Ch 15 – Flashcard Test Answers

1) What is the hybridization of the central carbon of allene (1,2-propadiene)?
A) sp
B) sp2
C) sp3
D) p
E) none of the above

2) What descriptive term is applied to the type of diene represented by 2,4-hexadiene?
A) conjugated diene
B) cumulated diene
C) isolated diene
D) alkynyl diene
E) none of the above

3) What descriptive term is applied to the type of diene represented by 1,5-octadiene?
A) conjugated diene
B) cumulated diene
C) isolated diene
D) alkynyl diene
E) none of the above

5) Which of the following molecules is chiral?
A) 1,2-pentadiene
B) 2,3-pentadiene
C) 2-methyl-2,3-pentadiene
D) 2-chloro-4-methyl-2,3-pentadiene
E) None of the above molecules is chiral.

6) Which of the following compounds has the most negative heat of hydrogenation?
A) 1,4-hexadiene
B) 1,5-hexadiene
C) 1,2-hexadiene
D) 1,3-hexadiene
E) hex-1-ene

7) Which compound has the smallest heat of hydrogenation?
A) 5-methyl-1,2-hexadiene
B) (E)-5-methyl-1,3-hexadiene
C) 5-methyl-1,4-hexadiene
D) 2-methyl-1,5-hexadiene
E) (E)-2-methyl-2,4-hexadiene

8) Rank the following dienes in order of increasing stability: trans-1,3-pentadiene, cis-1,3-pentadiene, 1,4-pentadiene, and 1,2-pentadiene.

9) Among a series of isomeric trienes, the more negative the ?H° of the hydrogenation reaction of a given triene, the ________ stable it is relative to the others in the isomeric series.

10) Which of the following compounds is the most stable?
A) (E)-2-methyl-1,3-pentadiene
B) 2-methyl-1,2-pentadiene
C) (Z)-2-methyl-1,3-pentadiene
D) 2-methyl-2,3-pentadiene
E) 2-methyl-1,4-pentadiene

11) Consider the hydrogenation of each compound listed and rank the compounds in order of increasing ?H°. The most negative ?H° should be listed first.

cis-pent-2-ene, 2,3-pentadiene, and trans-1,3-pentadiene

12) Consider the hydrogenation reaction of each compound listed and rank the compounds in order of increasing ?H° of this reaction. The most negative ?H° should be listed first.

2,5-dimethyl-1,3-cycloheptadiene,
1,4-dimethyl-1,3-cycloheptadiene, and
3,6-dimethyl-1,4-cycloheptadiene

13) When the relative energies of the s-cis and s-trans conformers of 1,3-butadiene are compared, one finds that:
A) the s-cis conformer is lower in energy than the s-trans.
B) the s-trans conformer is lower in energy than the s-cis.
C) the two conformers are of equal energy.

14) In the planar conformation of 1,3-butadiene, the electron density in the ?-system is ________ over the entire molecule.

19) How many nodes, other than the node coincident with the molecular plane, are found in the highest energy ? MO of 1,3-butadiene?
A) 0
B) 1
C) 2
D) 3
E) none of the above

27) When (S)-3-bromopent-1-ene is heated in water, which of the following compounds is not produced?
A) (S)-pent-1-en-3-ol
B) (R)-pent-1-en-3-ol
C) pent-4-en-1-ol
D) (E)-pent-2-en-1-ol
E) (Z)-pent-2-en-1-ol

28) Which of the following alkyl halides yields the most stable carbocation intermediate during solvolysis in hot ethanol?
A) methyl iodide
B) (S)-2-bromopentane
C) (R)-2-bromopentane
D) (S)-3-bromopent-1-ene
E) 1-chlorobutane

45) What is the major organic product which results when cycloheptene is irradiated in the presence of N-bromosuccinimide?
A) 1-bromocycloheptene
B) 2-bromocycloheptene
C) 1,2-dibromocycloheptane
D) 3-bromocycloheptene
E) 4-bromocycloheptene

47) Including all possible stereoisomeric forms, how many distinct allylic bromides could be produced when 2-methylpent-1-ene is treated with NBS under irradiation by a sunlamp?
A) 2
B) 3
C) 4
D) 5
E) 6

60) In the allyl radical, which ? molecular orbital is singly occupied?
A) The bonding ? molecular orbital.
B) The nonbonding ? molecular orbital.
C) The antibonding ? molecular orbital.
D) None of the above.

61) How many electrons are present in the nonbonding ? molecular orbital of the allyl cation?
A) 0
B) 1
C) 2
D) 3
E) 4

62) How many electrons are present in the nonbonding ? molecular orbital of the allyl cation?
A) 0
B) 1
C) 2
D) 3
E) 4

63) How many electrons populate the ? molecular orbitals of the allyl radical?
A) 0
B) 1
C) 2
D) 3
E) 4

64) Which substrate would react most rapidly in an SN2 reaction?
A) CH3CH2CH=CHCH2Br
B) BrCH2CH2CH=CHCH3
C) CH3CHBrCH=CH2CH3
D) CH3CH2CH2CH=CHBr

68) Which of the following terms best describes a Diels-Alder reaction?
A) a [4+2] cycloaddition
B) a [2+2] cycloaddition
C) a sigmatropic rearrangement
D) a 1,3-dipolar cycloaddition
E) a substitution reaction

69) When 1,3-butadiene reacts with CH2CHCN, which of the terms below best describes the product mixture?
A) a mixture of two diastereomers
B) a single compound
C) a racemic mixture
D) optically active
E) a mixture of bicyclic compounds

75) The Diels-Alder reaction is a concerted reaction; this means:
A) a mixture of endo and exo products are formed.
B) all bond making and bond breaking occurs simultaneously.
C) the products contain rings.
D) the reaction follows Markovnikov’s rule.
E) the reaction is highly endothermic.

76) Which of the following compounds is the most reactive dienophile in a Diels-Alder reaction with 1,3-butadiene?
A) CH2CHOCH3
B) CH2CHCHO
C) CH3CHCHCH3
D) (CH3)2CCH2
E) CH2CH2

121) Absorption of UV-visible energy by a molecule results in:
A) vibrational transitions
B) electronic transitions
C) rotational transitions
D) nuclear transitions
E) none of the above

122) Which of the following compounds absorbs the longest wavelength of UV-visible light?
A) (E)-but-2-ene
B) (Z)-but-2-ene
C) hex-1-ene
D) (Z)-1,3-hexadiene
E) (E)-1,3,5-hexatriene

123) UV spectroscopy measures the energy required to promote an electron from the ________ molecular orbital to the ________ molecular orbital.
A) highest occupied, lowest unoccupied
B) lowest occupied, lowest unoccupied
C) lowest occupied, highest unoccupied
D) highest occupied, highest unoccupied

125) How many nodes, other than the node coincident with the molecular plane, are present in the HOMO of 1,3,5-hexatriene?
A) 1
B) 2
C) 3
D) 4
E) 5

126) How many nodes, other than the node coincident with the molecular plane, are present in the LUMO of 1,3,5-hexatriene?
A) 1
B) 2
C) 3
D) 4
E) 5

127) In the UV-visible spectrum of (E)-1,3,5-hexatriene, the lowest energy absorption corresponds to a:
A) ? to ? transition.
B) ? to ?* transition.
C) ? to ?* transition.
D) ? to ?* transition.
E) ? to ?* transition.

129) Which has a lmax in its UV spectrum at a longer wavelength, 2,4-hexadiene, 1,4-hexadiene, or 1,5-hexadiene?