3 Conjugated Molecules

Organic Chemistry: Structure and Function

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1 Conjugated Molecules - Part 1 Lecture Supplement: Thinkbook page 19
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2 What is Conjugation? Structure dominated by minimization of electron repulsion Familiar examples : Other factors may also influence structure: conjugation, aromaticity Methane Ethane H C H H H H H C H H H C C H H H H H H H H H H H Square planar or tetrahedral? Staggered or eclipsed? What are these? For any molecule best structure = lowest energy
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3 What is Conjugation? Review these useful terms... Isomers : same formula, different structure •Heat of formation (enthalpy of formation; H o f ): hypothetical enthalpy change when a substance is synthesized from elements in their standard states Example: ls stability: lower H o f = more stable isomer 4 C ( graphite ) + 3 H 2 ( g ) H o f = 29.9 kcal mol -1 4 C ( graphite ) + 3 H 2 ( g ) H o f = 47.7 kcal mol -1 More stable Case #1: Relative stability of C 4 H 6 isomers H o f comparisons only valid among isomers
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4 What is Conjugation? Case #1: Relative stability of C 4 H 6 isomers 29.9 30.7 32.0 C C CH 3 H 3 C 37.5 91.8 1,2-butadiene (an allene) s -cis-1,3-butadiene 45.8 C C CH 2 CH 3 H s -trans-1,3-butadiene 47.7 64.6 67.7 H 2 C C C CH 3 H 1,3-dienes H e a t o f f o r m a t i o n ( H o f ) , k c a l m o l - 1 ( c a l c u l a t e d ) Why this order? Ring strain? No ring strain Position of pi bonds? pi-sigma-pi bond sequence Two pi bonds One pi bond Ring strain Two pi bonds No ring strain Number of pi bonds?
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5 What is Conjugation? Case #2: Catalytic Hydrogenation of 1,3-Dienes versus 1,4-Dienes Catalytic hydrogenation : Addition of H 2 to a pi bond with a catalyst Example : C C H CH 3 CH 2 H H H 2 Pt CH 3 CH 2 C H H C H H H Thermodynamics : Lose: H-H sigma bond, C-C pi bond Gain: 2 x C-H sigma bond Bond strength generalization: sigma > pi Therefore catalytic hydrogenation is exothermic ( H < 0) H = -30 kcal mol -1
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6 What is Conjugation? Case #2: Catalytic hydrogenation of 1,3-dienes versus 1,4-dienes H = -65.1 kcal mol -1 H = -56.5 kcal mol -1 1,3-butadiene more stable than 2-butyne E n t h a l p y ( H ; k c a l m o l - 1 ) C C H 3 C CH 3 Same molecule = same H + 2 H 2 + 2 H 2 Fact : 1,3-butadiene more stable than 3-butyne Observation : H (1,3-butadiene butane) < H (3-butyne butane) Conclusion : Lowest H (for cat H 2 ) belongs to most stable isomer How can we use catalytic hydrogenation to probe C 4 H 6 isomer stability?
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7 What is Conjugation?
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  • Pi Bond, Chemical bond, kcal mol-1

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