Chapter 14: Conjugated Dienes and Ultraviolet Spectroscopy Diene: molecule with two double bonds Conjugated diene: alternating double and single bonds

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1 Chapter 14: Conjugated Dienes and Ultraviolet Spectroscopy Diene: molecule with two double bonds Conjugated diene: alternating double and single bonds C-C single bond lkene Diene C=C double bonds Conjugate Diene When the carbons of a conjugate diene all lie in the same plane, the p-molecular orbitals overlap. Conjugation: a series of overlapping p-orbitals alkenes conjugated to alkenes n butadiene Vitamin (retinal) poly-acetylene rene alkenes conjugated to carbonyls acrolein (a,b-unsaturated aldehyde, enal) cyclohexenone (a,b-unsaturated ketone, enone) alkenes conjugated to non-bonding pairs of electrons 1

2 Preparation of conjugated dienes (1,3-dienes) from alkenes: allylic bromination followed by dehydrohalogenation S, hn (C 3 ) 3 C - K CCl 4 Stability of conjugated double bonds: the double bonds of conjugated dienes are more stable than isolated double bonds. Table 14.1 (p. 525) 2, Catalyst D (hydrogenation) -126 KJ/mol 2, Catalyst 2, Catalyst 2, Catalyst -253 KJ/mol (2 x 126 = 252) -110 KJ/mol ( = 16) -236 KJ/mol ( = 16) p-molecular orbitals of an alkene 2

3 p-molecular orbitals of butadiene 3 odes 0 bonding interactions 3 antibonding interactions TIDIG M 2 odes 1 bonding interactions 2 antibonding interactions TIDIG M 1 odes 2 bonding interactions 1 antibonding interactions DIG M 0 odes 3 bonding interactions 0 antibonding interactions DIG M y 2 is the ighest ccupied Molecular rbital (M) y 3 is the Lowest Unoccupied Molecular rbital (LUM) y 1 of utadiene (no nodes, bonding M) onding Interaction The four p-electrons in y 1 are delocalized over the four p-orbitals _ _ Table 14.2 (p. 528): ond lengths in pm 3 C C 3 2 C C 2 2 C C C 3 2 C C C C 2 2 C C C C

4 Electrophilic ddition to Conjugated lkenes: The addition of to butadiene ecall: Electrophilic addition to alkenes follows Markovnikov s ule - 3 C 3 C For a conjugated diene: C 3 not observed The observed product is derived from the most stable carbocation intermediate The distribution of products is dependent upon temperature - 1,2-addition product cis and trans 1,4-addition product 0 C 71% 29% 40 C 15% 85% The reaction goes through an allyl carbocation intermediate allyl carbocation is resonance stablized - - 1,2-addition product 1,4-addition product ther electrophilc additions give similar results 2 45 % 55 % 2 3 % 21 % 76 % 4

5 Kinetic vs. Thermodynamic Control of eacctions C - 0 C 71% 29% 40 C 15% 85% DG > DG C is formed faster than. ate (kinetics) favors DG < DG C C is more stable than. Thermodynamics favors C Thermodynamic Control (DG ): t higher temperatures, all reactions are readily reversible. n equilibrium distributions of products is obtained (DG = -T lnkeq). The product with the lowest DG is favored. Kinetic Control (DG ): t lower temperatures, the reactions are not readily reversible (irreversible). The product distributions is governed by the rates by which the products form. The product with the lowest DG is favored. 5

6 Sect Diene Polymers: atural and Synthetic ubbers (read) 14.8 Diels-lder Cycloaddition eaction (a very important reaction) eaction between a conjugated diene and an alkene (dienophile) to give a cyclohexene Diene Dienophile cyclohexene The Diels-lder reaction is favored by electron withdrawing groups on the dienophile and electron donating groups on the diene. ethylene (unreactive) conjugated carbonyls (aldehydes, ketones and esters) C C 2 Good dienophiles Diels-dlder eaction: Mechanism: Pericyclic eaction- proceeds in a single step via an "aromatic" transition state (pericyclic reaction). = Diels-lder Transition State enzene The diene must adopt an s-cis conformation to be reactive: s-trans (unreactive conformation) s-cis (reactive conformation) M diene very unreactive diene very reactive diene LUM dienophile 6

7 Endo vs. Exo Transition State: Generally, the endo transition state is favored. exo minor endo major Stereochemistry: In pericyclic reactions, the stereochemistry of the reactants is preserved in the product. ecall the cyclopropanation of alkenes by carbenes which is also a pericyclic reaction. groups are trans in the reactant C 2 I 2, Zn(Cu) groups are trans in the product groups are cis in the reactant C 2 I 2, Zn(Cu) groups are cis in the product Stereochemistry of the Diels-lder reaction: Dienophile: Groups that are cis on the dienophile will be cis in the product; groups that are trans on the dienophile will be trans in the product. Endo: cis dienophile trans dienophile Endo TS Endo TS = 's are cis = 's are trans Exo: cis dienophile trans dienophile Exo TS Exo TS = groups are cis = groups are trans 7

8 Diene: In the s-cis conformation: = inner rim of diene = outer rim of diene Groups on the inner rim of the diene will be cis in the product and groups on the outer rim of the diene will be cis in the product: In the product, the groups of the dienophile that are endo in the transition state will be cis to the groups on the outer rim of the diene (in the s-cis conformation). Endo TS Exo TS nimations of the Diels-lder eaction: UV-Vis Spectroscopy 1 nm = 10-9 m = 10-6 cm Infrared: molecular vibrations (stretches, bends) - identify functional groups (Ch. 12) adiowaves: nuclear spin an a magnetic field (M) - gives a and C map of the molecule (Ch. 13) UV-vis: valance electron transitions (Ch. 14) - gives information about p-bonds and conjugated systems 8

9 UV-Vis light causes electrons in lower energy molecular orbitals to be promoted to higher energy molecular orbitals. M LUM utadiene utadiene E = h n n = c/l E = h c l Chromophore: light absorbing portion of a molecule eer s Law: = e c l = absorbance c = concentration (M, mol/l) l = sample path length (cm) e = molar absorbtivity (extinction coefficient) a proportionality constant for a specific absorbance of a substance bsorbance is directly proportional to concentration 9

10 Molecular orbitals of conjugated polyenes Energy onding ntibonding 2 C C nm 217 nm 258 nm 290 nm Molecules with extended conjugation move toward the visible region 380 nm 400 nm 450 nm 500 nm 550 nm 600 nm 700 nm 780 nm violet-indigo blue green yellow orange red Color of absorbed light l Color observed violet 400 nm yellow blue 450 orange blue-green 500 red yellow-green 530 red-violet yellow 550 violet orange 600 blue-green red 700 green 10

11 b-carotene Chemistry of Vision trans Cis opsin G-protein coupled receptor hn opsin G-protein Cascade - P P - - P - GTP 2 - P cgmp opens Ca 2 ion channel 2 - P - GMP closes Ca 2 ion channel 2 11

12 Ca 2 light Ca 2 K cgmp cgmp GMP Ca 2 K a a - 2 C - 2 C C 2 - opsin C 2 - opsin ring locks double bond into the cis geometry C 12