CHAPTER 14 Delocalized Pi Systems: Investigation by Ultraviolet and mkaqt Visible Spectroscopy reermaenyterceen a学 -c H eao 14-1 he2pcaeenrl（ay5ystemisrepresentedby d时 R09g3977e6me 8 1
1 CHAPTER 14 Delocalized Pi Systems: Investigation by Ultraviolet and Visible Spectroscopy Overlap of Three Adjacent p Orbitals: Electron Delocalization in the 2-Propenyl (Allyl) System 14-1 Three key observations: 1. The primary C-H bond in propene is relatively weak. 2. Compared to saturated primary haloalkanes, 3-chloropropene dissociates relatively fast under SN1 conditions and undergoes rapid unimolecular substitution through a carbocation intermediate. The 3-propenyl cation is more stable than other primary carbocations. Its stability has been found to be about equal to that of a secondary carbocation. 3. The pKa of propene is about 40 (that of propane is about 50). The formation of the propenyl anion by deprotonation appears to be unusually favored. Overlap of Three Adjacent p Orbitals: Electron Delocalization in the 2-Propenyl (Allyl) System 14-1 Delocalization stabilizes 2-propenyl (allyl) intermediates. In each of the preceding three unusual observations, a reactive center (a radical, a carbocation or a carbanion, respectively) was formed adjacent to a double bond. This arrangement imparts special stability to the reactive center in question through delocalization of the reactive center: The 2-propenyl (allyl) π system is represented by three molecular orbitals. The molecular orbital description of resonance involves 3 parallel p orbitals, each on an adjacent carbon atom:
me6治 一 888 。非非 8日 888 The electron density in the allyl system: 14-2 Radical Allylic Halogenation NH Br nopaoog tC-CCG-GG 2
2 Hybridizing 3 atomic p orbitals results in 3 molecular π orbitals: 1. π1: bonding, no nodes 2. π2: nonbonding, one node (same energy as p orbitals) 3. π3: antibonding, two nodes The Aufbau Principle is used to fill up the available π orbitals in order of lowest to highest energy. The electron density in the allyl system: 14-2 Radical Allylic Halogenation Under conditions of low halogen concentration, halogens add to allylic molecules through a radical chain mechanism called radical allylic substitution. N-bromobutanimide (N-bromosuccinimide) suspended in chloroform is often used in allylic brominations. It is nearly insoluble and reacts with trace amounts of HBr to generate very small amounts of bromine. The reaction mechanism is through a radical chain mechanism, initiated by light or traces of radical initiators. The resonance-stabilized radical can react at either end of the allylic system to produce an allylic bromide and another bromine radical
14-3 Nucleophilic Substitution of Allylic Halides: Allylic halides can also undergo Sx2 reactions. nemllo he p orbital radicalformed,xture of CHCH-OH CH-CHCH.C+CH-CHCH C T3 OICH-CHCHO CH.CHCHC+CHCHCH Cr I 14-5 Two Neighboring Double Bonds:Conjugated arbon N Proronp vatives h iheeo double bonds are amed CH:-CH-CH-CH CH,-C-CH -aa4mG- 好 人 llylic lithium and Grignard reagents can function as nucleophile CH:-c-CH: 个，个 3
3 Nucleophilic Substitution of Allylic Halides: SN1 and SN2 14-3 Allylic halides undergo SN1 reactions. Different allylic halides may give identical products if they dissociate into the same allylic cation. If an unsymmetrical allylic radical is formed, a mixture of products may be obtained. Allylic halides can also undergo SN2 reactions. SN2 reactions of allylic halides with good nucleophiles are faster than those of the corresponding saturated haloalkanes due to transition-state stabilization by overlap of the double bond and the p orbital. Allylic Organometallic Reagents: Useful Three-Carbon Nucleophiles 14-4 Alkyl lithium reagents can be made from propene derivatives by proton abstraction by an alkyl lithium. The Grignard derivative can also be prepared: Allylic lithium and Grignard reagents can function as nucleophiles. Two Neighboring Double Bonds: Conjugated Dienes 14-5 Hydrocarbons with two double bonds are named dienes. Dienes can be either non-conjugated (separated by at least one saturated carbon atom) or conjugated (immediately adjacent to one another). In a conjugated system the single atomic p orbital on each outer sp2 hybridized carbon atom overlaps one of the two perpendicular p orbitals on the inner sp hybridized carbon atom. Naming of conjugated and non-conjugated dienes is straightforward. The longest chain incorporating both double bonds is found and numbered to indicate the locations of substituent groups. Cis-trans, or E,Z prefixes indicate the configuration about the double bonds
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4 Conjugated dienes are more stable than nonconjugated dienes. The heat of hydrogenation of a single non-conjugated double bond is about -30 kcal mol-1, regardless of its position within an alkene. In the case of a conjugated diene, such as 1,3-butadiene, however, the heat of hydrogenation is less: This difference in energy is known as the resonance energy of 1,3-butadiene. Conjugation in 1,3-butadiene results from overlap of the π bonds. The conformation of 1,3-butadiene places the p orbitals of C2 and C3 parallel to each other, which permits the formation of a weak π interaction of a few kilocalories per mole. The weak C2-C3 π interaction also raises the barrier to rotation about the single bond to more than 6 kcal mol-1. The s-cis conformation is almost 3 kcal mol-1 less stable than the s-trans conformation. The molecular orbital diagram for butadiene shows two bonding molecular orbitals and two antibonding molecular orbitals. Electrophilic Attack on Conjugated Dienes: Kinetic and Thermodynamic Control 14-6 Even though they are more stable than dienes with isolated double bonds, conjugated dienes are actually more kinetically reactive with electrophiles and other reagents
Many electrophilic additions to dienes yield product mixtures: 色aaa言 danfad eanao3sritetontons:hneticand hermidanbytiagptf6ramebecetoodby1okngal o-o-c- the Cgleo侧Ctoatonatackbythebromaesasteratce Kineti Compared with Ceatrul CH-CHH ·CHCH onds are in coniugation.the molecule is aghahiahydelbcaleredarbocate o-o 5
5 The formation of two products can be understood by examining the reaction mechanism: Many electrophilic additions to dienes yield product mixtures: Changing product rations: kinetic and thermodynamic control. When carried out at 40o C rather than at 0o C, the hydrobromination of 1,3-butadiene gives a 15:85 ratio of bromobutenes, rather than a 70:30 ratio: At the higher temperature, the two isomers are in equilibrium with concentrations reflecting their relative thermodynamic stability. Under these conditions, the reaction is said to be under thermodynamic control. The product distribution at 0o C can be understood by looking at the potential energy diagram for the reaction. After the initial protonation, attack by the bromide is faster at C3 because of two factors: •C3 (secondary) carries more positive charge than C1 (primary). •After losing its proton, the bromide ion is closer to C3 than to C1. Delocalization Among More than Two π Bonds: Extended Conjugation and Benzene 14-7 Extended π systems are thermodynamically stable but kinetically reactive. When more than two double bonds are in conjugation, the molecule is called an extended π system. 1,3,5-hexatriene is quite reactive, polymerizes readily in the presence of electrophiles, but is also relatively stable thermodynamically. Electrophilic additions proceed through a highly delocalized carbocation
Be8ae,atoniugatedorcictnenesunusuaW rn人人 b-a 14-8 A Special Trans mation of Conjugated Dienes The cycload ielsnAieiCeogiienesgves g Diel-Alder HC. CH AnexmpefDiels-Aldercyco t g一- e8esga38aee 0 6
6 Benzene, a conjugated cyclic triene, is unusually stable. Benzene and its derivatives are unusually stable, both thermodynamically and kinetically. Benzene’s stability is related to its two equally contributing resonance forms: Because of its stability towards catalytic hydrogenation, hydration, halogenation and oxidation, benzene can be used as a solvent in organic reactions. A Special Transformation of Conjugated Dienes: Diels-Alder Cycloaddition 14-8 The cycloaddition of dienes to alkenes gives cyclohexenes. The simplest example of a cycloaddition reaction between π systems is the addition of ethene to 1,3-butadiene in the gas phase. This reaction is known as a Diels-Alder reaction or a [4+2]cycloaddition. The yield of the reaction between 1,3-butadiene and ethene is low. Better yields result when an electron-poor alkene and an electron-rich diene are used. Other electron-poor alkenes result when the alkene substituents interact with the double bond by resonance, placing a positive charge on an alkene carbon atom: An example of an efficient Diels-Alder cycloaddition: 1,3-Butadiene, without activating substituents, is sufficiently electron-rich to undergo cycloadditions with electron-poor alkenes
The Diels-Alder reaction is concerted. The Diels-Alderreos The ste IM-M0C. 女- Theecnrnomrwec8naorim。ndosoctv e品o2 the r 7
7 The Diels-Alder reaction is concerted. Both new C-C single bonds and the new π bond form simultaneously, just as the three π bonds in the starting materials break. This type of one-step reaction is called concerted. The Diels-Alder reaction is stereospecific. The stereochemistry at the original double bond of the dienophile is retained in the product. The stereochemistry of the diene is also retained. Note that only one enantiomer is shown in each of the above reactions. The other enantiomer is equally probable and a racemic mixture is formed in each case. Diels-Alder cycloadditions follow the endo rule. In the reaction of 1,3-cyclopentadiene with dimethyl cis-2- butenedioate, two products are conceivable: an exo adduct and an endo adduct: The Diels-Alder reaction usually proceeds with endo selectivity. This is a kinetic effect known as the endo rule. As an aid to determining the positioning of the substituents in an adduct, use the following diagram:
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8 14-9 Electrocyclic Reactions Electrocyclic reactions involve the ring closure (or opening) of a single conjugated di-, tri- or polyene. These reactions belong to the class of reactions called pericyclic, because they exhibit transition states with a cyclic array of nuclei and electrons. The ring closure of cis-1,3,5-hexatriene takes place thermally. The ring opening of cyclobutene (a strained system) also takes place thermally. Both reactions can be reversed by irradiation with ultraviolet light. Electrocyclic reactions are concerted and stereospecific. The thermal isomeration of cis-3,4- dimethylcyclobutene gives only the cis,trans product: The thermal isomeration of trans-3,4- dimethylcyclobutene gives only the trans,trans product: In the thermal cyclobutene ring opening, the rehybridization from sp3 to sp2 is accomplished by both atoms rotating either clockwise or counterclockwise. This mode of reaction is called a conrotary process. The photochemical closure of butadiene to cyclebutene occurs with the opposite stereochemistry found in the ring opening. If one carbon rotates clockwise, the other rotates counterclockwise. The mode of movement is called disrotatory. In the case of the thermal ring closure of trans,cis,trans- and cis,cis,trans-Octatriene, the process is disrotatory:
b8aeretmoegncdgdbsureodtrans,ds,trane 14-10 Polymerization of Coniugated Dienes:Rubber 2。 for mers 。 CH-C-CCH-CHCHCH Ee器d 1.4-Paly mereatiom of cme ageaeoiymertaton eicrubbers are derived from poly-1,3-dines Crono-Bekin --1. nt olyns oub rn 股2a8 S8pememeanse3seng6wengeatpmc3io8etes 9
9 However, the photochemical ring closure of trans,cis,transoctatriene is conrotatory. The stereocontrol of these and many other electrocyclic transformations is governed by the symmetry properties of the relevant π molecular orbitals and is summarized by the Woodward-Hoffmann rules: 14-10 Polymerization of Conjugated Dienes: Rubber 1,3-Butadiene can form cross-linked polymers. 1,2-Butadiene can be polymerized at C1 and C4 to yield polyethenylethene (polyvinylethylene) or at C1 and C4 to yield trans-polybutadiene, cis-polybutadiene or a mixed polymer. Because the product of butadiene polymerization is unsaturated, irradiation or radical initiators can cause additional linkages between individual polymer chains. These substances are called cross-linked polymers. Cross-linking generally increases the density and hardness of polymeric materials. In addition, cross-linking increases the elasticity of polymeric materials, a property characteristic of rubbers. Synthetic rubbers are derived from poly-1,3-dienes. 2-Methyl-1,3-butadiene (isoprene), when polymerized using a Ziegler-Natta catalyst, results in a synthetic rubber called polyisoprene of almost 100% Z configuration. 2-Chloro-1,3-butadiene can be polymerized into an elastic, heat- and oxygen-resistant polymer having trans double bonds called neoprene. The elasticity of polyisoprene-like polymers can be increased by a process called vulcanization, in which the polymer is treated with hot elemental sulfur, which causes additional crosslinks. Copolymers of 1,3-butadiene with other alkenes result in polymeric materials having a wide range of physical properties. Polyisoprene is the basis of natural rubber. Natural rubber results from the polymerization of 3-methyl-2- butenyl pyrophosphate:
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10 (OOP = pyrophosphate) Many natural products are composed of 2-methyl- 1,3-butadiene (isoprene) units. Geraniol and farnesol are two widely distributed mono-terpenes based on the isoprene structure: The biosynthetic precursor of steroids, squalene, is formed by the coupling of two molecules of farnesyl pyrophosphate: Camphor and other higher terpenes are formed by enzymatically controlled electrophilic carbon-carbon bond formation: Electronic Spectra: Ultraviolet and Visible Spectroscopy 14-11 Ultraviolet and visible light give rise to electronic excitations. Spectroscopy of organic molecules is possible because the absorption of radiation is restricted to quanta having energies corresponding to specific molecular excitations: Ultraviolet (200-400 nm) and visible (400-800 nm) spectroscopy are particularly useful for investigating the electronic structures of unsaturated molecules and for measuring their extent of conjugation. UV and visible spectroscopic samples are usually dissolved in solvents having no absorption peaks above 200 nm (ethanol, methanol, cyclohexane). Absorption of UV and visible light leads to the excitation of electrons from filled bonding (and sometimes non-bonding) molecular orbitals to unfilled antibonding orbitals. The patterns of absorption for a molecule are recorded as an electronic spectra. Ultraviolet and visible light give rise to electronic excitations. In the ground electronic state of a molecule, all electrons (except for lone pairs) occupy bonding molecular orbitals. The absorption of a photon of UV or visible light transfers an electron from its ground electronic state in a bonding orbital, to an excited electronic state in an antibonding orbital. The dissipation of this absorbed energy can be in the form of a chemical reaction, the emission of light (fluorescence, phosphorescence) or as heat