chapter 12

Which are physical properties that describe arenes?

- Arenes are nonpolar. - Arenes have similar physical properties to other hydrocarbons.

Select all the statements that correctly describe the relative reactivity of benzylic halides toward nucleophilic substitution.

- Benzylic halides undergo SN2 reactions faster than allylic halides. - The rate of both SN1 and SN2 reactions is faster for benzylic halides than for simple halides.

Which of the following reagents and/or reaction conditions will promote free-radical halogenation of an alkylbenzene?

- Cl2, light - Br2, CCl4, light - NBS, benzoyl peroxide, heat

Select all the statements that correctly describe the behavior and characteristics of cyclobutadiene and cyclooctatetraene

- Cyclobutadiene is classed as antiaromatic. - Cyclobutadiene is extremely unstable.

Which of the following methods can be used to prepare an alkenylbenzene in one synthetic step?

- Dehydration of a benzylic alcohol - Dehydrohalgenation of a benzylic halide - Dehydrogenation of an alkylbenzene

Select all the statements that correctly represent the reagents and conditions required in the various steps in the synthesis of the target molecule from ethyl benzene.

- In Step A, the reagent is NBS with benzoyl peroxide in CCl4 at 80°C. -In Step B, the reagent is NaOC2H5 in C2H5OH at 50°C.

Select all the statements that correctly represent the reagents and conditions required in the various steps in the synthesis of the target molecule from ethyl benzene.

- In Step B, the reagent is NaOC2H5 in C2H5OH at 50°C. - In Step A, the reagent is NBS with benzoyl peroxide in CCl4 at 80°C.

Select all the statements that describe aromatic hydrocarbons.

- Many of these compounds have a benzene ring as a structural unit. - They are highly stable compounds. - They are unsaturated compounds.

Select all the statements that correctly describe the reactions involved in the two-step synthesis of benzyl acetate (CH3COOCH2C6H5) from toluene.

- Step 1 is the reaction of toluene with NBS in the presence of benzoyl peroxide in CCl4 at 80°C. - Step 2 is the SN2 reaction of benzyl bromide with the acetate ion in acetic acid as the solvent.

Select all the criteria that must be fulfilled for a monocyclic compound to be considered aromatic.

- The compound must be fully conjugated. - The compound must possess (4n + 2) π electrons. - The compound must be planar.

Select all the statements that correctly describe the structure shown.

- The compound possesses six π electrons. - The compound is planar.

Select all the statements that correctly describe the bonding in benzene.

- The π electrons of the benzene ring are delocalized. - Each ring C atom contributes one electron to the π system.

Select all the statements that correctly describe the bonding in cyclooctatetraene.

- This compound does not have a delocalized π-electron system. - This compound is not planar.

Select all the statements that correctly describe the comparison between CH4 and toluene, C6H5CH3, as acids.

- Toluene is relatively more acidic than methane. - Both methane and toluene are weak acids.

Huckel's rule states that a compound must have 4n+2 π electrons to be aromatic. Which of the following options explain why this number of electrons is important?

- With 4n π electrons, there will be unpaired electrons and/or electrons in antibonding molecular orbitals, raising the energy of the system. - With 4n+2 π electrons, there are no unpaired electrons in any of the molecular orbitals. - With 4n+2 π electrons, all bonding MOs are completely filled.

Select all the conditions that work well for the polymerization of styrene.

- Ziegler-Natta - Cationic - Anionic - Free radical

Select all the reagents that should be combined to carry out the side-chain oxidation of an alkylbenzene.

- aq. H2SO4 - Na2Cr2O7

Alkylbenzenes can be oxidized to benzoic acid derivatives by the action of certain strong oxidizing agents. Select all the reagents that can be used in this type of reaction.

- aq. Na2Cr2O7, H2SO4, heat - i) aq. KMnO4, -OH, Δ; ii) HCl

Which of the following correctly describes a benzylic carbon atom?

A carbon atom directly attached to a benzene ring

The pKa of toluene, C6H5CH3, is 42. Select the base that will remove the acidic hydrogen in toluene to form the resonance-stabilized benzylic anion. The pKa values of conjugate acids are provided.

CH3CH2CH2CH2Li (n-butyllithium); pKa (CH3CH2CH2CH3) = 60

The three-membered ring cation shown is a surprisingly stable species. Select all the statements that correctly describe this ion. (triangle)

Each C atom in this structure has a p orbital available for overlap. This system is planar. The system is stable because it is aromatic.

True or false: If a compound is cyclic and has conjugated double bonds, it is aromatic.

False Reason: Correct. Conjugated double bonds are not necessary for a compound to be fully conjugated. In addition, the compound must be planar and must have [4n+2] π electrons to be aromatic.

Select all the statements that correctly describe the structure shown

If this system were planar, it would be aromatic. The system is fully conjugated.

The compound [10]annulene (shown) is not aromatic, despite being cyclic and following the (4n + 2) rule. Which of the following correctly explains why?

Steric interactions between interior H atoms cause the system to be nonplanar.

Furan (shown) is an aromatic heterocyclic compound. Select all the statements that correctly describe furan.

The O atom in furan is sp2-hybridized. The O atom contributes only the lone pair in the p orbital to the π system. Each C atom contributes one electron to the π system.

Select all the statements that correctly describe the hetereocyclic compound shown.

The compound is not fully conjugated. The compound has six π electrons if the S atom is sp2-hybridized.

The dianion of cyclooctatetraene is shown. Assume this system is planar, and use a Frost circle to draw a π molecular orbital diagram for this anion. Select all the statements that correctly describe this system.

The system is aromatic with 10 π electrons. There are three π bonding MOs, two nonbonding MOs, and three π* antibonding MOs. There are four electrons in the nonbonding MOs.

The structure of [16]annulene is shown. Use a Frost circle to construct a molecular orbital energy diagram, and then select all the statements that correctly describe this system.

There are seven bonding and seven antibonding π MOs. The system would be antiaromatic if planar. There are two unpaired electrons in the π MOs of this system.

Why are the Kekule structures not adequate to represent the structure of benzene?

They cannot explain why the C-C bond lengths are equal.

Why are benzene and toluene referred to as aromatic hydrocarbons?

They were originally obtained from aromatic plant extracts.

An alkenylbenzene can be prepared conveniently by acid-catalyzed dehydration of a substituted benzylic _____ or by dehydrohalogenation of a suitable halide using a strong _____ as the reagent.

alcohol ; base

Match each type of compound to its characteristic reaction class.

alkene or alkyne = addition benzene = substitution

Which statement about the physical properties of arenes is NOT correct?

arenes are denser than water

Naphthalene is a(n) ______ compound because its structure is based on the benzene ring.

aromatic

Which hydrocarbon is an aromatic compound?

benzene

An arene is a hydrocarbon based on the _____ ring as a structural unit. Arenes are also called _______ hydrocarbons.

benzene, aromatic

A carbon atom that is directly attached to a benzene ring is called a(n) ______ carbon.

benzylic

Huckel's rule for aromaticity specifies that an aromatic system must contain 4n+2 π electrons. For a system that fulfills this requirement, all _____ molecular orbitals will be completely filled.

bonding

True or false: Kekulé structures adequately describe the structure of benzene.

false (Reason: Kekulé structures cannot explain the equal carbon-carbon bond lengths in benzene.)

True or false: Polymerization of styrene can only be carried out under free-radical conditions.

false Reason: Polymerization of styrene can be carried out under free-radical, cationic, anionic, or Ziegler-Natta conditions.

Benzene is more stable than expected for a system containing three isolated double bonds. The amount of heat released by the hydrogenation of benzene is therefore ______ than expected because benzene is already at a ______ energy level than an isolated triene system.

less, lower (Reason: Since benzene is more stable than expected, it is already at a lower energy than an isolated triene. Less energy will therefore be released during hydrogenation.)

Benzene is more stable than expected for a system containing three isolated double bonds. The amount of heat released by the hydrogenation of benzene is therefore ______ than expected because benzene is already at a ______ energy level than an isolated triene system.

less, lower (Reason: Since benzene is more stable than expected, it is already at a lower energy than an isolated triene. Less energy will therefore be released during hydrogenation.)

The heat of hydrogenation of an unsaturated compound can be used to estimate its stability. The heat of hydrogenation of benzene is ______ than expected by comparison with cyclohexene and 1,3-cyclohexadiene. This difference indicates that benzene is much ______ stable than a system containing three isolated double bonds.

lower, more (Reason: Since benzene is more stable than a system containing three isolated double bonds, it will release less energy when hydrogenated.)

Huckel's rule for determining aromaticity applies only to ____-cyclic systems. Tetracene (shown) has _____ π electrons and therefore does obey this rule and is aromatic. Benzo[a]pyrene (shown) has _____ π electrons but is nonetheless still aromatic.

mono, 18, 20

Reduction of benzene using sodium and an alcohol in liquid ammonia produces ______ ______.

nonconjugated dienes Reason: The Birch reduction produces nonconjugated dienes.

A hydrocarbon composed of two or more aromatic rings fused together is called a(n) _______ aromatic hydrocarbon

polycylic

If an alkylbenzene, such as toluene, is reacted with Cl2 or Br2 in the presence of light or heat, the reaction will proceed via a ____ mechanism, and halogenation will selectively occur at the ______ position(s) on the alkyl chain.

radical , benzylic

Benzene is a planar molecule with bond angles of 120°. All the C atoms in the ring are _____ -hybridized, leaving a single unhybridized ____ orbital on each C atom. These unhybridized orbitals overlap side-to-side to form a continuous ____ system.

sp2, p, pi

Benzene either failed to react or reacted by ______ with common reagents that led to ______ reactions with alkenes.

substitution; addition (Reason: Benzene undergoes substitution with reagents that undergo addition reactions with alkenes.)