Ch. 15 Benzene and Aromatic Compounds

Benzene has a total of 3 pi bonds. Since each pi bond includes _____ shared electrons, benzene has a total of _____ pi electrons

2, 6

Pyrrole is an aromatic heterocycle in which the N atom forms ___ sigma bond(s) and contributes ___ electron(s) to the pi system

3, 2

Huckel's rule states that in order to be aromatic, a compound must contain ____ pi electrons

4n+2

Molecular orbitals can be formed by the combination of multiple atomic orbitals. In benzene, there are ____ atomic p orbitals that combine to make ___ pi molecular orbitals

6, 6

The cyclopentadienyl anion is planar and completely conjugated, and contains ____ pi electrons. Therefore cyclopentadienyl anion is a(n) _____ ion

6, aromatic

Molecular orbitals can be formed by the combination of multiple atomic orbitals, such as the 6 orbitals that overlap to form the conjugated pi system of benzene. In this pi system, 6 atomic p orbitals combine to form ____ pi molecular orbitals. There are 3 pi ____ MOs and 3 pi* ___ MOs in this system

6, bonding, antibonding

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

Above the ring of the plane Below the ring of the plane

Match each type of compound with the appropriate reagents required for a successful halogenation reaction

Alkene: Br2 Benzene: Br2, FeBr3 (AlCl3 would also work)

Benzene is a resonance hybrid. What does this mean in terms of the C-C bonds in the ring?

All the C-C bonds are equal in length The C-C bonds are shorter than a typical C-C single bond

Rank nonaromatic, aromatic, and antiaromatic in order of increasing stability, with the most stable on the bottom and the least stable on the top

Antiaromatic Nonaromatic Aromatic

Aromatic hydrogens have a distinct chemical shift in 1H NMR spectroscopy and can be easily distinguished from aliphatic (nonaromatic) hydrogen atoms. Match the type of hydrogens with their shifting in an 1H NMR spectrum

Aromatic H atoms: downfield chemical shift Alphatic H atoms: upfield chemical shift

Describe the general system X=Y-Z, where Z has one or more lone pair

Atom Z will be sp2 hybridized One lone pair from atom Z will be in a p orbital

Describe the structure of benzene

Benzene is a planar molecule All the C atoms of benzene are sp2 hybridized Benzene is a fully conjugated system Benzene has 4 degrees of unsaturation

Describe the chemical reactivity of benzene

Benzene is much less reactive than other unsaturated hydrocarbons

An aromatic compound must be fully conjugated. What does this mean?

Each atom in the ring must have a p orbital available for continuous overlap

Describe the placement of electrons in molecular orbitals

Electrons fill the lowest energy orbitals first For orbitals of the same energy, electrons will not pair up until each orbital contains an electron

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

False 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 pi electrons to be aromatic

HOMO: LUMO:

Highest energy orbital that contains an electron Lowest energy orbital that contains an electron

A benzene substituent has the formula C6H5- and is called a(n) ____ group. It is often abbreviated as ____.

Phenyl, Ph

Pyrrole is an aromatic heterocycle. Describe pyrrole

The N atom in pyrrole forms 3 sigma bonds The N atom contributes its lone pair to the pi system Each C atom in this structure contributes one electron to the pi system

Furan is an aromatic heterocyclic compound. Describe furan

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

A hydrocarbon that is cyclic, planar, and fully conjugated contains 4 C=C double bonds, with no other pi bonds. What must be done to determine whether or not this compound fulfills Huckel's rule for aromaticity

The compound is antiaromatic because 4n+2=8 if n=2 The compound has 8 pi electrons and there is no integer value for n for which 4n+2 = 8

Describe pyridine

The lone pair on the N atom is in an sp2 hybridized orbital Pyridine has 6 pi electrons

Huckel's rule states that a system must contain 4n+2 pi electrons to be aromatic. This means that ______

The number of pi electrons must satisfy the formula 4n+2, where n=0 or any positive integer

Describe how to name a monosubstituted derivative of benzene

The parent name benzene can be used to name this type of compound Alkyl substituent names have the ending -yl and are written before the parent name Certain substituents give rise to common names that are used as the parent name

Why is benzene particularly reactive toward electrophiles?

The pi electron system is electron rich

Describe how to name a polysubstituted benzene derivative

The ring is numbered to give substituents the lowest possible numbers Substituent names are listen alphabetically

Describe how to name a disubstituted derivative of benzene where there is no common root

The substituent names should precede the word "benzene" The appropriate o,m, or p designation is used to represent the substituent pattern Substituents should be named alphabetically

Describe an antiaromatic system

The system contains 4n pi electrons The system is planar The system shows an unusual lack of stability

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

They cannot explain why the C-C bonds lengths are equal

Huckel's rule states that a compound must have 4n+2 pi electrons to be aromatic. Describe why this number of electrons is important

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

Match each type of compound to its characteristic reaction class: alkene benzene

addition substitution

A hydrocarbon that contains a single ring with alternating double and single bonds is specifically named a(n) ______

annulene

Compounds such as benzene, which are unsaturated and do not undergo the addition reactions typical of alkenes, are called ____ compounds

aromatic

The H's on cyclopentadiene are highly acidic in comparison with other hydrocarbon C-H bonds. This unique acidity is because the conjugate base is a(n) _____ anion making it unusually unstable

aromatic

NMR spectra of aromatic compounds show characteristic absorptions. Peaks in the 1H NMR spectrum that occur in the range of 6.5-8 ppm correspond to ____ protons, while peaks in the range 1.5-2.5 ppm are typical of ____ protons

aryl, benzylic

The parent name for monosubstituted benzene derivatives is

benzene

A group with the formula C6H5CH2- is called a(n) ____ group

benzyl

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

bonding

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

bonding

The molecular orbital model of bonding sates that when two atomic orbitals combine, they will form two molecular orbitals, one _____ and one ____ orbital

bonding antibonding

In order to be aromatic, a compound must be fully _____, meaning that there must be an available p orbital on every atom, with all overlapping in a continuous system. In order for all p orbitals to overlap, the compound must be _____ in shape; in order for the overlap to be continuous, the compound must be cyclic

conjugated, planar

When the 6 adjacent p orbitals overlap in benzene, the pi electrons are ____

delocalized

If the heteroatom is already part of a double bond: If the heteroatom is not part of a double bond:

its lone pair cannot occupy a p orbital and is not delocalized its lone pair can occupy a p orbital and be part of a delocalized pi system

Benzene is more stable than expected for a system containing 3 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

A molecular orbital is a region of space in any covalent species where electrons are likely to be found. The combination of two atomic orbitals always forms two molecular orbitals, the bonding orbital, which is ____ in energy, and the antibonding orbital, which is ____ in energy than the original atomic orbitals

lower, higher

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 3 isolated double bonds.

lower, more

When placing electrons into molecular orbitals, electrons are first placed in the orbitals of the ____ energy. For 2 orbitals with the same energy, electrons are initially place with ___ electron(s) in each orbital

lowest, 1

Huckel's rule for edtermining aromaticity applies only to ____-cyclic systems

mono

A cyclic system is fully conjugated when every atom in the ring has a _____ orbital in the same plane available for continuous overlap

p

For the general system X=Y-Z, where atom Z has one or more lone pairs, one lone pair will always be in a(n) ____ orbital, which allows the system to be conjugated

p

Valence bond theory is inadequate for describing systems with multiple adjacent ____ that overlap, as in aromatic molecules. A better explanation of these systems is given using ____ theory

p orbitals, molecular orbital

Benzene has 6 pi electrons and is an electron-____ system. It will therefore react readily with ____

rich, electrophiles

When determining whether or not a cyclic system is aromatic, we examine the ___ of the molecule as well as the number of ____ bonds

shape, pi

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

sp2, p

Pyridine is a heterocyclic compound containing a nitrogen atom. The N atom is ____ hybridized, and its lone pair resides in a(n) _____ orbital. The pyridine N atom has _____ electron(s) in a p orbital, which it contributes to the aromatic system so there is a total of 6 electrons

sp2, sp2, 1