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
C=C double bonds
Diene
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 (a,b-unsaturated aldehyde, enal)
cyclohexenone (a,b-unsaturated ketone, enone)
alkenes conjugated to non-bonding pairs of electrons O O
R 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. DH° (hydrogenation) Table 14.1 (p. 525) H2, Catalyst
H2, Catalyst
H2, Catalyst
H2, Catalyst
-126 KJ/mol -253 KJ/mol (2 x 126 = 252) -110 KJ/mol (126 - 110 = 16)
-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 H3C
CH3
154
H2C
CH2
133
H2C CH CH3
149
H2C CH CH CH2
148
H2C CH CH CH2
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 derived from the most stable carbocation intermediate
+
H3C H3C
CH3
H
not observed
For a conjugated diene: X
H-X
+
H
H
X cis and trans
1,2-addition product
The distribution of products is dependent upon temperature
0 °C 40 °C
1,4-addition product
71% 15%
29% 85%
The reaction goes through an allyl carbocation intermediate allyl carbocation is resonance stablized +
+
H X
H
H Br -
Br -
X
H
X
H
1,2-addition product
1,4-addition product
Other electrophilc additions give similar results Br2
Br Br
+
Br
Br 55 %
45 % Br2
Br Br
+
+
Br
Br
Br
Br 3%
21 %
76 %
4
Kinetic vs. Thermodynamic Control of Reacctions H-Br
Br H
0 °C 40 °C A
71% 15%
+
H
Br
29% 85%
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 ‡
Dienophile
Diene
cyclohexene
The Diels-Alder reaction is favored by electron withdrawing groups on the dienophile and electron donating groups on the diene. H H
H
O
O
H
ethylene (unreactive)
O R
H
OR
conjugated carbonyls (aldehydes, ketones and esters) O O N CO2R C 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 Transition State
Benzene
The diene must adopt an s-cis conformation to be reactive:
s-trans (unreactive conformation)
s-cis (reactive conformation)
HOMO diene
LUMO dienophile very unreactive diene
very reactive 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. R
R
CH2I2, Zn(Cu)
H
R R groups are cis in the reactant
R H R groups are trans in the product
R groups are trans in the reactant
CH2I2, Zn(Cu)
R
R
H
H R R R groups are cis 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
H
O
Endo:
R
H
R H
O
cis dienophile
H H
H
O
Endo TS
H
H
R
H O trans dienophile
=
R H
O
Endo TS
H
H
R
H R
H
H O cis dienophile
H
O trans dienophile
H
O
groups are cis R
R
R H
H O Exo TS
R
=
O
Exo TS
H
H
R
O
Exo: R
O
H's are trans
H H
H
R
H
O
O
H's are cis R
H
R
=
H
H
H
=
O
H
H
O
groups are trans
7
Diene:
B In the s-cis conformation:
A
A= inner rim of diene B= outer rim of diene
A 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
O A A
B
B
H
+
A H
O
O H O
B
H A Endo TS
B
O
+
A H
O O
O B
H O A 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
Butadiene
E=hn n = c/l E=
hc 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
Bonding
Energy
Antibonding
Molecular orbitals of conjugated polyenes
H2C CH2 180 nm
258 nm
217 nm
290 nm
Molecules with extended conjugation move toward the visible region
380 nm
400 nm
violet-indigo
450 nm blue
Color of absorbed light violet blue blue-green yellow-green yellow orange red
500 nm
550 nm
green
yellow
600 nm orange
l 400 nm 450 500 530 550 600 700
700 nm
780 nm
red
Color observed yellow orange red red-violet violet blue-green green
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b-Carotene
Chemistry of Vision OH
Cis O
G-protein coupled receptor
H + opsin N H
trans
+ opsin N H
hn
G-protein Cascade
O
O N
N O O -O P O P O O-
O O P O O-
N O OH
GTP
N
N
NH2
O O P -O
O O cGMP
opens Ca2+ ion channel
O N
NH
NH N
NH2
O -O P O O-
N
NH N
NH2
O OH GMP closes Ca2+ ion channel
11
Ca2+ light
Ca2+ K+
cGMP
cGMP
GMP
Ca2+ K+
Na+
Na+
-O2C
-O2C
+ opsin N H
CO2-
+ opsin N H CO2-
ring locks double bond into the cis geometry
CHO
12
