Question 1

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

Accepted Answer

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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.

Accepted Answer

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

Accepted Answer

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

Accepted Answer

Isolated dienes have double bonds separated by more than one single bond, as in 1,5-octadiene, where the double bonds are separated by two single bonds.

Question 5

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

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Question 6

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

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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

Accepted Answer

The s-trans conformer is lower in energy than the s-cis conformer due to steric hindrance. In the s-cis conformer, the hydrogens on each end of the molecule are too close together and experience repulsive interactions, while in the s-trans conformer, they are further apart and do not experience as much steric hindrance.

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

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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

Accepted Answer

The central carbon in allene is bonded to two other carbons and has no lone pairs, resulting in a linear geometry and sp hybridization.

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

Accepted Answer

The compound with the smallest heat of hydrogenation is the most stable diene. Conjugated dienes are more stable than isolated or cumulated dienes. (E)-2-methyl-2,4-hexadiene is a conjugated diene, making it more stable and thus having the smallest heat of hydrogenation.

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

Accepted Answer

The highest energy π MO of the given molecule has two nodes not coincident with the molecular plane. One node occurs along the x-axis and another occurs along the y-axis. Therefore, there are 3 nodes in total (including the node coincident with the molecular plane).

Question 12

What is the resonance energy of a system?

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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

Accepted Answer

(E)-2-methyl-1,3-pentadiene is the most stable due to the extended conjugation of the double bonds and the more stable trans (E) configuration, which reduces steric hindrance between substituents.

Question 14

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

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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

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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

Accepted Answer

The number of π molecular orbitals is equal to the number of p orbitals used to construct the π bonds. Each p orbital can form one half of a π bond, so the number of π bonds is equal to the number of p orbitals. However, each π bond involves two electrons, so the number of electrons in the π system is twice the number of π bonds. The number of hydrogen atoms in the molecule is not directly related to the number of π molecular orbitals.

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

Accepted Answer

2,4-hexadiene is a conjugated diene because it has two double bonds separated by a single bond, which allows for delocalization of electrons and increased stability.

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

Accepted Answer

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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.

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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

Accepted Answer

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

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

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

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

What descriptive term is applied to the type of diene represented by 1,5-octadiene?

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

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

When the relative energies of the s-cis and s-trans conformers of 1,3-butadiene are compared, one finds that ________.

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

What is the hybridization of the central carbon of allene (1,2-propadiene) ?

Which compound has the smallest heat of hydrogenation?

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

What is the resonance energy of a system?

Which of the following compounds is the most stable?

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

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

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

What descriptive term is applied to the type of diene represented by 2,4-hexadiene?

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

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.

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