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

C-C single bond

Alkene 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

O

H n butadiene Vitamin A (retinal) poly-acetylene Arene

alkenes conjugated to carbonyls O O

H

acrolein cyclohexenone (a,b-unsaturated aldehyde, enal) (a,b-unsaturated ketone, enone)

alkenes conjugated to non-bonding pairs of electrons

O R O N

1 Preparation of conjugated dienes (1,3-dienes) from alkenes: allylic bromination followed by dehydrohalogenation

Br - + NBS, hn (CH3)3CO K

CCl4 Stability of conjugated double bonds: the double bonds of conjugated dienes are more stable than isolated double bonds. Table 14.1 (p. 525) DH° (hydrogenation) H , Catalyst 2 -126 KJ/mol

H , Catalyst 2 -253 KJ/mol (2 x 126 = 252) H , Catalyst 2 -110 KJ/mol (126 - 110 = 16) H2, Catalyst -236 KJ/mol (252 - 236 = 16)

p-molecular orbitals of an alkene

2 p-molecular orbitals of butadiene 3 Nodes 0 bonding interactions 3 antibonding interactions ANTIBONDING MO

2 Nodes 1 bonding interactions 2 antibonding interactions ANTIBONDING MO

1 Nodes 2 bonding interactions 1 antibonding interactions BONDING MO

0 Nodes 3 bonding interactions 0 antibonding interactions BONDING MO

y2 is the Highest Occupied Molecular Orbital (HOMO) y3 is the Lowest Unoccupied Molecular Orbital (LUMO)

y1 of Butadiene (no nodes, bonding MO)

Bonding Interaction • • • • The four p-electrons in y1 are delocalized over the four p-orbitals

• •• _ • _ + +

Table 14.2 (p. 528): Bond lengths in pm

H C CH CH CH H3C CH3 H2C CH2 H2C CH CH3 H2C CH CH CH2 2 2 154 133 149 148 134

3 Electrophilic Addition to Conjugated Alkenes: The addition of HX to butadiene

Recall: Electrophilic addition to alkenes follows Markovnikov’s Rule

X X H-X The observed product is H C + 3 CH derived from the most stable H C 3 H 3 carbocation intermediate not observed For a conjugated diene:

H-X X + H H X cis and trans 1,2-addition product 1,4-addition product The distribution of products is 0 °C 71% 29% dependent upon 40 °C 15% 85% temperature

The reaction goes through an allyl carbocation intermediate allyl carbocation is resonance stablized

H X + + H H

Br - Br -

X H H X

1,2-addition product 1,4-addition product

Other electrophilc additions give similar results

Br2 Br + Br Br Br

45 % 55 %

Br Br 2 + Br + Br Br Br Br 3 % 21 % 76 %

4 Kinetic vs. Thermodynamic Control of Reacctions H-Br Br + H H Br 0 °C 71% 29% 40 °C 15% 85% A B + C

‡ ‡ DGB > DGC B is formed faster than A. Rate (kinetics) favors B

DG°B < DG°C C is more stable than B. Thermodynamics favors C

Thermodynamic Control (DG°): At higher temperatures, all reactions are readily reversible. An equilibrium distributions of products is obtained (DG° = -RT lnKeq). The product with the lowest DG° is favored. Kinetic Control (DG‡): At 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 Sect. 14.7 Diene Polymers: Natural and Synthetic Rubbers (read) 14.8 Diels-Alder Cycloaddition Reaction (a very important reaction) Reaction between a conjugated diene and an alkene (dienophile) to give a cyclohexene

‡

Diene Dienophile cyclohexene

The Diels-Alder reaction is favored by electron withdrawing groups on the dienophile and electron donating groups on the diene.

H H O O O

R H H H OR

ethylene (unreactive) conjugated carbonyls (aldehydes, ketones and esters) O O N CO R C 2 O

O O Good dienophiles

Diels-Adlder Reaction: Mechanism: Pericyclic Reaction- proceeds in a single step via an "aromatic" transition state (pericyclic reaction).

‡ =

Diels-Alder Benzene Transition State The diene must adopt an s-cis conformation to be reactive:

HOMO s-trans s-cis diene (unreactive conformation) (reactive conformation)

LUMO dienophile

very unreactive very reactive diene diene

6 Endo vs. Exo Transition State: Generally, the endo transition state is favored.

exo minor H H

H endo H major

Stereochemistry: In pericyclic reactions, the stereochemistry of the reactants is preserved in the product. Recall the cyclopropanation of alkenes by carbenes which is also a pericyclic reaction.

CH2I2, CH2I2, R Zn(Cu) H R R Zn(Cu) H H R R H R R R R groups are trans R groups are trans R groups are cis R groups are cis in the reactant in the product in the reactant in the product

Stereochemistry of the Diels-Alder 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. H R R R H R = H H H H H O H O O O

Endo: cis dienophile Endo TS H's are cis

R H H H H R = R R H H H O H O O O trans dienophile Endo TS H's are trans

H R H H H R = R R H H H H O O O O Exo TS cis dienophile groups are cis

Exo: R R R R H H = H H H H H H O O O O Exo TS trans dienophile groups are trans

7 Diene: B In the s-cis conformation: A A= inner rim of diene A B= outer rim of diene

B 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:

B B A A O H O B O H H A + O O + A O H H O O B B A H O B A Endo TS Exo TS 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) .

Animations of the Diels-Alder Reaction: http://www.brunel.ac.uk/depts/chem/ch241s/re_view/barry/diels2.htm

UV-Vis Spectroscopy

1 nm = 10-9 m = 10-6 cm Infrared: molecular vibrations (stretches, bends) - identify functional groups (Ch. 12) Radiowaves: nuclear spin an a magnetic field (NMR) - gives a H and C map of the molecule (Ch. 13) UV-vis: valance electron transitions (Ch. 14) - gives information about p-bonds and conjugated systems

8 UV-Vis light causes electrons in lower energy molecular orbitals to be promoted to higher energy molecular orbitals. HOMO LUMO

Butadiene

E = h n n = c/l Butadiene h c E = l

Chromophore: light absorbing portion of a molecule

Beer’s Law: A = e c l A= 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

Absorbance is directly proportional to concentration

9 Molecular orbitals of conjugated polyenes g n i d n o b i t n A y g r e n E g n i d n o B

H2C CH2 180 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 Color absorbed light l 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 b-Carotene

Chemistry of Vision

OH

Cis

O G-protein coupled H receptor

+ opsin trans N H

+ hn opsin N G-protein Cascade H

O O O N NH N N NH NH N N N NH2 N O O O N NH2 O N NH2 O O -O P O -O P O P O P O O O - - - O- O O O O P O OH OH -O cGMP GTP GMP opens Ca2+ closes Ca2+ ion channel ion channel

11 Ca2+ light

cGMP Ca2+ cGMP GMP Ca2+ K+ K+

Na+ Na+

-O2C -O2C

+ opsin N H

+ opsin N H - - CO2 CO2

ring locks double bond into the cis geometry

CHO

12