Title: Conjugated Systems
1Conjugated Systems
Chapter 20
220.1 Conjugated Dienes, Table 20.1
- Comparison of heats of hydrogenation and relative
- stabilities of conjugated and unconjugated dienes
3Stability of Conjugated Dienes
- conjugation of the double bonds in 1,3-butadiene
gives an extra stability of approximately 17 kJ
(4.1 kcal)/mol.
1-butene
butadiene
4Conjugated Carbonyls ?,?-unsaturated
- systems containing conjugated double bonds, not
just those of dienes, are more stable than those
containing unconjugated double bonds.
520.2 A. 1,2- and 1,4-Addition (HBr)
- Addition of 1 mol of HBr to butadiene at -78C
gives a mixture of two constitutional isomers. - we account for these products by the following
two-step mechanism.
61,2- and 1,4-Addition (HBr)
- the key intermediate is a resonance-stabilized
allylic carbocation.
71,2- and 1,4-Addition (Br2)
- Addition of 1 mole of Br2 to butadiene at -15 C
also gives a mixture of two constitutional
isomers. - we account for the formation of these 1,2- and
1,4-addition products by a similar mechanism. - Note at 40C distribution favors the
1,4-product.
1,3-Butadiene
8Experimental Information
- for addition of HBr at -78C and Br2 at -15C,
the 1,2-addition products predominate at higher
temperatures (40 to 60C), the 1,4-addition
products predominate. - if the products of the low temperature addition
are warmed to the higher temperature, the product
composition becomes identical to the higher
temperature distribution the same result can be
accomplished using a Lewis acid catalyst, such as
FeCl3 or ZnCl2. - if either pure 1,2- or pure 1,4- addition product
is dissolved in an inert solvent at the higher
temperature and a Lewis acid catalyst added, an
equilibrium mixture of 1,2- and 1,4-product
forms the same equilibrium mixture is obtained
regardless of which isomer is used as the
starting material.
9B. 1,2- and 1,4-Addition
- We interpret these results using the concepts of
kinetic and thermodynamic control of reactions. - Kinetic control the distribution of products is
determined by their relative rates of formation - in addition of HBr and Br2 to a conjugated diene,
1,2-addition occurs faster than 1,4-addition.
101,2- and 1,4-Addition
- Thermodynamic control the distribution of
products is determined by their relative
stabilities. - in addition of HBr and Br2 to a butadiene, the
1,4-addition product is more stable than the
1,2-addition product.
111,2- and 1,4-Addition
- Figure 20.3 Kinetic vs thermodynamic control
121,2- and 1,4-Addition
- Is it a general rule that where two or more
products are formed from a common intermediate,
that the thermodynamically less stable product is
formed at a greater rate? - No.
- whether the thermodynamically more or less stable
product is formed at a greater rate from a common
intermediate depends very much on the particular
reaction and reaction conditions.
1320.3 A. UV-Visible Spectroscopy
- Absorption of radiation in these regions give us
information about conjugation of carbon-carbon
and carbon-oxygen double bonds and their
substation.
14UV-Visible Spectroscopy, Fig 20.4
- typically, UV-visible spectra consist of one or a
small number of broad absorptions
15UV-Visible Spectroscopy
- Beer-Lambert law the relationship between
absorbance, concentration, and length of the
sample cell (cuvette). - A absorbance (unitless) a measure of the
extent to which a compound absorbs radiation of a
particular wavelength. - e molar absorptivity (M-1cm-1) a
characteristic property of a compound - values range from zero to 106 M-1cm-1
- C concentration in Molarity (M)
- l length of the sample tube (cm).
16UV-Visible Spectroscopy
- the visible spectrum of b-carotene (the orange
pigment in carrots) dissolved in hexane shows
intense absorption maxima at 463 nm and 494 nm,
both in the blue-green region.
17B. UV-Visible Spectroscopy
- Absorption of UV-Vis energy results in move-ment
of an electron from a lower-energy occupied MO to
a higher-energy unoccupied MO. - the energy of the uv-vis is sufficient to promote
electrons from a pi (p) bonding MO to a pi
antibonding (p) MO. - but is generally not sufficient to affect
electrons in the much lower-energy sigma bonding
(s) MOs - following are three examples of conjugated
systems.
18Orbitals of Conjugated Dienes
- the pi system of butadiene is derived from the
combination of four 2p atomic orbitals there are
two bonding MOs and two antibonding MOs.
19Orbitals of Conjugated Dienes
- bond order
- three nodes -3
- (3 antibonding)
- two nodes -1
- (2 antibonding
- and 1 bonding)
- one node 1
- (1 antibonding
- and 2 bonding)
- no nodes 3
- (3 bonding)
20B. UV-Visible Electronic Transitions
- s -----------------------
-
- p -----------------------
-
- n -----------------------
-
- p -----------------------
-
- s -----------------------
21UV-Visible Spectroscopy
- UV-Visible spectroscopy of carbonyls.
- simple aldehydes and ketones show only weak
absorption in the UV due to an n to p electronic
transition of the carbonyl group. - if the carbonyl group is conjugated with one or
more carbon-carbon double bonds, intense
absorption occurs due to a p to p transition.
22UV-Visible Spectroscopy
- Figure 20.5 A p to p transition in excitation
of ethylene
23UV-Visible Spectroscopy
- Figure 20.6 A p to p transition in excitation
of 1,3-butadiene
24UV-Visible Spectroscopy, Table 20.3
- Wavelengths and energies required for p to p
transitions of ethylene and three conjugated
polyenes.
2524.6 Diels-Alder Reaction
- Diels-Alder reaction a cycloaddition reaction of
a conjugated diene and certain types of double
and triple bonds - dienophile diene-loving
- Diels-Alder adduct the product of a Diels-Alder
reaction
26 Pericyclic reactions
- Pericyclic reaction a reaction that takes place
in a single step, without intermediates, and
involves a cyclic redistribution of bonding
electrons. There three classes of pericyclic
reactions - 1. Cycloaddition two molecules combine to form
a ring. - 2. Electrocyclic one molecule, an
intramolecular cyclization. - 3. Sigmatropic an intramolecular rearrangement.
27 Pericyclic reactions
28 Pericyclic reactions
29Diels-Alder Reaction
- alkynes also function as dienophiles
- cycloaddition reaction a reaction in which two
reactants add together in a single step to form a
cyclic product
Diels-Alder adduct
30A. Diels-Alder Reaction
- the conformation of the diene must be s-cis
31B. Diels-Alder Reaction
- reaction is facilitated by a combination of
electron-withdrawing substituents on one reactant
and electron-releasing substituents on the other
pressure
Cyclohexene
Ethylene
1,3-Butadiene
O
O
3-Buten-2-one
1,3-Butadiene
O
O
3-Buten-2-one
32Diels-Alder Reaction
33C. Diels-Alder Reaction
- the Diels-Alder reaction can be used to form
bicyclic systems
H
170C
H
Diene
Dienophile
34Diels-Alder Reaction
- exo and endo are relative to the double bond
derived from the diene
exo (outside)
endo (inside)
35Diels-Alder Reaction
- for a Diels-Alder reaction under kinetic control,
endo orientation of the dienophile is favored
36D. Diels-Alder Reaction
- the configuration of the dienophile is retained
37E. Diels-Alder Reaction
- the configuration of the diene is retained
38Diels-Alder Reaction
- Figure 24.1 Mechanism of the Diels-Alder reaction
39Diels-Alder Reaction
Orbital overlap must obey orbital symmetry rules.
40F. Diels-Alder Reaction
- The 1,3 substitution product is not observed.
41F. Diels-Alder Reaction
- A 1,3 substitution product is not observed.
42Hexatriene Molecular Orbitals
two nodes
five nodes
one node
four nodes
no nodes
three nodes
43Conjugated Systems
End Chapter 20