Chapter 20 Flashcards
a conjugated diene is one in which the double bonds are separated by … so that the 2p orbitals of the adjacent pi bonds overlap
one single bond
an unconjugated diene is one in which the double bonds are separated by
two or more single bonds
a cumulated diene is one in which the two double bonds share an … hybridized C. In a cumulated diene, the 2p orbitals of the pi bonds do not …; so they are not …
sp; overlap; conjugated
the increased stability of conjugated double bonds results from … of the four pi electrons over the set of four parallel 2p orbitals
delocalization
in order for maximal overlap to occur, the 2p orbitals must be parallel; so the sp2 atoms of the conjugated systems must be
coplanar
the ratio of 1,2 addition to 1,4 addition for conjugated dienes is …, with … often predominating at lower temperature and … predominating at higher temperature
temperature dependent; 1,2 addition; 1,4 addition
1,2 addition to butadiene leads to the predominant product formed at lower temperature under kinetic control, bc there is usually greater … at the 2 position of the allylic cation intermediate, lowering the activation barrier for rxn at this position. the lower temperature prevents … between products, so relative product … is not important
positive charge; equilibration; stability
1,4 addition to butadiene leads to the predominant product formed at higher temperature under thermodynamic control, bc the double bond of the 1,4 addition product is more … and therefore of … the higher temperature allows equilibration of products so that product distribution depends on relative product …
substituted; lower energy; stability
the UV region of the electromagnetic spectrum has wavelengths extending between … nm and … nm, and the visible region has wavelengths btwn … nm and … nm
200; 400; 400; 700
ultraviolet and visible spectral data are plotted as absorbance vs wavelength where absorbance is calculated as the log base 10 of the ratio (I0/ I) where I0 is the … of light at a given wavelength irradiating a sample and I is the light … through the sample
intensity; transmitted
Absorption of electromagnetic radiation in the ultraviolet-visible region results in promotion of an electron from a …. to a ….
lower energy, occupied molecular orbital; higher energy, unoccupied molecular orbital.
sigma bonding electrons are usually too … to take part in UV or visible light absorption
low in E
The greater the number of conjugated p bonds, the … the pi –> pi* energy gap, so the …the wavelength of absorbed light.
smaller; longer
Conjugated p systems have pi –> pi* transitions that can be seen in the ultraviolet or even visible absorption region, because conjugation … the energy difference between filled and unfilled p orbitals
decreases
… groups can take part in conjugation along with C”C double bonds
Carbonyl
pericyclic rxns occur in a single step involving a transition state that has a closed
loop of orbitals
frontier molecular orbital theory is used to determine if a pericyclic rxn is … or …
allowed; forbidden
frontier molecular orbital theory:
first a reaction geo is proposed
second, the homo and lumo of the reacting partners are written
third, the interaction between the homo and lumo of the partners is examined in order to reveal whether an … number of phase changes (most commonly 0) or an … number (most commonly one) exist at the points of the interaction
if the number of phase changes is even, the reaction is …, and if it is odd, the reaction is .. and is generally not observed
even; odd; allowed; forbidden
frontier molecular orbital theory predicts that … cycloadditions in which the reactants interact in a … manner are allowed but that analogous … cycloadditions are forbidden
4 + 2; suprafacial; 2 + 2
The Diels-Alder reaction is facilitated by having … groups such as carbonyls on one reactant (usually the …) and … groups on the other (usually the ….)
electron-withdrawing; dienophile; electron-releasing; diene
The diene must be in the … conformation to react, and dienes such as cyclopentadiene that are constrained to be in this conformation are particularly reactive.
s-cis
When cyclic dienes are used, a …Diels-Alder adduct is produced
bicyclic
The terms exo and endo are used with bicyclic Diels-Alder adducts. Exo substituents are on the … side of the newly formed ring from the diene-derived two-carbon bridge, and endo substituents are on the … For reactions that give kinetic products (not at equilibrium), the … orientation of the dienophile is preferred.
opposite; same side; endo
The configuration of the … (i.e., E or Z ) is retained in the Diels-Alder reaction, as is the relative orientation of groups on the …., indicating a highly concerted reaction mechanism
dienophile; diene
The mechanism of the reaction is concerted in that there is a single six-membered ring transition state in which the … pi bonds are … at the same time as the .. new … bonds and … new … bond are being created.
three; breaking; two; sigma; one; pi;
The Diels-Alder reaction has high stereoselectivity. One way to create enantiomerically pure target molecules is to use a … , which is a chiral molecule available as a single enantiomer that is bonded to the … The use of a chiral auxiliary can influence the resulting stereochemistry of a Diels-Alder reaction, producing a desired enantiomer in excess. The chiral auxiliary is then removed.
chiral auxiliary; starting material
Sigmatropic shifts involve the migration of a … bond across one or more … systems.
sigma; pi
Frontier molecular orbital analysis shows that the reaction is allowed in a geometry that creates a … transition state, although boatlike transition states can alsooccur.
chairlike
One example of the Claisen rearrangement transforms allyl phenyl ethers to… through the redistribution of six electrons in a cyclic transition state.
o-allylphenols
The Cope rearrangement of 1,5-dienes produces an equilibrium mixture of …. through the redistribution of six electrons in a cyclic transition state.
isomeric 1,5-dienes
