene More reactive: k(rel) 123 CH2 CH3 CH3 CHCCl tert-Butyl chloride Less reactive: k(rel) 1.0 CH3 CH3 CH3CCl CH2 CHCH2 Allyl cation CH3CH CHCHCH3 1-Methyl-2-butenyl cation 2-Cyclopentenyl cation 3-Methyl-2-buten-1-ol (an allylic alcohol) HOCH2CH CH3 CH3 C 3-Chloro-3-methyl-1-butene (an allylic chloride) CH2 CH3 CH3 CHCCl H H CH3 H C C Vinylic hydrogens Allylic hydrogens Vinylic hydrogen Allyl alcohol (2-propen-1-ol) CH2œCHCH2OH Allyl chloride (3-chloro-1-propene) CH2œCHCH2Cl Allyl bromide (3-bromo-1-propene) CH2œCHCH2Br 366 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
10.2 Allylic Carbocations Both compounds react by an SNI mechanism, and their relative rates reflect their acti- vation energies for carbocation formation. Since the allylic chloride is more reactive, we reason that it ionizes more rapidly because it forms a more stable carbocation. Struc urally, the two carbocations differ in that the allylic carbocation has a vinyl substituent on its positively charged carbon in place of one of the methyl groups of tert-butyl cation CHa=CH 1, 1-Dimethylallyl cation ferl-Butyl cation (less stable) A vinyl group stabilizes a carbocation more than does a methyl group. Why? rule of thumb is that a A vinyl group is an extremely effective electron-releasing substituent. A resonance C-Csubstituent stabilizes a interaction of the type shown permits the T electrons of the double bond to be delocal- carbocation about as well as ed and disperses the positive charge vo alkyl groups. Although carbocation it is H3 CH,=CH—C ←>CH2CH=C、 condary carbocation such isopropyl cation, (CH3)2CH It's important to recognize that the positive charge is shared by the two end carbons in the C-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 following carbocations (a)CH3 CH= CHCH (b )CH2=CCH C(CH3)2 SAMPLE SOLUTION(a) When writing resonance forms of carbocations, elec- rons are moved in pairs from sites of high electron density toward the positively charged carbon CH3CH=CH-cH2←>CH3CH一CH=CH2 Electron delocalization in allylic carbocations can be indicated using a dashed line to show the sharing of a pair of T electrons by the three carbons. The structural formula is 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. H Two dashed-line representations of 1, 1-dimethylallyl In the case of the parent cation CH2=CH-CH2 both the terminal carbons are equivalently substituted, and so each bears exactly half of a unit positive charge Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Both compounds react by an SN1 mechanism, and their relative rates reflect their activation energies for carbocation formation. Since the allylic chloride is more reactive, we reason that it ionizes more rapidly because it forms a more stable carbocation. Structurally, the two carbocations differ in that the allylic carbocation has a vinyl substituent on its positively charged carbon in place of one of the methyl groups of tert-butyl cation. A vinyl group stabilizes a carbocation more than does a methyl group. Why? A vinyl group is an extremely effective electron-releasing substituent. A resonance interaction of the type shown permits the electrons of the double bond to be delocalized and disperses the positive charge. It’s important to recognize that the positive charge is shared by the two end carbons in the CœC±C unit; the center carbon does not bear a positive charge in either of the resonance 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 following carbocations: (a) (b) (c) SAMPLE SOLUTION (a) When writing resonance forms of carbocations, electrons are moved in pairs from sites of high electron density toward the positively charged carbon. Electron delocalization in allylic carbocations can be indicated using a dashed line to show the sharing of a pair of electrons by the three carbons. The structural formula is completed by placing a positive charge above the dashed line or by adding partial positive charges to the carbons at the end of the allylic system. In the case of the parent cation CH2œCH±CH2 both the terminal carbons are equivalently substituted, and so each bears exactly half of a unit positive charge. C CH3 C C H H H CH3 C CH3 C C H H H CH3 or Two dashed-line representations of 1,1-dimethylallyl cation CH3CH CH2 CH3CH CH CH CH2 C(CH3)2 CH3 CH2 CCH2 CH3CH CHCH2 CH CH3 CH3 CH2 C CH CH3 CH3 CH2 C 1,1-Dimethylallyl cation (more stable) CH CH3 CH3 CH2 C tert-Butyl cation (less stable) CH3 CH3 CH3 C 10.2 Allylic Carbocations 367 A rule of thumb is that a CœC substituent stabilizes a carbocation about as well as two alkyl groups. Although allyl cation (CH2œCHCH2 ) is a primary carbocation, it is about as stable as a typical secondary carbocation such as isopropyl cation, (CH3)2CH. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TEN Conjugation in Alkadienes and Allylic Systems H + This same sharing of positive charg low in. the first and third carbons in an electrostatic potential map(Fig An orbital overlap description of electron delocalization in 1, I-dimethylallyl cation CH2=CH-C(CH3) is given in Figure 10.2. Figure 10.2a shows the T bond and the give an extended T orbital that encompasses all three carbons. This permits the ap t vacant p orbital as independent units. Figure 10.2b shows how the units can overlap to 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, there are two potential sites for attack by a nucleophile. Thus, hydrolysis of 3-chloro-3-methyl 1-butene gives a mixture of two allylic alcohols (CH3),CCH=CH,->(CH3),CCH=CH, +(CH)C=CHCH,OH 3-Chloro-3-methyl 2-Methyl-3-buten-2-ol 3-Methyl-2-buten-1-ol 1-butene FIGURE 10.1 An elec- trostatic potential map fo allyl cation. The middle car bon(red region)has the least positive charge of the three carbons: the end carbor (blue regions) have the most positive charg FIGURE 10.2 Electron delocalization in an allylic carbocation. (a) The m orbital of the doubl bond, and the vacant 2p orbital of the positively charged carbon. ( b)Overlap of the orbital and the 2p orbital gives an extended t orbital that encompasses all three carbons. The two electron n the t bond are delocalized over two carbons in(a) and over three carbons in(b) Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
This same sharing of positive charge between the first and third carbons in CH2œCH±CH2 is shown by the use of colors in an electrostatic potential map (Figure 10.1). An orbital overlap description of electron delocalization in 1,1-dimethylallyl cation CH2œCH± C(CH3)2 is given in Figure 10.2. Figure 10.2a shows the bond and the vacant p orbital as independent units. Figure 10.2b shows how the units can overlap to give 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, there are 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 (CH3)2CCH Cl CH2 2-Methyl-3-buten-2-ol (85%) (CH3)2CCH OH CH2 3-Methyl-2-buten-1-ol (15%) (CH3)2C CHCH2OH H2O Na2CO3 Allyl cation C H C C H H H H 1 2 1 2 368 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems 2 p (a) (b) π π FIGURE 10.1 An electrostatic potential map for allyl cation. The middle carbon (red region) has the least positive charge of the three carbons; the end carbons (blue regions) have the most positive charge. FIGURE 10.2 Electron delocalization in an allylic carbocation. (a) The orbital of the double bond, and the vacant 2p orbital of the positively charged carbon. (b) Overlap of the orbital and the 2p orbital gives an extended orbital that encompasses all three carbons. The two electrons in the bond are delocalized over two carbons in (a) and over three carbons in (b). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
10.2 Allylic Carbocations Both alcohols are formed from the same carbocation. Water may react with the carbo- cation to give either a primary alcohol or a tertiary alcohol. H3C +C-CH=CH Use Learning By Model- arge distributed among its 少→(CH3)CCH=CH2+(CH)2C=CHCH2OH H3C C=CH—CH 2-Methyl-3-buten-2-ol 3-Methyl-2-buten-1-ol H3C (85%) (15%) B It must be emphasized that we are not dealing with an equilibrium between two isomeric carbocations. There is only one carbocation. Its structure is not adequately represented by either of the individual resonance forms but is a hybrid having qualities of both of them The carbocation has more of the character of a than b because resonance struc- ture A is more stable than B. Water attacks faster at the tertiary carbon because it bears more of the positive charge The same two alcohols are formed in the hydrolysis of 1-chloro-3-methy l-2-butene (CH,,C=CHCH2CI-H20 (CH3) CCH=CH +(CH3)2C=CHCH,OH OH 1-Chloro-3-methy 2-Methyl-3-buten-2-ol 3-Methyl-2-buten-1-ol (15%) The carbocation formed on ionization of 1-chloro-3-methyl-2-butene is the same allylic carbocation as the one formed on ionization of 3-chloro-3-methyl-1-butene and gives the same mixture of products. Reactions of allylic systems that yield products in which double-bond migration has occurred are said to have proceeded with allylic rearrangement, or by way of an allylic shift. PROBLEM 10.2 From among the following compounds, choose the two that yield the same carbocation on ionization CHa CI CH H3 Later in this chapter we'll see how allylic carbocations are involved in trophilic addition to dienes and how the principles developed in this section Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Both alcohols are formed from the same carbocation. Water may react with the carbocation to give either a primary alcohol or a tertiary alcohol. It must be emphasized that we are not dealing with an equilibrium between two isomeric carbocations. There is only one carbocation. Its structure is not adequately represented by either of the individual resonance forms but is a hybrid having qualities of both of them. The carbocation has more of the character of A than B because resonance structure A is more stable than B. Water attacks faster at the tertiary carbon because it bears more of the positive charge. The same two alcohols are formed in the hydrolysis of 1-chloro-3-methyl-2-butene: The carbocation formed on ionization of 1-chloro-3-methyl-2-butene is the same allylic carbocation as the one formed on ionization of 3-chloro-3-methyl-1-butene and gives the same mixture of products. Reactions of allylic systems that yield products in which double-bond migration has occurred are said to have proceeded with allylic rearrangement, or by way of an allylic shift. PROBLEM 10.2 From among the following compounds, choose the two that yield the same carbocation on ionization. Later in this chapter we’ll see how allylic carbocations are involved in electrophilic addition to dienes and how the principles developed in this section apply there as well. Cl CH3 CH3 Br CH3 Br CH3 Cl CH3 Br 1-Chloro-3-methyl- 2-butene (CH3)2C CHCH2Cl 2-Methyl-3-buten-2-ol (85%) (CH3)2CCH OH CH2 3-Methyl-2-buten-1-ol (15%) (CH3)2C CHCH2OH H2O Na2CO3 2-Methyl-3-buten-2-ol (85%) (CH3)2CCH OH CH2 3-Methyl-2-buten-1-ol (15%) (CH3)2C CHCH2OH H2O CH H3C H3C C CH2 A CH H3C H3C C CH2 B 10.2 Allylic Carbocations 369 Use Learning By Modeling to view the carbocation represented by resonance structures A and B. How is the positive charge distributed among its carbons? Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website