Chapter 12: Arenes and Aromaticity

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

-Na2Cr2O7 -aq. H2SO4

In which parts of the benzene ring is the electron density highest?

-above the plane of the ring -below the plane of the ring

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

Which of the following correctly describes a heterocyclic compound?

A cyclic compound that has an atom other than carbon within the ring

Match each cyclopentadienyl ion to its number of π electrons

Cyclopentadienyl cation - 4 π electrons Cyclopentadienyl anion - 6 π electrons

A benzylic acid reacts at a _____ rate in an SN1 reaction than a simple alkyl halide because a phenyl substituent stabilized a carbocation _____ than a methyl or alkyl group

greater; more

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

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

Match the hydrocarbons with their classification as aliphatic or aromatic hydrocarbons

Aliphatic hydrocarbons - alkanes, alkenes, alkynes Aromatic hydrocarbons - benzene and toulene

Match the reagent with it regioselectivity in the addition to an alkylbenzene

HBr - follows Markovnikov's rule HBr, peroxides - opposite to Markovnikov's rule

Which of the following is a correct description of an antiaromatic compound?

a compound that is destabilized by π-electron delocalization

An antiaromatic compound is _____ by π-electron delocalization and is much _____ reactive than benzene

destabilized; more

A benzylic halide reacts at a _____ rate in an SN1 reaction than a simple alkyl halide because a phenyl substituent stabilized a carbocation

greater; more

The heat of hydrogenation for benzene is 36 kcal/mol less than the expected value, based on a comparison with other conjugated systems. This energy difference is called the _____ energy of benzene and is a measure of the _____ of this compound

resonance; stability

Molecular orbitals can be formed by the combination of multiple atomic orbitals. In the π system of benzene, six atomic p orbitals combine to form _____ π molecular orbitals. There are three π _____ MOs and three π* _____ MOs in the system

six, bonding, antibonding

Benzene is a _____-membered ring compound in which each C atom is sp2-hybridized. All six C atoms lie in the same _____, and the molecule is fully conjugated, since each C atom also has an available _____ orbital for delocalization of electrons

six; plane; p

Pyrrole is an aromatic heterocycle in which the N atom is _____-hybridized with its lone pair located in a(n) ______ orbital. the N atom contributes _____ electron(s) to the π system

sp2; p; 2

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

sp2; p; π

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

substitution; addition

Which statement about the Birch reduction of arenes is correct?

the mechanism involves two electron transfer and two proton transfer steps

True or false: cyclopentadiene is more acidic than other hydrocarbons because its conjugate base is aromatic

true

Unlike pyridine, pyrrole is not basic because the N lone pair is part of the _____ system and is therefore not available to accept a proton

π

Select all the statements that correctly describe the reaction of an alkylbenzene with a strong oxidizing agent

-an alkyl group with benzylic H atoms is oxidized to a COOH group, regardless of the chain length -an alkyl group that lacks benzylic H atoms will not be oxidized -the oxidation product will be benzoic acid or a substituted benzoic acid

Select all the conditions that work well for the polyermization of styrene

-anionic -free radical -cationic -Ziegler-Natta

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

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

-benzylic halides udnergo SN2 reactions faster than allylic halides -the rate of both SN1 and SN2 reactions is faster for benzylic halides than for simple halides

Select all the statements that correctly describe the comparison between CH4 and toulene, C5H5CH3, as acids

-both methane and toulene are weak acids -toulene is relatively more acidic than methane

Which of the following methods can be used to prepare an alkylbenzene in one synthetic step? Select all that apply

-dehydration of a benzylic alcohol -dehydrogenation of an alkylbenzene -dehydrohalogenation of a benzylic halide

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

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

Select all the statements that correctly describe the π molecular orbital system of benzene

-in benzene, all the bonding MOs are filled -six atomic 2p orbitals combine to form six π MOs

Select all the statements that correctly describe the procedure used for naming a disubstituted derivative of benzene

-the compound is named as a derivative of the base compound -the substitution pattern could be indicated by numbering the ring carbon atoms -the appropriate designation o, m, or p can be used as a prefix to indicate the substitution pattern

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

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

Pyridine is a common aromatic heterocyclic compound. Select all the statements that correctly describe the structure and bonding of pyridine

-the lone pair on the N atom is in an sp2-hybridized orbital -pyridine has six π electrons

Select all the statements that correctly describe the procedure for naming a benzene derivative

-the ring is numbered to give substituents the lowest possible numbers -the designations o, m, and p are used for naming disubstituted systems only -if a simple base name other than benzene is used, the base name determines where numbering begins

Select all the statements that correctly describe an antiaromatic system

-the system contains 4n π electrons -the system is planar -the system is extremely unstable

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 inscribed polygon method for predicting aromaticity

-where each vertex touches the circle, draw a line corresponding to a molecular orbital -MOs below the center of the circle are bonding, and MOs above the center are antibonding -aromatic systems have all bonding MOs completely filled up with no unpaired electrons

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

-with (4n+2) π electrons, all bonding MOs are completely filled -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

Arrange the intermediates of the Birch reduction in the order in which they form, with reference to the mechanism. Place the first step at the top of the list

1. benzene anion radical 2. cyclohexadienyl radical 3. cyclohexadienyl anion

Order the steps required when using the inscribed polygon method for predicting aromaticity. Place the first step at the top of the lists

1. draw a polygon that has the same number of vertices as the carbon atoms in the cyclic compound, and place the polygon with a vertex pointing down 2. draw a circle around the polygon so that each vertex touches the circle; the points at which the polygon intersects the circle represents the MOs 3. place the electrons in the MOs following normal rules for placing electrons in orbitals 4. a system that has all bonding MOs completely filled is aromatic

Match the regions of electron density in the benzene ring

Above and below the ring - high electron density Inside the ring - low electron density

Match each category of multiple aromatic rings to its name

Biphenyl - a compound in which two benzene rings are connected by a single bond Polycyclic aromatic hydrocarbon - a compound in which two or more aromatic rings are fused together

Match each type of compound correctly according to the reaction it undergoes when treated with a strong oxidizing agent such as aqueous acidic Na2Cr2O7 or hot aqueous KMnO4

No reaction: alkanes, benzene Oxidation at benzylic position: alkylbenzenes with benzylic hydrogens

Which of the following correctly describes a benzylic carbon atom?

a carbon atom directly attached to a benzene ring

Electrophilic addition to the side-chain double bond in an alkylbenzene is governed by the ability of the aromatic ring to stabilize a positive charge on the _____ carbon atom. These reactions usualy produce a single product

adjacent

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

alcohol; base

Unlike _____ halides, _____ halides do not undergo rearrangements in SN1 reactions beacuse the ring's aromaticity is retained only if the nucleophile bonds to the benzylic carbon

allylic; benzylic

A completely conjugated monocyclic hydrocarbon with more than six carbons is called a(n) ______

annulene

A compound that is cyclic, planar, fully conjugated, and contains 4n π electrons is said to be _____. Such a compound will be much _____ stable than benzene.

antiaromatic; less

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

aromatic

Allylic halides give a mixture of products in SN1 reactions. An analogous "benzylic rearrangement" does not occur in an SN1 reaction of a benzylic halide because _____

aromatic stabilization would be lost if the nucleophile bonded to a ring C atom

Which statement correctly describes the chemical reactivity of benzene?

benzene is much less reactive than other unsaturated hydrocarbons

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

benzene; aromatic

The reaction of an alkylbenzene with a strong oxidizing agent such as chromic acid will produce a(n) _____ acid. Any alkyl chain will undergo the same reaction regardless of the chain length, unless the alkyl chain lacks _____

benzoic; benzylic

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

benzylic

Arenes readily undergo reactions that involve _____ carbocation or radical intermediates because these species are highly stabilized by _____

benzylic; resonance

Hückel'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

A benzylic halide reacts faster than a simple halide in an SN1 reaction because the _____ intermediate formed can be stabilized by _____

carbocation; resonance

The _____ _____ is an exampel of an aromatic species, with six π electrons delocalized over seven carbon atoms

cycloheptatrienyl cation

Why is a benzylic C-H bond much weaker and more reactive than a C-H bond in an alkane?

dissociation of the C-H bond produces a benzylic radical, which is stabilized by resonance

A cyclic compound that contains at least one atom other than carbon within the ring is called a(n) _____ compound

heterocyclic

Benzoyl peroxide is a common _____ to start free-radical halogenation reactions

initiator

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 cyclohexane and 1,3-cyclohexadiene. This difference indicated that benzene is much _____ stable than a system containing three isolated double bonds

lower; more

Hückel's rule for determining aromaticity applies only to _____-cyclic systems. Phenanthrene has _____ π electrons and therefore does obey this rule and is aromatic. Pyrene has _____ π electrons but is nonetheless still aromatic

mono; 14; 16

Benzylic carbocations and radicals are _____ stabilized than their allylic counterparts because a phenyl group is a _____ conjugating substituent than a vinyl group

more; better

For a monocyclic compound to be aromatic, it must possess (4n+2) π electrons. What does this mean?

n is any whole number, and the number of π electrons must be a solution to the expression (4n+2)

When a benzene ring is named as a substituent, it is called a(n) _____ group, whereas a general arene substituent is termed a(n) _____ group

phenyl; aryl

Azoles are heterocyclic aromatic compounds related to _____ in which a second carbon atom in the ring is replaced by another _____

pyrrole; heteroatom

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

radical; benzylic

Pyridine is a heterocyclic compound containing a nitrogen atom. The N is _____-hybridized, and its lone pair resides in a(n) _____ orbital. Since the lone pair electrons do not contribute to the aromatic π system, pyridine has a total of _____ π electrons

sp2; sp2; 6