Chapter 8 Flashcards
1) Which of the following statements is incorrect about benzene?
A) All of the carbon atoms are sp hybridized.
B) It has delocalized electrons.
C) The carbon-carbon bond lengths are all the same.
D) The carbon-hydrogen bond lengths are all the same.
E) All twelve atoms lie in the same plane.
Answer: A
Which of the following statements about benzene is correct?
A) All of the carbon atoms are sp3 hybridized.
B) It has no delocalized electrons.
C) The carbon-carbon bond length is longer than that of ethane.
D) It is a planar molecule.
E) The carbon-hydrogen bonds are not the same length.
Answer: D
The delocalized π system in benzene is formed by a cyclic overlap of 6 ________ orbitals.
A) s
B) p
C) sp
D) sp2
E) sp3
Answer: B
Give the hybridization, shape, and bond angle of a carbon in benzene.
Answer: sp2, trigonal planar, 120°
Identify the resonance hybrid.
Answer: D
Which of the following pairs are resonance structures?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the following pairs are resonance structures?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the following is not a resonance structure of the species shown?
A) I
B) II
C) III
D) IV
Answer: D
In which of the following does resonance delocalization of electron density not play a role?
A) allyl cation
B) benzyl anion
C) CO32- D) O3
E) cyclohexyl radical
Answer: E
Draw the arrows to go from one resonance structure to another.
Due to electron delocalization, one would predict that the carbon-oxygen bond in acetamide, CH3CONH2
A) is nonpolar.
B) has more double bond character than the carbon-oxygen bond of acetone, (CH3)2CO.
C) is longer than the carbon-oxygen bond of dimethyl ether, (CH3)2O.
D) is longer than the carbon-oxygen bond of acetone, (CH3)2CO.
E) is formed by overlapping sp3 orbitals.
Answer: D
Draw two other resonance contributors for the following compound.
Does the following structures represent different compounds or resonance contributors?
Answer: They are resonance contributors.
Draw the important resonance structures of
Draw the important resonance structure of
Draw the important resonance structures of
Draw the important resonance contributing forms for the structure shown below.
Draw the important resonance contributing forms for the structure shown below.
Draw the important resonance contributing forms for the structure shown below.
Mark the most electron-rich carbon atom in the compound below with an asterisk.
CH3CH=CHNHCH3
Draw all major resonance contributors of the species below.
Draw all major resonance contributors of the species below.
Draw all major resonance contributors of the species below.
Draw all major resonance contributors of the species below.
Provide the major organic product of the following reaction.
Provide the major organic product of the following reaction.
Which of the following is the strongest acid?
Draw the arrows to go from one resonance structure to another.
Identify the correctly drawn arrows.
Answer: D
Which of the following is the most stable resonance contributor to acetic acid?
Answer: E
Draw four resonance contributors for the following structure and indicate which is the most important contributor. Explain.
Answer: C is the most important contributor because all the atoms have complete octets and the negative charges are on the most electronegative atoms.
Rate the following molecules in decreasing order of stability.
Answer: a>c>b
aromatic > linear > antiaromatic
Identify the most stable structure(s).
Answer: A
Stabilization of a charged species usually happens when this species can be more accurately depicted as a hybrid of several resonance contributing forms. Why is this the case?
Answer: Stabilization results from delocalization of charge.
In the acetate ion (CH3CO2-), which of the oxygen atoms bears a greater negative charge? Explain your answer.
Answer: The acetate ion is a hybrid of the two resonance contributors below. Each oxygen in this ion has the same charge (-1/2); the oxygen atoms are indistinguishable and the carbon- oxygen bonds are identical.
Which of the following statements concerning resonance contributors and resonance hybrids is not correct?
A) The fewer nearly equivalent resonance contributors are in the structure, the greater the resonance energy.
B) A resonance hybrid is more stable than the predicted stability of any of its resonance contributors.
C) The greater the number of relatively stable resonance contributors, the greater the resonance energy.
D) The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid.
E) none of the above
Answer: A
What is suggested by the fact that benzene’s molar heat of hydrogenation is 36 kcal less than three times the molar heat of hydrogenation of cyclohexene?
Answer: This suggests that the type of π bonding in benzene lends special stability to the molecule.
Aromatic molecules contain ________ π electrons.
A) no
B) 4n + 2 (with n an integer)
C) 4n + 2 (where n = 0.5)
D) 4n (with n an integer)
E) unpaired
Answer: B
The following molecule is not aromatic. Why?
Answer: It does not have an odd number of pairs of pi electrons (i.e., it does not obey Hucklel’s rule) and it is not planar.
Is the following molecule aromatic? Explain.
Answer: Yes, it is aromatic since the molecule is planar, monocyclic with 5 pairs of π electrons.
Is the following molecule aromatic? Explain.
Answer: No, it is not aromatic since the molecule is not a conjugated system.
List the criteria which compounds must meet in order to be considered aromatic.
Answer:
1) The structure must be cyclic.
2) Each atom in the ring must have an unhybridized p orbital.
3) The structure must be planar or nearly planar so that overlap of these p orbitals is effective.
4) The π network must contain 4n + 2 electrons (where n is a whole number), so that delocalization of the π electrons results in a lowering of the molecule’s electronic energy.
Which of the following is an aromatic hydrocarbon?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the following is aromatic?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the structures below would be aromatic?
A) I and IV
B) I, III, and IV
C) III and IV
D) II
Answer: C
Which of the following is aromatic?
A) I
B) II
C) III
D) IV
E) V
Answer: B
Which of the following is aromatic?
A) cyclopentadienyl cation
B) 1,3-cyclohexadiene
C) cyclobutenyl anion
D) 1,3,5-hexatriene
E) cycloheptatrienyl cation
Answer: E
Cyclic hydrocarbons which can be represented as structures containing alternating single and double bonds are called ________.
Answer: annulenes
Which of the following compounds is aromatic?
A) I
B) II
C) III
D) IV
E) V
Answer: D
Which of the following is aromatic?
A) I
B) II
C) III
D) IV
E) V
Answer: A
One of the following compounds is aromatic and the other is antiaromatic. Which is which?
Answer: a) is aromatic, b) is antiaromatic.
Provide the structure of the biologically significant heterocycle purine.
Provide the structure of the biologically significant heterocycle pyrimidine.
Provide the structure of indole.
Which of the following is antiaromatic?
A) I
B) II
C) III
D) IV
E) V
Answer: D
When cycloheptatriene is deprotonated, an anion with seven resonance forms of equal energy can be drawn. Given this fact, explain why cycloheptatriene is only slightly more acidic than propene.
Answer: Each resonance form of the cycloheptatrienyl anion is antiaromatic.
Classify cyclopentadienyl cation as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.
Answer: antiaromatic
Classify cyclopheptatrienyl cation as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.
Answer: aromatic
Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.
Answer: aromatic
Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.
Answer: aromatic
Is cyclooctatetraene a planar molecule? Explain.
Answer: No. In addition to increasing other forms of strain, a planar conformation would make this molecule antiaromatic.
Indicate whether the following molecule is aromatic, antiaromatic, or nonaromatic.
Answer: aromatic
Which species is represented by the following distribution of p electrons in the molecular energy diagram?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the following statements about the π molecular orbital description of cyclobutadiene is not correct?
A) Cyclobutadiene has two degenerate nonbonding π molecular orbitals.
B) Cyclobutadiene has a single bonding π molecular orbital.
C) Cyclobutadiene has two electrons in nonbonding π molecular orbitals.
D) Cyclobutadiene has one electron in an antibonding π molecular orbital which makes it antiaromatic.
E) none of the above
Answer: D
Draw the distribution of π electrons in the molecular orbitals of cyclobutadiene.
Which of the following is an allylic cation?
A) I
B) II
C) III
D) IV
E) V
Answer: B
Which of the following is a benzylic cation?
A) I
B) II
C) III
D) IV
E) V
Answer: D
Which of the following is the most stable cation?
A) I
B) II
C) III
D) IV
E) V
Answer: C
How many allylic hydrogen atoms are present in the following molecule?
A) 2
B) 3
C) 4
D) 5
E) zero
Answer: E
How many benzylic hydrogens are present in the following molecule?
A) 2
B) 3
C) 4
D) 5
E) zero
Answer: E
Which of the following is the most stable diene?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the following compounds contains the longest carbon-carbon single bond?
A) allene
B) 1, 3-butadiyne
C) 1, 3-butadiene
D) propyne
E) ethane
Answer: E
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
Answer: A
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
Answer: A
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
Answer: C
Which of the following is a cumulated diene?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Which of the following is a conjugated diene?
A) I
B) II
C) III
D) IV
E) V
Answer: A
Which of the following numbered hydrogens is most easily abstracted by heterolytic cleavage? Explain.
Answer: Hydrogen #3 is most easily abstracted by heterolytic cleavage because the carbocation produced from the cleavage is allylic.
Which of the following compounds is more stable? Explain.
Answer: (a) and (b) are both benzylic cations. (b) is more stable because it is more alkyl substituted.
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: 1,2-pentadiene < 1,4-pentadiene < cis-1,3-pentadiene < 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.
Answer: less
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.
cis-2-pentene, 2,3-pentadiene, and trans-1,3-pentadiene
Answer: 2,3-pentadiene < trans-1,3-pentadiene < cis-2-pentene
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: 3,6-dimethyl-1,4-cycloheptadiene < 2,5-dimethyl-1,3-cycloheptadiene < 1,4-dimethyl-1,3-cycloheptadiene
Arrange the following unsaturated hydrocarbons in order of decreasing reactivity toward HBr addition.
Answer: III>II>IV>I
Indicate the compound that has the shortest bond length between the two middle carbon atoms.
Answer: E
Identify the most stable carbocation.
Answer: E
Identify the most stable carbocation.
Answer: C
In the molecular orbital representation of benzene, how many π molecular orbitals are present?
A) 1
B) 2
C) 4
D) 6
E) 8
Answer: D
Which species is represented by the following distribution of π electrons in the molecular orbitals and energy levels diagram?
A) I
B) II
C) III
D) IV
E) V
Answer: C
Excluding the horizontal node which is coincident with the molecular plane, how many nodes are present in Ψ4 of the 1, 3, 5-hexatriene π system?
A) 6
B) 5
C) 4
D) 3
E) 0
Answer: D
Provide a diagram which depicts the relative energies of the π molecular orbitals of benzene. Show which molecular orbitals are filled in benzene’s ground state.
Show how the p orbitals overlap to form the nonbonding π molecular orbital of the allyl system.
Which π MOs are bonding MOs in the ground state of 1,3,5-hexatriene?
Answer: Ψ1, Ψ2, Ψ3
Draw the LUMO of 1,3,5-hexatriene.
Draw the HOMO of 1,3,5-hexatriene.
Which of the following is/are the major product(s) of the following reaction?
A) I
B) I and II
C) I and III
D) II
E) III
Answer: A
Which of the following is the strongest acid?
Answer: E
Which of the following is the strongest acid?
A) I
B) II
C) III
D) IV
E) V
Answer: D
Which of the following ions is most acidic?
A) C6H5NH3+
B) C6H5CH2NH3+
C) (CH3)2CHNH3+
D) CH3CH2NH3+
E) CH3NH3+
Answer: A
Which of the following is the weakest base?
A) phenolate ion (C6H5O-)
B) ethoxide ion (CH3CH2O )
C) hydroxide ion (HO-)
D) acetate ion (CH3CO2-)
E) methoxide ion (CH3O-)
Answer: D
Provide the structure of the major organic product(s) in the reaction below.
Why is phenol a stronger acid than cyclohexanol?
Answer: The anion of phenol is stabilized by resonance, causing the proton to be more easily abstracted than that of cyclohexanol.
Which of the following is/are the main product(s) of the following reaction?
Answer: E
What is/are the product(s) from the following reaction?
A) I only
B) II only
C) I is minor, II is major
D) I is major, II is minor
E) equal amounts of I and II
Answer: D
What is/are the product(s) of the following reaction?
A) I only
B) II only
C) III only
D) I and II
E) I and III
Answer: E
What compound results from the 1, 4-addition of one equivalent of HBr to 1, 3-butadiene?
A) 1-bromo-1-butene
B) 2-bromo-2-butene
C) 4-bromo-1-butene
D) 3-bromo-1-butene
E) 1-bromo-2-butene
Answer: E
Which of the following is/are the major product(s) of the following reaction?
A) I
B) I and III
C) II
D) II and III
E) III
Answer: C
What are the products from the following reaction?
A) I and II
B) I and III C) I and IV D) II and III E) II and IV
Answer: A
Provide the structure of the major product which results from 1,2-addition of HBr to the diene shown below.
Provide the structure of the major product which results from 1,4-addition of Br2 to the diene shown below.
Give the major organic product for the reaction.
Answer: D
Give the major organic product for the reaction.
Answer: D
Give the major organic product(s) for the reaction. You may choose more than one answer.
Answer: A, B
Draw the product(s) for the reaction.
Give the major organic product(s) for the reaction. You may choose more than one answer.
Answer: D, E
Provide the major organic product.
What is the thermodynamic product for the following reaction?
A) I
B) II
C) III
D) IV
E) V
Answer: D
What is the kinetic product for the following reaction?
A) I
B) II
C) III
D) IV
E) V
Answer: E
Which of the following is a correct statement considering thermodynamic versus kinetic control of organic reactions?
A) When a reaction is under thermodynamic control, the relative amounts of products depend on the activation energies of the steps leading to their formation.
B) The kinetic product always predominates when the reaction is reversible.
C) Higher temperatures and longer reaction times typically favor the kinetic product over the thermodynamic product.
D) When the products are in equilibrium under the reaction conditions, the relative amounts of products depend on their stabilities and the reaction is under thermodynamic control.
E) none of the above
Answer: D
What is/are the product(s) from the following reaction?
A) II Minor, III Major
B) II Major, III Minor
C) III only
D) I and II
E) II and III
Answer: B
What is/are the product(s) from the following reaction?
A) I only
B) II only
C) III only
D) I and II
E) II and III
Answer: C
What is the major product obtained from the addition of one mole of HCl at high temperature to 1,3-butadiene?
Answer: 1-chloro-2-butene (thermodynamic product)
Draw the structure of the major product which results when the diene shown is treated with HBr at 40°C.
Draw the structure of the major product which results when the diene shown is treated with HBr at -80°C.
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?
Answer: The product of kinetic control typically results from 1,2-addition.
Provide the major organic product.
Provide the major organic product.
What is the product of the following reaction?
A) I
B) II
C) III
D) IV
E) V
Answer: E
What is the product from the following reaction?
A) I
B) II
C) III
D) IV
E) V
Answer: B
Which of the following conjugated dienes would not react with a dieneophile in a Diels- Alder reaction?
A) I
B) II
C) III
D) IV
E) I and III
Answer: E
Which of the following dienes is the most reactive in a Diels-Alder reaction?
A) I
B) II
C) III
D) IV
E) V
Answer: C
What diene and what dieneophile would be used to synthesize the following compound?
A) I
B) II
C) III
D) IV
E) V
Answer: B
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
Answer: A
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
Answer: C
The Diels-Alder reaction is a concerted reaction; this means that
A) a mixture of endo and exo products is 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
Answer: B
Which of the following statements is true of the Diels-Alder reaction?
A) When the diene is electron-rich, the reaction works best when the dienophile contains one or
more electron-withdrawing groups conjugated to its C
B) Substituents which are cis in the dienophile remain cis in the product.
C) Dienes which cannot achieve an s-cis conformation do not react in Diels-Alder reactions.
D) Secondary orbital interactions typically cause the endo product to be favored kinetically over the exo.
E) all of the above
Answer: E
When 1, 3-cyclopentadiene reacts with the cis-isomer of NCCH=CHCN, the major product is
A) optically active
B) a meso compound
C) a racemic mixture
D) a spirocyclic compound
E) a fused bicyclic compound
Answer: D
Draw (Z)-1,3-hexadiene in its s-trans conformation.
What is the product of the following Diels-Alder reaction?
What diene and what dieneophile could be used to prepare the following?
How could the following compound be synthesized using a Diels-Alder reaction?
Cyclopentadiene can react with itself in a Diels-Alder reaction. Draw the endo and exo products.
Provide the structure of the major organic product in the following reaction.
Provide the structure of the major organic product in the following reaction.
Provide the structure of the major organic product in the following reaction.
Provide the structure of the major organic product in the following reaction.
Provide the structure of the major organic product in the following reaction.
Provide the structure of the major organic product in the following reaction.
What diene and dienophile would react to give the product below?
What diene and dienophile would react to give the product below?
What diene and dienophile would react to give the product below?
What diene and dienophile would react to give the product below?
Why does the (E,E)-isomer of 2,4-hexadiene react much more rapidly with dienophiles than does the (Z,Z)-isomer?
Answer: The s-cis conformation, which is required for participation in a Diels-Alder reaction, is hindered by a severe CH3/CH3 steric interaction in the (Z,Z)-isomer.
What diene and dienophile should be used to synthesize the compound below?
What diene and dienophile should be used to synthesize the compound below?
What diene and dienophile should be used to synthesize the compound below and its enantiomer?
What diene and dienophile should be used to synthesize the compound below and its enantiomer?
Give the major organic product for the reaction.
Answer: C
Give the major organic product for the reaction.
Answer: A
Give the major organic product for the reaction.
Answer: E
Draw the major product of the reaction.
Draw the s-cis conformation of (2E,4Z)-4-methyl-2,4,-heptadiene.
Draw the s-trans conformation of (2E,4Z)-4-methyl-2,4,-heptadiene.
Which of the following is an isolated diene?
A) I
B) II
C) III
D) IV
E) V
Answer: D
What is the hybridization of the carbons labeled A, B, and C?
Answer: sp2, sp3, sp
Give the major organic product for the reaction.
Answer: C
The delocalized π system of benzene is located
A) above the plane of the ring
B) below the plane of the ring
C) in the plane of the ring
D) above and below the plane of the ring
E) in the center of the ring
Answer: D
Which of the following statement is incorrect about drawing resonance contributors?
A) can move π-electrons
B) can move lone pairs
C) cannot move atoms
D) net charge in various contributors remains same
E) can break single bonds
Answer: E
Scientists are interested in organic compounds that can serve as conductors of electricity and/or as components of electric circuits. Compound A has properties that may be useful as a rectifier (a device that controls which way electrons flow through it). Draw a resonance structure of compound A that has no separation of charge. (R represents an alkyl chain)
The C-C single bond in the following molecule is formed by the overlap of what type of orbitals?
A) sp3-sp2
B) sp2-sp2
C) sp-sp2
D) sp-sp
E) sp3-sp
Answer: C
Explain the reason for the difference in bond lengths below:
Answer: The C-C bond in the first molecule is shorter because it is formed by a sp-sp overlap
whereas, C-C bond in the second molecule is formed by sp2-sp2 overlap. The sp orbitals have higher s-character which means tightly held electrons and closer to the nucleus. As a result,
bonds formed from sp-sp overlap are shorter and stronger than sp2-sp2 overlap.
What is the most acidic hydrogen on cyclopentadiene?
A) I
B) II
C) III
D) IV
E) V
Answer: A
Is the following molecule aromatic or antiaromatic? Explain.
Answer: Aromatic. It has p-orbital on every atom of the ring and has odd number of -electron pairs. Also, molecule is planar.
There is large dipole moment associated with the fulvenoid shown below. Suggest a reason.
Answer: Because there is separation of charges in the resonance structure on the right. It is the major contributor because both the rings are aromatic.
Draw the resonance hybrid of the structures below.
Which of the following structures contributes most to the resonance hybrid.
A dienophile with an electron withdrawing group is a better A) nucleophile.
B) electrophile.
C) catalyst.
D) solvent.
E) none of the above.
Answer: B
Predict the product of the following reactions.
Predict the product of the following reactions.
Predict the product of the following reactions.
The type of reaction that benzene undergoes with electrophiles is a(n)
A) addition reaction.
B) substitution reaction.
C) subtraction reaction.
D) multiplication reaction.
E) elimination reaction.
Answer: B
Protonation of pyridine, shown below, generates a conjugate acid that is
A) aromatic.
B) antiaromatic.
C) nonaromatic.
D) nucleophile.
E) none of the above.
Answer: A
How would you convert cyclopentadiene to the dialdehyde shown?
Which of the following is the weakest acid?
A) I
B) II
C) III
D) IV
E) V
Answer: A
Which of the following is the strongest acid?
A) I
B) II
C) III
D) IV
E) V
Answer: B
Show the missing reagents A and the compound B.
