Chapter 20 Flashcards
Conjugated Systems
–Systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds.
conjugated dienes
–From heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes.
butadiene
–Conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol .
•Conjugation of double bonds in butadiene gives the molecule an additional stability of approximately 17 kJ/mol
Structure of Butadiene MOs
•Combination of four parallel 2p atomic orbitals gives two pi-bonding MOs (this screen) and two pi-antibonding MOs (the next screen).
-the two pi-antibonding MOs of butadiene.
electromagnetic Spectrum
Note: Blue is higher energy than red; keep it in mind!UV-Vis
•Absorption of radiation in these regions give us information about conjugation of carbon-carbon and carbon-oxygen double bonds and their substitution.
•UV-Vis spectral data are plotted as absorbance (A) versus wavelength (nm)
–Typically, UV-visible spectra consist of one or a small number of broad absorptions.
–Wavelengths and energies required for pi to pi* transitions of ethylene and three conjugated polyenes
–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
–A pi to pi* transition in excitation of ethylene
Absorbance:
a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation at a particular wavelength
I00is the intensity of the incident radiation on the sampleI I is the intensity transmitted through the sample
Transmission:
a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation at a particular wavelength
Where:
II00is the intensity of the incident radiation on the sampleI I is the intensity transmitted through the sample
Beer-Lambert law
-the relationship between absorbance, concentration, and length of the sample tube
-A = Ecl
A = absorbance
c = concentration (mol • liter -1 )l = length of the sample tube (cm)
E = molar absorptivity (liter • mol-1 • cm -1 ). Experimental values of range from 0 to 106
Electronic Transitions
- Wavelengths and energies required for to * transitions of ethylene and three conjugated (alternating) polyenes
- The “longer the wire, the redder the absorbance”. This is a qualitative expression of Woodward’s rule.
UV-Visible Spectroscopy
•Absorption of UV-Vis radiation results in promotion of electrons from a lower-energy, occupied MO to a higher-energy,unoccupied MO.
–The energy of this radiation is sufficient to promote electrons in a pi- bonding (pi) MO to a pi-antibonding (pi) MO.
–Electrons in sigma bonding MOs are lower in energy and the UV radiation energy is no longer sufficient to promote the electrons to the empty anti-bonding MOs.
–Following are three examples of conjugated systems.
•UV-Visible spectroscopy of carbonyls.
–Simple aldehydes and ketones show only weak absorption in the UV due to an n to pi 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 pi to pi* transition.
1,2- and 1,4-Addition
•Addition of one mol of HBr to butadiene at -78°C gives a mixture of two constitutional isomers.
–We account for these products by the following two-step mechanism.
–The key intermediate is a resonance-stabilized allylic carbocation.
•Addition of one 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.
•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.
•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.
experimental information
–For addition of HBr at -78°C and Br2 at -15°C, the 1,2-addition products predominate; at higher temperatures (40° to 60°C), 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 FeBr3 or ZnBr2.
–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.
Extended Conjugation and Bromination
HBr (HCl) and bromine (Br2) will add to most extended pi systems that contain two or more double bonds in conjugation; generally the products formed at low temperature are kinetically inert but thermodynamically unstable relative to those with the two addition partners further apart.
Diels Alder and Other Pericyclic Reactions
- Transformations that include cycloadditions and electrocyclic reactions
- Exhibit transition states with a cyclic array of nuclei and electrons
All Diels-Alder reactions have the following features in common:
- They are initiated by heat; that is, the Diels-Alder reaction is a thermal reaction.
- They form new six-membered rings.
- Three pi bonds break, and two new C—C bonds and one new C—C pi bond forms.
- They are concerted; that is, all old bonds are broken and all new bonds are formed in a single step.
