Orgo 16.6-16.9

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Orgo 16.6-16.9
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2014-01-07 09:17:55
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CHM 202
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  1. In using __ rule, we must be certain that the compound under consideration meets the criteria for an aromatic or antiaromatic system. 

    To qualify as aromatic or antiaromatic, what?

    Once these criteria are met __ applies.
    • Huckel's rule
    • a cyclic compound must have a continuous ring of overlapping p orbitals, usually in a planar conformation
    • Huckel's rule
  2. Huckel's rule?

    Common __ have 2,6, or 10 pi electrons, for N= 0, 1, or 2

    __ might have 4, 8, or 12 pi electrons, for N=1,2,3.

    Benzene is __, __, with a continuous ring of overlapping p orbitals. There are six pi electrons in benzene (three double bonds in the classical structure), so it is a (4N+2) system, with N=1. __ predicts benzene to be __.
    • if the number of pi electrons in the cyclic system is (4N+2), the system is aromatic
    • (4N)
    • N is an integer, commonly 0, 1, 2, or 3

    • [6]annulene
    • cyclic
    • Huckel's rule
    • aromatic
  3. Like benzene, cyclobutadiene (__) has a continuous ring of overlapping p orbitals. But it has four pi electrons (two double bonds in the classical structure), which is a (4N) system with N=1. __ predicts it to be __.

    Cyclooctatetraene is __, with 8 pi electrons (4 double bonds) in the classical structure. It is a (4N) system with N=2. If __ were applied to cyclooctatetraene, it would predict __. However, it is a stable hydrocarbon with a boiling point of 153 degrees Celsius. It does not show the high reactivity associated with __, yet it is not __ either. Its reactions are typical of alkenes.
    • [4]annulene
    • Huckel's rule
    • antiaromtaic

    • [8]annulene
    • Huckel's rule
    • antiaromaticity x2
    • aromatic
  4. Cyclooctatetraene would be __ if Huckel's rule applied, so the conjugation of its double bonds is energetically unfavorable. Remember that __ applies to a compound only if there is a continuous ring of overlapping p orbitals, usualyl in a __. Cyclooctatetraene is more flexible than cyclobutadiene, and it assumes a __ taht avoids most of the overlap between adjacent pi bonds. __ simply doesnt apply.
    • antiaromatic
    • Huckel's rule
    • planar system
    • nonplanar tub conformation
    • Huckel's rule
  5. Like cyclooctatetraene, why dont larger annulenes with (4N) systems show antiaromaticity?
    because they have the flexibility to adopt nonplanar conformations; they all react as partially conjugated polyenes
  6. __ in the larger (4N+2) annulenes depends on what? 

    In the all-cis[10]annulene, the planar conformaiton requries an excessive amount of __. The [10] annulene isomer with __ cannot adopt a planar conformation either, becuase two hydrogen atoms interfere with each other. Neither of these [10]annulene isomers is __, even though each has {4N+2} pi electrons, with N=2. If the interfering hydrogen atoms in the partially trans isomer are removed, the molecule can be __. When these hydrogen atoms are replaced with a bond, the aromatic compound __results.

    Some of the larger annulenes with (4N+2) pi electrons can achieve __.
    • aromaticity
    • whether the molecule can adopt the necessary planar conformation

    • angle strain
    • two trans double bonds
    • aromatic
    • planar
    • naphthalene
    • planar conformation
  7. Benzene is aromatic because it has what? The __ are filled and all electrons are paired. Cyclobutadiene, by contrast, has an __. There are two half-filled orbitals easily capable of donating or accepting electrons. To derive __, we must show under what general condition there is a filled shell of orbitals.
    • a filled shell of equal-energy orbitals
    • degenerate orbitals π2 and π3
    • open shell of electrons
    • Huckel's rule
  8. The pattern of MOs in a cyclic conjugated system:
    - There is one __, followed by __. The __ is always filled (two electrons). Each additional shell consists of two __, requiring four electrons to fill a shell.
    • all-bonding, lowest lying MO
    • degenerate pairs of bonding MOs
    • lowest-lying MO
    • degenerate MOs
  9. A compound has a filled shell of orbitals if it has two electrons for hte __, plus (4N) electrons, where N is the number of __. The total number of pi electrons in this case is (4N+2). If the system has a total of only (4N) electrons, it is two electrons short of filling N pairs of __. There are only two electrons in the Nth air of __. There is a __ shell, and __ predicts these will be __ (a __)>
    • lowest-lying orbital 
    • filled pairs of degenerate orbitals. 
    • degenerate orbitals x2
    • half-filled shell 
    • unpaired (diradical)
  10. __ are uncharged molecules having even numbers of carbon atoms with alternating single and double bonds. __ also applies to systems having odd numbers of carbon atoms and bearing positive or negative charges. 

    We can draw a five-membered ring of __ with all the __ lined up to form a __. With five pi electrons, this system would be __, but it would be a radical because an __ of electrons cannot all be paired. With four pi electrons (a __), what does Huckel's rule predict? With six pi electrons (an __), what does Huckel's rule predict?
    • annulenes
    • Huckel's rule
    • sp2 hybrid carbon atoms
    • unhybridized p orbitals
    • continuous ring
    • neutral
    • cation
    • this system to be antiaromatic
    • anion
    • aromaticity
  11. Because the cyclopentadienyl anion (six pi electrons) is __, it is unusually stable compared with other carbanions. It can be formed by __ from cyclopentadiene, which is unusually __for an alkene. Cyclopentadiene has a pKa of __ compared with a pKa of __ for cyclohexene. In fact, cyclopentadiene is nearly as acidic as water and more acidic than many alcohols. It is entirely __by potassium tert-butoxide.
    • aromatic
    • abstracting a proton
    • acidic 
    • 16
    • 46
    • ionized
  12. Cyclopentadiene is unusually acidic because what?
    Cyclopentadiene contains an __ carbon atom wiithout an __, so there can be no continuous ring of p orbitals. __ of the -CH2- group leaves an orbital occupied by a pair of electrons. This orbital can __ to a __, completing a ring of p orbitals containing six pi electrons: the two electrons on the __, plus the four electrons in the original double bonds.
    loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion.

    • sp3 hybrid
    • unhybridized p orbital
    • deprotonation
    • rehybridize
    • p orbital
    • deprotonated carbon
  13. Even though cyclopentadienyl anion is __,, it isn't as stable as benzene. As a carbanion the cyclopentadienyl anion reacts readily with __. Because this ion is __, however, it is more stable than the corresponding __.

    __ predicts that the cyclopentadienyl cation, with four pi electrons, is __. In agreement with this, the cyclopentadienyl cation is not easily formed. Protonated cyclopenta-2,3-dien-1-ol does not __ (to give the cyclopentadienyl cation), even in concentrated sulfuric acid
    • aromatic
    • electrophiles
    • aromatic
    • open-chain ion

    • huckel's rule
    • antiaromatic
    • lose water
  14. What approach is bad when dealing with cyclopentadienyl ions and their stability?

    As with the five-membered ring, we can imagine a flat seven-membered ring with seven p orbitals aligned. The cation has six pi electrons, and the anion has eight pi electrons. Once again, we can draw __ that seem to show either hte positive charge of the cation or the negative charge of the anion __ over all seven atoms of the ring. By now, however, we know that the six-electron system is __ and the eight is __.
    • resonance approach
    • resonance forms
    • delocalized 
    • aromatic
    • antiaromatic
  15. THE CYclohelptatrienyl caation is easily formed by treating the __ with dilute aqueous sulfurinc acid. This hydrocarbon cation is __ in aqueous solution.

    The cycloheptatrienyl cation is called the __. This __ ion is much __ than most carbocations. Some __ salts can be isolated and stored for months without decomposing. Nevertheless the __ ion is not necessarily as stable as benzene. Its __ simply implies that the __ ion is more stable than the corresponding __ ion.
    • corresponding alcohol
    • stable
    • troplium ion
    • aromatic ion
    • less reactive
    • troylium  x2
    • aromaticity
    • cyclic
    • open-chain
  16. Although the __ forms easily, the corresponding anion is difficult to form because it is __. Cycloheptatriene is barely more __ than propene and the __ is very __. This result agrees with the prediction of __ that the cycloheptatrienyl anion is _ if it is __.

    Aromatic stabilization leads to what? __ of hydrocarbons are rare and are usually much more difficult to form. Cyclooctatetraene reacts with potassium metal, however, to form an __.
    • tropylium ion
    • antiaromatic
    • acidic
    • anion
    • reactive
    • Huckel's rule
    • antiaromatic 
    • planar
    • unusually stable hydrocarbon anions
    • dianions 
    • aromatic dianion
  17. The cyclooctatetraene __ has a __, regular __ structure with C-C bond lengths of 1.40 Angstroms close to the 1.397 Angstrom bond lengths in benzene. Cyclooctatetraene itself has eight pi electrons, so the __ has ten: (4N+2) with N=2. The __ is easily prepared because it is __.
    • dianion
    • planar
    • octagonal
    • dianion
    • cyclooctatetraene dianion
    • aromatic
  18. The criteria for __ require a __, all with unhybridized p orbitals overlapping in a continuous ring. __, with rings containing sp2-hybridized atoms of other elements, can also be __. Nitrogen, oxygen,, and sulfur are the most common __ in __.
    • Huckel's rule
    • ring of atoms
    • Heterocyclic compounds
    • aromatic
    • heteroatoms
    • heterocyclic aromatic compounds
  19. __ is an aromatic nitrogen analogue of benzene. It has a six-membered __ with six pi electrons. __ has a N in place of one of the six C-H units of benzene, and the nonbonding pair of electrons on nitrogen replaces benzene's bond to a hydrogen atom. These nonbonding electrons are in an __ hybrid orbital in the plane of the ring. They are __ to the pi system and do not overlap with it.
    • pyridine
    • heterocyclic ring 
    • pyridine
    • sp2
    • perpendicular
  20. __ shows all the characteristics of aromatic compounds. It has __ of 113 kJ/mol and it usually undergoes __ rather than __. Because it has an available pair of nonbonding electrons, __ is basic. In an acidic solution, __ protonates to give the __ ion. The __ ion is still aromatic because the additional proton has no effect on the electrons of the __: it simply bonds to pyridine's nonbonding pair of electrons.
    • pyridine
    • resonance energy
    • substitution
    • additoin
    • pyridine x2
    • pyridinium x2
    • aromatic sextet
  21. __ is an aromatic 5- membered __, with one nitrogen atom and two double bonds. Although it may seem that __ has only four pi electrons, the N atom has a lone pair of electrons. The __ N atom is __ hybridized, and its unhybridized p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring. The lone pair on nitrogen occupies hte p orbital, and (unlike the llone pair of __) these electrons take part in the pi bonding system. These two electrons, added to the four pi electrons of the two double bonds, complete an __. __ has a __ of 92 kJ/mol.
    • pyrrole
    • heterocycle x3
    • sp2
    • pyridine
    • aromatic sextet
    • pyrrole
    • resonance energy
  22. __ is a much weaker base than __. This difference is due to the __. To form a bond to a proton requires the use of one of hte electron pairs in the __. In the __, the N atom is bonded to four different atoms (two carbons and two H atoms), requiring __ hybridization and leaving no unhybridized p orbital. The protonated __ is __. In fact, a sufficiently strong acid actually protonates __ at the 2-position, on one of the carbon atoms of the ring, rather than on nitrogen.
    • pyrrole
    • pyridine
    • structure of the protonated pyrrole
    • aromatic sextet
    • protonated pyrrole
    • sp3
    • pyrrole
    • nonaromatic
    • pyrrole
  23. __ is a six-membered __ with two N atoms situated in a 1,3-arrangement. Both N atoms are like the pyridine nitrogen. Each has its lone pair of electrons in the __ hybrid orbital in the plane of hte __. These lone pairs are not needed for hte __, and they are __, like the lone pair of pyridine.
    • pyrimidine
    • heterocycle
    • sp2
    • aromatic ring
    • aromatic sextet
    • basic
  24. __ is an aromatic five-membered __ with two N atoms. The lone pair of one of the nitrogen atoms (the one not bonded to a hydrogen) is in an sp2 obbital that is not involved in the __: this lone pair is __. The other nitrogen uses its third sp2 orbital to bond to hydrogen, and its lone pair is part of hte __. Like the pyrrole N atom, this __ N-H nitrogen is not very basic. Once __ is protonated, the two nitrogens become chemically equivalent. Either nitrogen can lose a proton and return to an __ molecule.
    • imidazole
    • heterocycle
    • aromatic system
    • basic
    • aromatic sextet
    • imidazole x3
  25. __ has an __ ring fused to a __ ring. __ has three basic nitrogen atoms and one __- like nitrogen.

    __ and __ derivatives serve in DNA and RNA to specify genetic code. __ derivatives enhance the catalytic activity of enzymes.
    • purine
    • imidazole
    • pyrimidine
    • purine
    • pyrrole
    • pyrimidine
    • purine
    • imidazole
  26. Like pyrrole, __ is an aromatic five-membered __, but in __, the heteroatom is __ instead of nitrogen. The classical structure for __ shows that the oxygen atom has two lone pairs of electrons. The oxygen atom is __ hybridized, and one of the lone pairs occupies an sp2 hybrid orbital. The other lone pair occupies the unhybridized p orbital, combining with the four electrons in the double bonds to give an aromatic sextet. __ has a __ of 67 kJ/mol.
    • furan
    • heterocycle
    • furan
    • O
    • furan
    • sp2
    • furan
    • resonance energy
  27. __ is similar to __ with a __ in place of the __. The bonding in __ is similar ot that in __, except that the __ atom uses an unhybridized 3p orbital to overlap with the 2p orbitals on the carbon atoms. The __ of thiophene is 121 kJ/mol.
    • thiophene
    • furan
    • S
    • O
    • thiophene
    • furan
    • S
    • resonance energy

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