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Ch 6 - Structure and Syntehesis of Alkenes

Ch 6 - Structure and Syntehesis of Alkenes - Structure and...

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7 Structure and Synthesis of Alkenes Alkenes are hydrocarbons with carbon-carbon double bonds. Alkenes are some± times caLLed oLefns, a teRm derIved fRom olefiant gas, meanIng "oIL²foRmIng gas.³ ThIs term oRIgInated wIth earLy experImentalIsts who notIced the oiLy appeaRance of aLkene derIvatIves´ ALkenes are among the most important industrial compounds (see Section 7µ6), and many alkenes aRe also found in pLants and anImaLs¶ Ethylene is the LaRgest·volume ¸ndu¹tRºal oRganic compound, used to make poLyethyLene aNd a vaRIety of other industriaL and consumeR chemIcals´ PInene Is a majoR component of turpent±ne, the paInt solvent distIlled fRom extracts of evergReen tRees´ Muscalure (C»¼µ9µtricosene) is the sex attractant of the common housefly. H" / H C=C ± ' H H ethylene (eThene) a-pinene cIs-9-tr±CoSene, " muS²ALuRe³ The bond eneRgy of a carbon·½arbon double bond is about 611 kJ/moL (146 kcaL/moL), compared with the sIngLeµbond eneRgy of about 347 kJ/moL (83 kcaL/moL). FRom these eneRgIes, we can caLculate the appRoxImate eneRgy of a pI bond: double¾bond dIssociatIon energy subtract sigma bond dIssociatIon energy pI bond dissociatIon energy 6¿¿ kJ/mol À - Á347 kJ i moL 264 kJ/moL (146 kcaL/mol) À ÂÁ(83 kcaLÃmol) (63 kcal/moL) This vaLue of 264 kJ/moL is much Less than the sIgma bond eneRgy of 347 kJ/moL, showIng that pI bonds should be more ReactIve than sigma bonds. Because a carbon-carbon double bond is ReLatIvely Reactive, It Is consIdeRed to be AFunct±onal group, and Its ReactIons aRe characterIstic of alkenes¶ Än previous chapÅ ters, we saw alkene synthesis by eliminatIon reactIons and we encounteRed a few ReacÆ tions of alkenes´ Än this chapteR, we study aLkenes In moRe detaiLÇ concentRatIng on theIR pRopeRtIes and the ways they are synthesIzed. 7-1 Introduction 279
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280 Chapter 7: StRuctUre and Synthesis of Alkenes 7-2 The Orbital DescriptIon of the ALkene Double Bond In a Lewis stRucture, the doub±e bond of an a±kene is represented by two pairs of elec- tRons between the caRbon atoms. The Pau±i exc±usion princip±e tells us that two paiRs of e±ectRons can go into one region of space between the caRbon nuc±ei on±y if each pair has its own molecu±ar orbita±² ³sing ethy±ene as an example, let's consider how the e±ectrons aRe distributed in the double bond´ 7-2A The Sigma Bond Framework µn Section 2¶4, we saw how we can visualize the sigma bonds of organic molecules using hybRid atomic orbitals´ µn ethylene, each carbon atom is bonded to three othe· atoms (one caRbon and two hydrogens), and theRe are no non bonding e±ectrons² Three hybrid orbita±s are needed, implying ¸¹ º hybridization» We have seen (Section 2¶4) that ¸¹ º hybridization coRresponds to bond ang±es of about 120°¼ giving optimum sep½ aration of thRee atoms bonded to the carbon atom.
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Ch 6 - Structure and Syntehesis of Alkenes - Structure and...

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