370 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems 10.3 ALLYLIC FREE RADICALS Just as allyl cation is stabilized by electron delocalization, so is allyl radical HCcH2→ HC—CH=CH Allyl radical Allyl radical is a conjugated system in which three electrons are delocalized over three carbons. The unpaired electron has an equal probability of being found at C-l or C-3. Reactions that generate allylic radicals occur more readily than those involving simple alkyl radicals. Compare the bond dissociation energies of the primary C-H bonds of propane and propene 302:→cHHc+H△=+410(+9 cal) c=C一CH=CCn+△P=+36(+8a Propene All Hydrogen radical It requires less energy, by 42 k/mol (10 kcal/mol), to break a bond to a primary hydro- gen atom in propene than in propane. The free radical produced from propene is allylic and stabilized by electron delocalization; the one from propane is not PROBLEM 10.3 Identify the allylic hydrogens (a)Cyclohexene (c)2, 3, 3-Trimethyl-1-butene (b)1-Methylcyclohexene (d 1-Octene SAMPLE SOLUTION (a) Allylic hydrogens are bonded to an allylic carbon. An allylic carbon is an sp-hybridized carbon that is attached directly to an sp hybridized carbon of an alkene. Cyclohexene has four allylic hydrogens hese are These are allylic allylic These are vinyl 10.4 ALLYLIC HALOGENATION Of the reactions that involve carbon radicals the most familiar are the chlorination and bromination of alkanes (Sections 4.15 through 4. 19) Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
10.3 ALLYLIC FREE RADICALS Just as allyl cation is stabilized by electron delocalization, so is allyl radical: Allyl radical is a conjugated system in which three electrons are delocalized over three carbons. The unpaired electron has an equal probability of being found at C-1 or C-3. Reactions that generate allylic radicals occur more readily than those involving simple alkyl radicals. Compare the bond dissociation energies of the primary C±H bonds of propane and propene: It requires less energy, by 42 kJ/mol (10 kcal/mol), to break a bond to a primary hydrogen atom in propene than in propane. The free radical produced from propene is allylic and stabilized by electron delocalization; the one from propane is not. PROBLEM 10.3 Identify the allylic hydrogens in (a) Cyclohexene (c) 2,3,3-Trimethyl-1-butene (b) 1-Methylcyclohexene (d) 1-Octene SAMPLE SOLUTION (a) Allylic hydrogens are bonded to an allylic carbon. An allylic carbon is an sp3 -hybridized carbon that is attached directly to an sp2 - hybridized carbon of an alkene. Cyclohexene has four allylic hydrogens. 10.4 ALLYLIC HALOGENATION Of the reactions that involve carbon radicals, the most familiar are the chlorination and bromination of alkanes (Sections 4.15 through 4.19): These are vinylic hydrogens H H H H H H These are allylic hydrogens These are allylic hydrogens CH3CH2CH2 H Propane CH3CH2CH2 Propyl radical H Hydrogen atom H° 410 kJ (98 kcal) H Hydrogen atom CH2 CHCH2 H Propene CH2 CHCH2 Allyl radical H° 368 kJ (88 kcal) H or 2C CH CH 2 C H C C H H H H 1 2 1 2 H2C CH CH2 Allyl radical 370 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
10.4 Allylic Halogenation RH X? > RX HX Alkane Halogen Alkyl Hydrogen halide Although alkenes typically react with chlorine and bromine by addition at room tem- perature and below(Section 6. 14), substitution becomes competitive at higher tempera- tures, especially when the concentration of the halogen is low. When substitution does occur, it is highly selective for the allylic position. This forms the basis of an industrial preparation of allyl chloride CH2=CHCH3 Clz CH2=CHCH2CI+ Chlorine Hydrogen chloride The reaction proceeds by a free-radical chain mechanism, involving the following prop agation step CH2-CHCH2 H+Ni CI →CH=CHCH,+H:Cl Allyl radical Hydrogen chloride CHa=CHCH 9+g→CH CHCH,CI:+ Allyl radical Chlorine Allyl chloride Chlorine atom Allyl chloride is quite reactive toward nucleophilic substitutions, especially those that proceed by the Sn2 mechanism, and is used as a starting material in the synthesis of a variety of drugs and agricultural and industrial chemicals. Allylic brominations are normally carried out using one of a number of specialized reagents developed for that purpose. N-Bromosuccinimide(NBS)is the most frequently used of these reagents. An alkene is dissolved in carbon tetrachloride. N-bromo- succinimide is added, and the reaction mixture is heated, illuminated with a sunlamp, or both. The products are an allylic halide and succinimide Br reagent for selective bromination in Sec- NB NH tion11.12. Cyclohexene N-Bromosuccinimide Bromocyclohexane Succinimide (NBS) (82-87%) N-Bromosuccinimide provides a low concentration of molecular bromine, which reacts with alkenes by a mechanism analogous to that of other free-radical halogenations PROBLEM 10. 4 Assume that N-bromosuccinimide serves as a source of Br2, and write equations for the propagation steps in the formation of 3-bromocyclohex ene by allylic bromination of cyclohexene Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Although alkenes typically react with chlorine and bromine by addition at room temperature and below (Section 6.14), substitution becomes competitive at higher temperatures, especially when the concentration of the halogen is low. When substitution does occur, it is highly selective for the allylic position. This forms the basis of an industrial preparation of allyl chloride: The reaction proceeds by a free-radical chain mechanism, involving the following propagation steps: Allyl chloride is quite reactive toward nucleophilic substitutions, especially those that proceed by the SN2 mechanism, and is used as a starting material in the synthesis of a variety of drugs and agricultural and industrial chemicals. Allylic brominations are normally carried out using one of a number of specialized reagents developed for that purpose. N-Bromosuccinimide (NBS) is the most frequently used of these reagents. An alkene is dissolved in carbon tetrachloride, N-bromosuccinimide is added, and the reaction mixture is heated, illuminated with a sunlamp, or both. The products are an allylic halide and succinimide. N-Bromosuccinimide provides a low concentration of molecular bromine, which reacts with alkenes by a mechanism analogous to that of other free-radical halogenations. PROBLEM 10.4 Assume that N-bromosuccinimide serves as a source of Br2, and write equations for the propagation steps in the formation of 3-bromocyclohexene by allylic bromination of cyclohexene. Cyclohexene N-Bromosuccinimide (NBS) O NBr O heat CCl4 3-Bromocyclohexene (82–87%) Br Succinimide NH O O CH2 CHCH2 H Propene CH2 CHCH2 Allyl radical H Cl Hydrogen chloride Cl Chlorine atom CH2 CHCH2 Allyl radical Cl Cl Chlorine Allyl chloride CH CHCH2Cl 2 Cl Chlorine atom Propene CH2 CHCH3 Chlorine Cl2 Allyl chloride (80–85%) CH2 CHCH2Cl Hydrogen chloride HCl 500°C Alkane RH Halogen X2 Alkyl halide RX Hydrogen halide HX heat or light 10.4 Allylic Halogenation 371 N-Bromosuccinimide will be seen again as a reagent for selective bromination in Section 11.12. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Although allylic brominations and chlorinations offer a method for attaching a reactive functional group to a hydrocarbon framework, we need to be aware of two important limitations. For allylic halogenation to be effective in a particular synthesis 1. All the allylic hydrogens in the starting alkene must be equivalent 2. Both resonance forms of the allylic radical must be equivalent. In the two examples cited so far, the chlorination of propene and the bromination of cyclohexene, both criteria are met. All the allylic hydrogens of propene CH,=CH—CH The two resonance forms of allyl CH,←>CH一CH=CH radical are equivalent. All the allylic hydrogens of exene are equivalent. The two resonance forms of H 2-cyclohexenyl radical are equivalent. H Unless both criteria are met, mixtures of constitutionally isomeric allylic halides result PROBLEM 10.5 The two alkenes 2, 3, 3-trimethyl-1-butene each subjected to allylic halogenation with N-bromosuccinit e and 1-octene were One of these alkenes yielded a single allylic bromide, whereas the other gave a mixture of two constitutionally isomeric allylic bromides. Match the d behavior to the rect alkene and give the structure of the allylic bromide(s)formed from each 10.5 CLASSES OF DIENES Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates in chemical reactions. The third type of conjugated system that we will examine, conjugated dienes, consists of stable molecules A hydrocarbon that contains two double bonds is called an alkadiene, and the rela tionship between the double bonds may be described as isolated, conjugated, or cumu lated Isolated diene units are those in which two carbon-carbon double bond units are separated from each other by one or more sp-hybridized carbon atoms. 1, 4-Pentadiene and 1,5-cyclooctadiene have isolated double bonds CH,=CHCH,CH=CH, 1. 4-Pentadiene 1, 5-Cyclooctadiene Conjugated dienes are those in which two carbon-carbon double bond units are directly connected to each other by a single bond. 1, 3-Pentadiene and 1, 3-cyclooctadiene cor tain conjugated double bonds Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Although allylic brominations and chlorinations offer a method for attaching a reactive functional group to a hydrocarbon framework, we need to be aware of two important limitations. For allylic halogenation to be effective in a particular synthesis: 1. All the allylic hydrogens in the starting alkene must be equivalent. 2. Both resonance forms of the allylic radical must be equivalent. In the two examples cited so far, the chlorination of propene and the bromination of cyclohexene, both criteria are met. All the allylic hydrogens of propene are equivalent. The two resonance forms of allyl radical are equivalent. All the allylic hydrogens of cyclohexene are equivalent. The two resonance forms of 2-cyclohexenyl radical are equivalent. Unless both criteria are met, mixtures of constitutionally isomeric allylic halides result. PROBLEM 10.5 The two alkenes 2,3,3-trimethyl-1-butene and 1-octene were each subjected to allylic halogenation with N-bromosuccinimide. One of these alkenes yielded a single allylic bromide, whereas the other gave a mixture of two constitutionally isomeric allylic bromides. Match the chemical behavior to the correct alkene and give the structure of the allylic bromide(s) formed from each. 10.5 CLASSES OF DIENES Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates in chemical reactions. The third type of conjugated system that we will examine, conjugated dienes, consists of stable molecules. A hydrocarbon that contains two double bonds is called an alkadiene, and the relationship between the double bonds may be described as isolated, conjugated, or cumulated. Isolated diene units are those in which two carbon–carbon double bond units are separated from each other by one or more sp3 -hybridized carbon atoms. 1,4-Pentadiene and 1,5-cyclooctadiene have isolated double bonds: Conjugated dienes are those in which two carbon–carbon double bond units are directly connected to each other by a single bond. 1,3-Pentadiene and 1,3-cyclooctadiene contain conjugated double bonds: 1,5-Cyclooctadiene CH2 CHCH2CH CH2 1,4-Pentadiene H H H H H H H H H H H H H H H H CH2 CH CH2 CH2 CH CH2 CH2 CH CH3 372 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
10.5 Classes of dienes CH2=CH—CH=CHCH3 1, 3-Cyclooctadiene Cumulated dienes are those in which one carbon atom is common to two carbon-ca bon double bonds. The simplest cumulated diene is 1, 2 -propadiene, also called allene, Allene is an acceptable and compounds of this class are generally referred to as allenes UPAC name for 1.2 CH,-C-CH PROBLEM 10.6 Many naturally occurring substances contain several carbon-car- bon double bonds: some isolated, some conjugated, and some cumulated. Iden fy the types of carbon-carbon double bonds found in each of the following sub stances. (a)B-Springene (a scent substance from the dorsal gland of springboks (b)Humulene(found in hops and oil of cloves) H3CCH3 (c)Cembrene (occurs in pine resin) (CH3)2CH- (d) The sex attractant of the male dried-bean beetle CH3(CH2)6 CH2 CH==C-CH CO2CH3 SAMPLE SOLUTION (a)B-Springene has three isolated double bonds and a pair of conjugated double bonds Conjugated double bonds Isolated double bonds Isolated double bonds are separated from other double bonds by at least one sp hybridized carbon. Conjugated double bonds are joined by a single bond Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Cumulated dienes are those in which one carbon atom is common to two carbon–carbon double bonds. The simplest cumulated diene is 1,2-propadiene, also called allene, and compounds of this class are generally referred to as allenes. PROBLEM 10.6 Many naturally occurring substances contain several carbon–carbon double bonds: some isolated, some conjugated, and some cumulated. Identify the types of carbon–carbon double bonds found in each of the following substances: (a) -Springene (a scent substance from the dorsal gland of springboks) (b) Humulene (found in hops and oil of cloves) (c) Cembrene (occurs in pine resin) (d) The sex attractant of the male dried-bean beetle SAMPLE SOLUTION (a) -Springene has three isolated double bonds and a pair of conjugated double bonds: Isolated double bonds are separated from other double bonds by at least one sp3 - hybridized carbon. Conjugated double bonds are joined by a single bond. Isolated double bonds Conjugated double bonds CH3(CH2)6CH2CH C H CO2CH3 C H C CH (CH3)2CH CH3 CH3 CH3 CH3 CH3 H3C CH3 CH2 C CH2 1,2-Propadiene 1,3-Cyclooctadiene CH2 CH CH CHCH3 1,3-Pentadiene 10.5 Classes of Dienes 373 Allene is an acceptable IUPAC name for 1,2- propadiene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TEN Conjugation in Alkadienes and Allylic Systems Alkadienes are named according to the IuPAC rules by replacing the -ane of an alkane with -adiene and locating the position of each double bond by nt Compounds with three carbon- carbon double bonds are called alkatrienes and cordingly, those with four double bonds are alkatetraenes, and so on 10.6 RELATIVE STABILITIES OF DIENES Which is the most stable arrangement of double bonds in an alkadiene--isolated, con jugated, or cumulated? As we saw in Chapter 6, the stabilities of alkenes may be assessed by comparing their heats of hydrogenation. Figure 10.3 depicts the heats of hydrogenation of an iso- lated diene (1, 4-pentadiene) and a conjugated diene(1, 3-pentadiene), along with the alkenes 1-pentene and(E)-2-pentene. The figure shows that an isolated pair of double bonds behaves much like two independent alkene units. The measured heat of hydro- genation of the two double bonds in 1, 4-pentadiene is 252 kJ/mol(60.2 kcal/mol), exactly twice the heat of hydrogenation of 1-pentene. Furthermore, the heat evolved on hydro- genation of each double bond must be 126 kJ/mol (30.1 kcal/mol), since 1-pentene is an intermediate in the hydrogenation of 1, 4-pentadiene to pentane By the same reasoning, hydrogenation of the terminal double bond in the conju gated diene(E)-1, 3-pentadiene releases only Ill kJ/mol (26.5 kcal/mol) when it is hydrogenated to (E)-2-pentene. Hydrogenation of the terminal double bond in the con- nal double bond in the diene with isolated double bonds. A conjugated double bond is 15 kJ/mol (3.6 kcal/mol) more stable than a simple double bond. We call this increased stability due to conjugation the delocalization energy, resonance energy, or conjuga- tion energy The cumulated double bonds of an allenic system are of relatively high energy The heat of hydrogenation of allene is more than twice that of propene 252 kI/mol (60.2 kcal/mol) (54.1 kcal/mol) 1. 4-Pentadiene (0.1 kcal/mol 115 kJ/mol- -Pentene (E)-2-Pentene Pentane Pentane FIGURE 10.3 Heats of hydrogenation of some CsHio alkenes and CsHa alkadienes. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Alkadienes are named according to the IUPAC rules by replacing the -ane ending of an alkane with -adiene and locating the position of each double bond by number. Compounds with three carbon–carbon double bonds are called alkatrienes and named accordingly, those with four double bonds are alkatetraenes, and so on. 10.6 RELATIVE STABILITIES OF DIENES Which is the most stable arrangement of double bonds in an alkadiene—isolated, conjugated, or cumulated? As we saw in Chapter 6, the stabilities of alkenes may be assessed by comparing their heats of hydrogenation. Figure 10.3 depicts the heats of hydrogenation of an isolated diene (1,4-pentadiene) and a conjugated diene (1,3-pentadiene), along with the alkenes 1-pentene and (E)-2-pentene. The figure shows that an isolated pair of double bonds behaves much like two independent alkene units. The measured heat of hydrogenation of the two double bonds in 1,4-pentadiene is 252 kJ/mol (60.2 kcal/mol), exactly twice the heat of hydrogenation of 1-pentene. Furthermore, the heat evolved on hydrogenation of each double bond must be 126 kJ/mol (30.1 kcal/mol), since 1-pentene is an intermediate in the hydrogenation of 1,4-pentadiene to pentane. By the same reasoning, hydrogenation of the terminal double bond in the conjugated diene (E)-1,3-pentadiene releases only 111 kJ/mol (26.5 kcal/mol) when it is hydrogenated to (E)-2-pentene. Hydrogenation of the terminal double bond in the conjugated diene evolves 15 kJ/mol (3.6 kcal/mol) less heat than hydrogenation of a terminal double bond in the diene with isolated double bonds. A conjugated double bond is 15 kJ/mol (3.6 kcal/mol) more stable than a simple double bond. We call this increased stability due to conjugation the delocalization energy, resonance energy, or conjugation energy. The cumulated double bonds of an allenic system are of relatively high energy. The heat of hydrogenation of allene is more than twice that of propene. 374 CHAPTER TEN Conjugation in Alkadienes and Allylic Systems — 252 kJ/mol — (60.2 kcal/mol) 1,4-Pentadiene (E)-1,3-Pentadiene H2 H2 1-Pentene H2 (E)-2-Pentene H2 Pentane Pentane Energy — 226 kJ/mol — (54.1 kcal/mol) — 115 kJ/mol — (27.6 kcal/mol) — 126 kJ/mol — (30.1 kcal/mol) FIGURE 10.3 Heats of hydrogenation of some C5H10 alkenes and C5H8 alkadienes. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website