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

Question 1

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

Answer 1

Question 2

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

Answer 2

1,2-pentadiene < 1,4-pentadiene < cis-1,3-pentadiene < trans-1,3-pentadiene

Question 3

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

Answer 3

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

Question 4

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 5

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

Question 6

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

Question 7

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

Question 8

In 1,3,5-hexatriene, there are ________ nodes in the HOMO and ________ nodes in the LUMO for this molecule.

A) 1; 2
B) 2; 3
C) 2; 0
D) 2; 1

Question 9

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 10

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 11

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

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

Question 12

What is the resonance energy of a system?

Question 13

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 14

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

Question 15

Rank the following compounds in order of increasing heats of hydrogenation. (The smallest ΔH is first.)

A) A < B < C
B) C < B < A
C) C < A < B
D) A < C < B

Question 16

The number of π molecular orbitals in a molecule is always equal to the number of ________.

A) π bonds
B) p orbitals used to construct the π bonds
C) electrons in the π system
D) hydrogen atoms in the molecule

Question 17

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 18

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

Question 19

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 20

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 21

Name the two major products which are formed when
undergoes solvolysis in hot methanol.

Question 22

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 23

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

Question 24

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 25

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 26

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

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

Question 27

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 28

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 29

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

Question 30

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 31

Provide the two major organic products of the following reaction.

Question 32

Provide the two major organic products of the following reaction.

Question 33

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

A)
B)
C)
D)

Question 34

Draw the LUMO of 1,3-butadiene.

Question 35

The structure of cholesterol is shown below. How many allylic carbons are there in this structure?

A) 2
B) 3
C) 4
D) 5

Question 36

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

Question 37

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

Question 38

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

Question 39

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

Question 40

Draw the HOMO of 1,3-butadiene.

Question 41

Draw the HOMO of allyl anion.

Question 42

Draw the LUMO of pentadienyl anion.

Question 43

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

Question 44

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

Question 45

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

Question 46

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

Question 47

Draw the HOMO of pentadienyl anion.

Question 48

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

Question 49

Draw the 2 organic products expected from this reaction, label them as thermodynamic or kinetic and tell which predominates at -80°C.

Question 50

In the diene shown below, there are two possible double bonds that could react when 1 eq. of HBr is added. However, the double bond labeled "A" reacts preferentially to give the major product. Using resonance structures, justify this observation.

Question 51

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 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

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 54

Draw the HOMO of pentadienyl cation.

Question 55

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

Question 56

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

Question 57

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 58

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

Question 59

Draw the LUMO of pentadienyl cation.

Question 60

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

Question 61

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

Question 62

Using resonance structures, explain the regiochemistry observed in this reaction:

Question 63

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 64

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

Question 65

For what does the acronym HOMO stand?

Question 66

Draw mechanism for the following reaction:

Question 67

In a Diels-Alder reaction, what is the endo rule?

Question 68

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

Question 69

Provide the major organic product of the following reaction.

Question 70

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 71

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

Question 72

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

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

Question 73

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 74

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following:

Question 75

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 76

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

Question 77

Provide the major organic product of the following reaction.

Question 78

Draw the major organic product that results from the intramolecular Diels-Alder reaction.

Question 79

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 80

Which substrate would react most rapidly in an S_N2 reaction?

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