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Slides6 - Chapter 6 ALKENES STRUCTURE AND REACTIVITY...

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Chapter 6 ALKENES: STRUCTURE AND REACTIVITY
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INDUSTRIAL PREPARATION OF ALKENES Alkenes (or olefins) are hydrocarbons (C,H) that contain a C=C double bond. H C C H H H H C C H CH 3 H Ethene (Ethylene) Propene (Propylene) 26 million tons per year 14 million tons per year H C C C C H H H H H H H H H 2 H C C H H H + H H D (C-C) = 376 kJ/mol D (C-H) = 438 kJ/mol D ( π C=C) = 281 kJ/mol D (H-H) = 436 kJ/mol Δ H o = + 376 + 2(438) - 2(281) - 436 = + 254 kJ/mol but Δ S o is positive and T is high that makes T Δ S o > Δ H o and therefore Δ G o = Δ H o - T Δ S o < 0 Thermal cracking is an example of a reaction whose energetics are dominated by entropy ( Δ S o ) rather than by enthalpie ( Δ H o ). H C C H H H H 2 900 o C
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MOLECULE'S DEGREE OF UNSATURATION Because of double bonds, alkenes have fewer H than alkanes with the same number of C, and are therefore referred as unsaturated. Each double bond corresponds to a loss of 2 H from the alkane formula C n H 2n+2 Each ring corresponds to a loss of 2 H from the alkane formula C n H 2n+2 butene C 4 H 8 H 2 C CH CH 2 CH 3 H 2 C H 2 C CH 2 CH 2 cyclobutane C 4 H 8 H 2 C CH CH CH 2 butadiene C 4 H 6 HC H 2 C CH 2 CH cyclobutene C 4 H 6 HC HC CH CH cyclobutadiene C 4 H 4 H 2 C HC CH 2 CH bicyclobutane C 4 H 6 DOU = 0 DOU = 1 DOU = 1 DOU = 2 DOU = 2 DOU = 2 DOU = 3 H 3 C CH 2 CH 2 butane C 4 H 10 CH 3
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Degree of unsaturation: Number of double bonds + Number of rings DEGREE OF UNSATURATION OF MOLECULES CONTAINING OTHER ELEMENTS THAN C AND H ! Add the number of halogens ( F , Cl , Br ) to the number of H. Ignore the number of oxygens. Substract the number of nitrogens from the number of H. saturated C n H 2n+2 1 unsaturation C n H 2n 2 unsaturations C n H 2n -2 H 2 C CH CH Br CH 2 Br C 4 H 6 Br 2 C 4 H 6 H 2 H 2 C CH CH H CH 2 H C 4 H 8 = C n H 2n 1 unsaturation HC H 2 C CH 2 C O H C 4 H 6 O HC H 2 C CH 2 C H C 4 H 6 C n H 2n -2 2 unsaturations C 4 H 7 N HC H 2 C CH 2 C N H 2 C 4 H 6 HC H 2 C CH 2 C H C n H 2n -2 2 unsaturations DOU = 1 DOU = 2 DOU = 2
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ELECTRONIC STRUCTURE OF ALKENES AND CONSEQUENCES σ -bond π -bond 3 sp 2 orbital + 1 p orbital They are stereoisomers CC single bond: 376 kJ/mol CC double bond ( σ + π ): 657 kJ/mol CC π -bond: 281 kJ/mol
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