Ch 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Ch 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

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15 Conjugated Systems, OrbiTal SYMMeTrY, and ULtravIoLet SpectRoscoPy Double bonds can interact with each other if they aRe sepaRated by just one singLe bond. Such InteRacting doubLe bonds aRe said to be conjugated. DoubLe bonds with two oR moRe singLe bonds separatIng them have Little inteRaction and aRe caL±ed isolated double bonds± FoR exampLe, 1,3-pentadiene has conJugated doubLe bonds² and 1³4´pentadiene has isoLated doubLe bonds. conjugated double boNds (more stAbLe ThAN isolated do u b±e boNds) -"c=c/ ±²C=C/ / / -.±² C H2 Iso²Ated doUble boNds Because of the Interaction between the doubLe bondsµ systems containing conJu¶ gated doubLe bonds tend to be more stable than similaR systems with isoLated doubLe bonds. ·n this chapteR, we consideR the unique pRoperties of conjugated systems, the theoRetIcaL Reasons foR this extra stabILity, and some of the chaRacteRistic reactIons of molecuLes containIng conJugated doubLe bonds¸ We aLso study uLtRavIoLet spec¹ tRoscopy, a too± foR deteRmining the structuºes of conjugated systems. ·n Chapter 7² we used heats of hydrogenation to compare the Re±ative stabiLities of alkenes» For example¼ the heats of hydRogenatIon of 1´pentene and T½AN¾-2-pentene show that the disubstituted doubLe bond In ¿½AN¾´2´pentene is À0 kÁ/moL (2¸5 kcaL/moL) more stabLe than the monosUbstituted doubLe bond In 1-pentene tO = Ã]26 kÁ ÄÃÅ0Â Æ kca±) J -peN³eNe TW = Ã1Æ6 kÁ (Ã27.6 kcaÇ) ´Rµ¶S·2·peNteNe 15-1 Introduction 15-2 Stabilities of Dienes 663
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