36 Chap 20 : Conjugated Systems 20.1 Stability of Conjugated Dienes Diene : 2 개의 C=C 이중결합을가진화합물 C 1,4-Pentadiene 1,3-Pentadiene 1,2-Pentadiene (unconjugated diene) (conjugated diene) (cumulated diene) ( 비콘쥬게이션다이엔 ) ( 콘쥬게이션다이엔 ) ( 연이은다이엔 ) 문제 20.1 : * Conjugated diene 안정성의측정법 - eat of hydrogenation ( 수소화반응열 ) 을측정하여 conjugation energy를 예측함. 수소화반응 - 발열반응 ( < 0) Table 20.1 : eat of hydrogenation of alkenes Name Structural formula, kj (kcal)/mol 1-Butene -127 (-30.3) 1-Pentene -126 (-30.1) cis-2-butene -120 (-28.6) trans-2-butene -115 (-27.6) 1,3-Butadiene -237 (-56.5) trans-1,3-pentadiene -226 (-54.1) 1,4-Pentadiene -254 (-60.8) 2 + 2 2 Pt + 2 2 Pt = 2 x (-127 kj/mol) = -254 kj/mol = -237 kj/mol Conjugation energy = 254-237 = 17 kj/mol Figure 20.1 :
37 Figure 20.2 : Molecular orbital of 1,3-butadiene 20.2 Electrophilic Addition to Conjugated Dienes A. 1,2-Addition and 1,4-addition Addition of to 1,3-butadiene : C 2 C C C 2 + C 2 C C C 2 + C 2 C C C 2 3-omo-1-butene (1,2-addition) 1-omo-2-butene (1,4-addition) -78 o C : 90% 10% -15 o C : 54% 46% Mechanism : C 2 C C C 2 + C 2 C C C 2 allylic, 2 o (more stable) C 2 C C C 2 allylic, 1 o (less stable) C 2 C C C 2 1,2-addition (less stable) C 2 C C C 2 1,4-addition (more stable) -78 o C : 상대적으로온도가낮아에너지공급이충분하지못하므로안정한중간체를거쳐서반응이먼저일어남 Kinetic control ( 속도론적지배 ) or Rate control -15 o C : 상대적으로온도가높아에너지공급이충분하므로생성물이안정한쪽으로반응이일어남 Thermodynamic control ( 열역학적지배 ) or Equilibrium control ( 평형지배 ) Energy diagram :
38 예제 20.3 : Addition of to 2,4-hexadiene B. Kinetic versus Thermodynamic Control of Electrophilic Addition Electrophilic addition of conjugated diene : 1) 일반적으로낮은온도에서는 1,2-첨가반응생성물이 1,4-첨가반응생성물보다많이생성된다. - 안정한중간체가반응을지배 C 2 C C C 2 allylic, 2 o (more stable) C 2 C C C 2 allylic, 1 o (less stable) 2) 일반적으로높은온도에서는 1,4-첨가반응생성물이 1,2-첨가반응생성물보다많이생성된다. - 안정한생성물이반응을지배 C=C의이중결합에치환된알킬기의개수가많을수록알켄의열역학적인안정성은증가함. (Saytzeff rule) 3 C C 3 C C 2 and C C C 2 less stable more stable 2 C C 2 CC C 2 and C C C 2 less stable more stable 예외 : Addition of 2 to 1,4-dimethyl-1,3-cyclohexadiene C 3 2 높은온도 C 3 + C 3 C 3 C 3 1,4-addition product (less stable) C 3 1,2-addition product (more stable)
39 3) 낮은온도에서생성된 1,2- 첨가반응생성물을산촉매하에서높은온도로가열 하면, 생성물의조성이변화하여그화학반응이높은온도에서일어났을때의 조성과같게된다. C 2 C C C 2 + + C 2 C C C 2 + C 2 C C C 2 54% 46% 20.3 Uv-Visible Spectroscopy A. Introduction Near ultraviolet ( 근자외선 ) : 200-400 nm 에너지 : 299-598 kj/mol or 71.5-143 kcal/mol Visible ( 가시광선 ) : 400 nm ( 보라색 )-700 nm ( 적색 ) (or 800~1000 nm) 171-299 kj/mol or 40.9-71.5 kcal/mol 예제 20.4 : Polyene 폴리엔의농도는 Uv 스펙트럼을측정하여농도를결정할수있음 Beer-Lambert law : Absorbance ( 흡광도 ) = log( I o ) = ε c l I A : 화합물이특정파장의복사선을흡수하는척도를가늠하는척도 0 ~ 1 (2) I o : 입사광의세기 I : 투과광의세기 ε : molar absorptivity ( 몰흡광계수 ) - 화합물 1M 용액의흡광도화합물의특성이며, 농도및빛의경로길이의영향을받지않음 c : 농도 M/liter (L) l : cell의길이 ( 대개 1 cm) Figure 20.4 : Uv spectrum of 2,5-dimethyl-2,4-hexadiene
40 예제 20.5 : 문제 20.5 : -Carotene Carbonyl : Simple aldehyde & ketone : n π * electronic transition - weak Conjugated carbonyl compound : π π * elecctronic transition - strong 2-pentanone 3-penten-2-one acetophenone λ max, 180 nm(ε, 900) λ max, 224 nm(ε, 12590) λ max, 246 nm(ε, 9800) * Curry Curcuma longa L. 의뿌리에서얻어지는천연염료 Me Me Me Me Curcumin (Natural yellow-3)
41 B. The rigin of Transitions between Electronic Energy Levels Table 20.3 : Wavelengths and Energies Required for π π * transitions of Ethylene and Three Conjugated Polyenes Name Structural formula λ max (nm) Energy(kcal/mol ) Ethylene C 2 =C 2 165 173 1,3-Butadiene C 2 =CC=C 2 217 132 (3E)-1,3,5-exatriene C 2 =CC=CC=C 2 268 107 (3E,5E)-1,3,5,7-ctatetraene C 2 =C(C=C) 2 C=C 2 290 92 Electronic transition : M LUM M - ighest ccupied Molecular rbital LUM - Lowest Unoccupied Molecular rbital Molecular orbital energy level of ethylene and 1,3-butadiene :
42 Problems 20.9 : Major product of 1,4-addition of Cl and isoprene 20.11 : Major product of 1,2-addition of Cl and cyclopentadiene + Cl Cl or Cl 3-chlorocyclopentadiene 4-chlorocyclopentadiene 20.14 : Kinetic or thermodynamic product of diene and 2 20.15 : Identification of 1,3-cyclohexadiene and 1,4-cyclohexadiene by uv spectroscopy