Deck 15: Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Question 1

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.

Answer 1

the s-trans conformer is lower in energy than the s-cis.

Question 2

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.

Answer 2

2,3-pentadiene

Question 3

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

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

Answer 3

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

Question 4

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

Question 5

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

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

Question 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

Question 7

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

Question 8

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

Question 9

What descriptive term is applied to the type of diene shown?

A) conjugated diene
B) cumulated diene
C) isolated diene
D) alkynyl diene
E) none of the above

Question 10

Show how the carbon p orbitals overlap to form the lowest energy π molecular orbital of 1,3-butadiene.

Question 11

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

Question 12

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

Question 13

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

Question 14

What is the resonance energy of a system?

Question 15

Draw the most stable conformation of (2Z,4E)-2,4-hexadiene.

Question 16

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

Question 17

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

Question 18

Draw (Z)-1,3-hexadiene in its s-trans conformation.

Question 19

Draw (E)-1,3-hexadiene in its s-cis conformation.

Question 20

How many nodes, other than the node coincident with the molecular plane, are found in the highest energy π MO of 1,3-butadiene?

A) 0
B) 1
C) 2
D) 3
E) none of the above

Question 21

In the addition of HBr to conjugated dienes, is the product which results from 1,2-addition or that which results from 1,4-addition typically the product of kinetic control?

Question 22

When 3-bromobut-1-ene is heated in a good ionizing solvent, reactions occur through a carbocation intermediate. Draw all reasonable resonance contributors of this cation and indicate which is the major contributor.

Question 23

Provide the structure of the major product which results from 1,4-addition of Br₂ to the diene shown below.

Question 24

Which of the following represents the highest occupied molecular orbital for the conjugated pi system in Vitamin D3?

A)
B)
C)
D)

Question 25

Give a representation of the highest occupied π MO of 1,3-butadiene in its ground state.

Question 26

Provide the structure of the major product which results from 1,2-addition of HBr to the diene shown below.

Question 27

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

Question 28

How many molecular orbitals are needed to represent the conjugated pi system of β-carotene?

A) 1
B) 10
C) 11
D) 20
E) 22

Question 29

When 3-bromo-1-methylcyclohexene is heated in a good ionizing solvent, reactions occur through a carbocation intermediate. Draw all reasonable resonance contributors of this cation and indicate which is the major contributor.

Question 30

Draw the HOMO of 1,3-butadiene.

Question 31

Name the two major products which are formed when 3-bromo-1-methylcyclohexene undergoes solvolysis in hot methanol.

Question 32

When 1-methylcyclohex-2-en-1-ol is treated with HBr, two constitutionally isomeric bromides are produced. Provide a detailed, stepwise mechanism which explains this observation.

Question 33

Give a representation of the lowest occupied π MO of 1,3-butadiene in its ground state.

Question 34

Draw structures for the two major products of the following reaction.

Question 35

When 1 mole of anhydrous HCl is reacted with excess 1,3-pentadiene, both the 1,2 and the 1,4-addition products are formed. Which of the following structures shown below is the least likely to be one of these products? (Note: When a chiral carbon is formed in this reaction a racemic mixture results, only one of the two possible enantiomers is shown.)

A)

B)

C)

D)

E)

Question 36

Provide the two major organic products of the following reaction.

Question 37

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

Question 38

Draw the LUMO of 1,3-butadiene.

Question 39

Provide a detailed, stepwise mechanism for the reaction shown below.

Question 40

Provide the two major organic products of the following reaction.

Question 41

Draw the HOMO of pentadienyl anion.

Question 42

Draw the resonance structures of the intermediate and then predict the two major products in the following reaction.

Question 43

Give a representation of the bonding π molecular orbital of the allyl anion.

Question 44

Draw the LUMO of pentadienyl cation.

Question 45

Provide the necessary synthetic sequence for preparing 1,3-cyclopentadiene from cyclopentane.

Question 46

Draw the structure of the major product which results when the diene shown is treated with HBr at 40°C.

Question 47

Draw the HOMO of allyl anion.

Question 48

Three halogenated organic products result from the reaction below. Draw the structure of each product and then circle the most stable product.

Question 49

Draw the HOMO of pentadienyl cation.

Question 50

Draw the resonance structures of the intermediate and then predict the two major products in the following reaction and label them as a kinetic or thermodynamic product.

Question 51

Show how the carbon p orbitals overlap to form the π₂ molecular orbital of the allyl cation.

Question 52

In the allyl radical, which π molecular orbital is singly occupied?

A) The bonding π molecular orbital.
B) The nonbonding π molecular orbital.
C) The antibonding π molecular orbital.
D) None of the above.

Question 53

Into what is NBS converted as it reacts in allylic bromination reactions?

Question 54

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

Question 55

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

Question 56

Draw the LUMO of pentadienyl anion.

Question 57

Give a representation of the antibonding π molecular orbital of the allyl cation.

Question 58

What name is given to the type of "control" which arises in a reaction which does not achieve equilibrium and in which the product distribution is determined by the relative activation energies of the pathways which produced them?

Question 59

Draw the structure of the major product which results when the diene shown is treated with HBr at -80°C.

Question 60

Show how the carbon p orbitals overlap to form the LUMO of the allyl anion.

Question 61

Draw the s-trans conformation of (Z)-1,3-hexadiene.

Question 62

How many electrons are present in the nonbonding π molecular orbital of the allyl cation?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 63

Which of the following compounds is the most reactive dienophile in a Diels-Alder reaction with 1,3-butadiene?

A) CH₂=CHOCH₃
B) CH₂=CHCHO
C) CH₃CH=CHCH₃
D) (CH₃)₂C=CH₂
E) CH₂=CH₂

Question 64

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

Question 65

How many electrons populate the π molecular orbitals of the allyl radical?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 66

What diene and dienophile would react to give the product below?

Question 67

Provide the structure of the major organic product in the following reaction.

Question 68

Provide the major organic product of the following reaction.

Question 69

Provide the major organic product of the following reaction.

Question 70

What diene and dienophile would react to give the product below?

Question 71

How many electrons are present in the nonbonding π molecular orbital of the allyl cation?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 72

When 1,3-butadiene reacts with CH₂=CHCN, 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

Question 73

What reagent could be used to convert allyl tosylate directly into oct-1-ene?

Question 74

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.

Question 75

For what does the acronym HOMO stand?

Question 76

Provide the major organic product of the following reaction.

Question 77

Provide the structure of the major organic product in the following reaction.

Question 78

Which substrate would react most rapidly in an S_N2 reaction?

A) CH₃CH₂CH=CHCH₂Br
B) BrCH₂CH₂CH=CHCH₃
C) CH₃CHBrCH=CH₂CH₃
D) CH₃CH₂CH₂CH=CHBr

Question 79

Explain why S_N2 reactions on allyl bromide proceed faster than corresponding reactions on ethyl bromide.

Question 80

Draw the s-cis conformation of (E)-1,3-hexadiene.