Organic Chemistry: Benzene and Aromatic Compounds (Chap 15)

Hückel's Rule: Four structural criteria must be satisfied for a compound to be aromatic:

1. A molecule must be cyclic. →To be aromatic, each p orbital must overlap with p orbitals on two adjacent atoms. 2. A molecule must be planar. →All adjacent p orbitals must be aligned so that the π electron density can be delocalized. 3. A molecule must be completely conjugated. →Aromatic compounds must have a p orbital on every atom in the ring. 4. A molecule must satisfy Hückel's rule, and contain a particular number of π electrons. →An aromatic compound must contain 4n + 2 π electrons (n = 0, 1, 2, and so forth). →Cyclic, planar, and completely conjugated compounds that contain 4n π electrons are especially unstable, and are said to be antiaromatic.

When naming benzene derivatives, if two groups on the benzene ring are different:

1. Alphabetize the names of the substituents preceding the word benzene. 2. If one of the substituents is part of a common root, name the molecule as a derivative of that mono substituted benzene. Examples: o-bromochlorobenzene, m-fluoronitrobenzene, p-bromotoluene, and o-nitrophenol.

Any structure for benzene must account for the following:

1. Benzene contains a six-membered ring and three additional degrees of unsaturation. 2. Benzene is planar. 3. All C-C bond lengths are equal.

Heats of hydrogenation can be used to estimate the stability of benzene:

1. Cyclohexene → cyclohexane ΔH° observed = -120 kJ/mol 2. Cyclohexa-1,3-diene → cyclohexane ΔH° observed = -232 kJ/mol ΔH° predicted = 2(-120) = -240 kJ/mol *Small difference → slightly more stable than two isolated double bonds. 3. Benzene → cyclohexane ΔH° observed = -208 kJ/mol ΔH° predicted = 3(-120) = -360 kJ/mol *Large difference → much more stable than three isolated double bonds.

To name a benzene ring with one substituent:

1. Name the substituent. 2. Add the word benzene. Example, ethylbenzene, tert-butylbenzene, and chlorobenzene.

The most important features of the six benzene MOs are as follows:

1. The larger the number of bonding interactions, the lower in energy the MO. The lowest-energy molecular orbital (ψ1) has all bonding interactions between the p orbitals. 2. The larger the number of nodes, the higher in energy the MO. The highest-energy MO (ψ6*) has all nodes between the p orbitals. 3. Three MOs are lower in energy than the starting p orbitals, making them bonding MOs (ψ1, ψ2, and ψ3), whereas three MOs are higher in energy than the starting p orbitals, making them antibonding MOs (ψ4*, ψ5*, and ψ6*). 4. The two pairs of MOs (ψ2 and ψ3; ψ4* and ψ5*) with the same energy are called degenerate orbitals. 5. The highest-energy orbital that contains electrons is called the highest occupied molecular orbital (HOMO). For benzene, the degenerate orbitals ψ2 and ψ3 are the HOMOs. 6. The lowest-energy orbital that does not contain electrons is called the lowest unoccupied molecular orbital (LUMO). For benzene, the degenerate orbitals ψ4* and ψ5* are the LUMOs.

Not aromatic or nonaromatic

A compound that lacks one (or more) of the four requirements to be aromatic or antiaromatic.

Aromatic

A cyclic, planar, completely conjugated compound with 4n + 2 π electrons.

Antiaromatic

A cyclic, planar, completely conjugated compound with 4n π electrons.

When forming molecular orbitals from atomic orbitals, keep in mind:

A set of n atomic orbitals forms n molecular orbitals. *If two atomic orbitals combine, two molecular orbitals are formed.

What are some examples of aliphatic hydrocarbons?

Aliphatic hydrocarbons include the alkanes, alkenes, and alkynes, as well as the conjugated dienes and polyenes.

As the number of fused benzene rings increases, the number of resonance structures increases as well.

Although two resonance structures can be drawn for benzene, naphthalene is a hybrid of three resonance structures.

To fill the MOs of benzene, the six electrons are added, two to an orbital, beginning with the lowest-energy orbital.

As a result, the six electrons completely fill the bonding MOs, leaving the antibonding MOs empty. This is what gives benzene and other aromatic compounds their special stability, and this is why six π electrons satisfies Hückel's 4n + 2 rule.

The molecular orbital description of benzene is much more complex than the two MOs formed from the combination of two p orbitals.

Because each of the six carbon atoms of benzene has a p orbital, six atomic p orbitals combine to form six π molecular orbitals.

Does benzene undergo addition reactions typical of other highly unsaturated compounds?

Benzene does not undergo addition reactions typical of other highly unsaturated compounds, including conjugated dienes. Benzene does not react with Br2 to yield an addition product. Instead, in the presence of a Lewis acid, bromine substitutes for a hydrogen atom, thus yielding a product that retains the benzene ring. *This behavior is characteristic of aromatic compounds.

How many degrees of unsaturation does benzene have?

Benzene has four degrees of unsaturation, making it a highly unsaturated hydrocarbon. *Whereas unsaturated hydrocarbons, such as alkenes, alkynes, and dienes, undergo addition reactions, benzene does not. For example, bromine adds to ethylene to form a dibromide, but benzene is inert under similar conditions.

When does benzene react with bromine?

Benzene reacts with bromine only in the presence of FeBr3 (a Lewis acid), and the reaction is a substitution, not an addition.

Helicene and twistoflex are two synthetic PAHs with unusual shapes.

Both helicene and twistoflex are chiral molecules - that is, they are not superimposable on their mirror images, even though neither of them contains a stereogenic center. It's their shape that makes them chiral, not the presence of carbon atoms bonded to four different groups. Each ring system is twisted into a shape that lacks a mirror plane, and each structure is rigid, thus creating the chirality. *They are nonplanar.

Conjugated dienes are ______ stable than two isolated carbon-carbon double bonds.

Conjugated dienes are more stable than two isolated carbon-carbon double bonds.

What are the characteristic 13-C NMR absorptions of benzene derivatives?

Csp2 of arenes → 120 - 150 ppm

What are the characteristic IR absorptions of benzene derivatives?

Csp2-H → 3150 - 3000 cm^-1 C=C (arene) → 1600, 1500 cm^-1

Many mono substituted benzenes, such as those with methyl (CH3-), hydroxy (-OH), and amino (-NH2) groups, have common names that we must learn.

Examples: Toluene (methylbenzene) Phenol (hydroxybenzene) Aniline (aminobenzene)

Pyridine

Pyridine is a heterocycle containing a six-membered ring with three π bonds and one nitrogen atom. Similar to benzene, two resonance structures (with all neutral atoms) can be drawn. -cyclic -planar -completely conjugated -satisfies Hückel's rule (6 π electrons) -aromatic

If the lone pair on the N atom occupies a p orbital:

Pyrrole has a p orbital on every adjacent atom, so it is completely conjugated. Pyrrole has six π electrons - four from the π bonds and two from the lone pair. Because pyrrole is cyclic, planar, completely conjugated, and has 4n + 2 π electrons, pyrrole is aromatic.

The cyclopentadienyl anion and the tropylium cation both illustrate an important principle:

The number of π electrons determines aromaticity, not the number of atoms in a ring or the number of p orbitals that overlap. The cyclopentadienyl anion and the tropylium cation are aromatic because they each have six π electrons.

In a system X=Y-Z:,

Z is generally sp2 hybridized and the lone pair occupies a p orbital to make the system conjugated.

The benzyl group contains a benzene ring bonded to:

a CH2 group. Thus, a benzyl group and a phenyl group differ by the presence of a CH2 group.

The cyclopentadienyl anion is:

a cyclic and planar anion with two double bonds and a non bonded electron pair. In this way, it resembles pyrrole. The two π bonds contribute four electrons, and the lone pair contributes two more, for a total of six. By Hückel's rule, having six π electrons confers aromaticity. Like the N atom in pyrrole, the negatively charged carbon atom must be sp2 hybridized, and the non bonded electron pair must occupy a p orbital for the ring to be completely conjugated.

A π* antibonding MO is higher in energy than the two atomic p orbitals from which it is formed because:

a destabilizing node results when orbitals of opposite phase combine. A destabilizing interaction pushes nuclei apart.

The tropylium cation is:

a planar carbocation with three double bonds and a positive charge contained in a seven-membered ring. This carbocation is completely conjugated, because the positively charged carbon is sp2 hybridized and has a vacant p orbital that overlaps with the six p orbitals from the carbons of the three double bonds. *Because the tropylium cation has three π bonds and no other nonbonded electron pairs, it contains six π electrons, thereby satisfying Hückel's rule.

When two p orbitals of similar phases overlap side-by-side:

a π bonding molecular orbital results. Like phases interact → increased electron density between the nuclei.

Hydrocarbons containing a single ring with alternating double and single bonds are called:

annulenes.

There are two different ways to join three rings together, forming:

anthracene and phenanthrene. These have 14 π electrons.

In the nineteenth century, many other compounds having properties similar to those of benzene were isolated from natural sources. These compounds were called:

aromatic compounds because they possessed strong and characteristic odors. It is their chemical properties, however, that make these compounds special - not their odor.

Compounds that contain 2, 6, 10, 14, 18, and so forth π electrons are:

aromatic. Benzene is aromatic and especially stable because it contains 6 π electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 π electrons.

We must use resonance and orbitals to describe the structure of benzene because:

benzene is conjugated. *The resonance description of benzene consists of two equivalent Lewis structures, each with three double bonds that alternate with three single bonds. The electrons in the π bonds are delocalized around the ring.

In the last half of the nineteenth century, August Kekulé proposed structures that were close to the modern description of benzene. In the Kekulé model:

benzene was thought to be a rapidly equilibrating mixture of two compounds, each containing a six-membered ring with three alternating π bonds. These structures are now called Kekulé structures. In the Kekulé description, the bond between any two carbon atoms is sometimes a single bond and sometimes a double bond.

In aromatic compounds, all bonding MOs (and HOMOs) are:

completely filled. No π electrons occupy antibonding MOs.

The structure of diamond consists of a continuous tetrahedral network of sp3 hybridized carbon atoms, thus creating an infinite array of chair cyclohexane rings. The structure of graphite, on the other hand:

consists of parallel sheets of sp2 hybridized carbon atoms, thus creating an infinite array of benzene rings. The parallel sheets are then held together by weak intermolecular interactions. Diamond → an "infinite" array of six-membered rings, covalently bonded in three dimensions Graphite → an "infinite" array of benzene rings, covalently bonded in two dimensions *Graphite exists in planar sheets of benzene rings, held together by weak intermolecular forces.

The cyclopentadienyl anion is readily formed from:

cyclopentadiene by a Brønsted-Lowry acid-base reaction. Cyclopentadiene itself is not aromatic because it is not fully conjugated. *The cyclopentadienyl anion, however, it aromatic, so it is a very stable base. As such, it makes cyclopentadiene more acidic than other hydrocarbons. In fact, the pKa of cyclopentadiene is 15, much lower (more acidic) than the pKa of any C-H bond discussed thus far.

In a benzene molecule, the six adjacent p orbitals overlap, which:

delocalizes the six electrons over the six atoms of the ring and makes benzene a conjugated molecule. Because each p orbital has two lobes, one above and one below the plane of the benzene ring, the overlap of the p orbitals creates two "doughnuts" of electron density.

Heterocycles containing oxygen, nitrogen, or sulfur - atoms that also have at least one lone pair of electrons - can also be aromatic. With heteroatoms, we must always:

determine whether the lone pair is localized on the heteroatom or part of the delocalized π system. Two examples, pyridine and pyrrole, illustrate these different possibilities.

The two most common elemental forms of carbon are:

diamond and graphite. Diamond, one of the hardest substances known, is used for industrial cutting tools, whereas graphite, a slippery black substance, is used as a lubricant. Their physical characteristics are so different because their molecular structures are very different.

13-C NMR spectroscopy is used to determine the substitution patterns in disubstituted benzenes because:

each line in a spectrum corresponds to a different kind of carbon atom. For example, o-, m-, and p-dibromobenzene each exhibit a different number of lines in its 13-C NMR spectrum. o-dibromobenzene → three types of C's → three 13-C NMR signals m-dibromobenzene → four types of C's → four 13-C NMR signals p-dibromobenzene → two types of C's → two 13-C NMR signals *The number of signals (lines) in the 13-C NMR spectrum of a disubstituted benzene with two identical groups indicates whether they are ortho, meta, or para to each other.

Benzene's six π electrons make it electron rich, so it reacts with:

electrophiles.

Benzene has a high degree of unsaturation and a lack of reactivity toward:

electrophilic addition.

Benzene can be hydrogenated under:

forcing conditions, and even then the reaction is extremely slow.

Cyclooctatetraene resembles benzene in that:

it is a cyclic molecule with alternating double and single bonds. Cyclooctatetraene is tub shaped, however, not planar, so overlap between adjacent π bonds is impossible. Cyclooctatetraene, therefore, is not aromatic, so it undergoes addition reactions like those of other alkenes.

The tropylium cation is aromatic because:

it is cyclic, planar, completely conjugated, and has six π electrons delocalized over the seven atoms of the ring.

Hückel's rule for determining aromaticity can be applied only to:

monocyclic systems. *But many aromatic compounds containing several benzene rings joined together are also known!

The total number of MOs always equals the:

number of vertices of the polygon.

Although five resonance structures can also be drawn for both the cyclopentadienyl cation and radical:

only the cyclopentadienyl anion has six π electrons, a number that satisfies Hückel's rule. The cyclopentadienyl cation has four π electrons, making it antiaromatic and especially unstable. The cyclopentadienyl radical has five π electrons, so it is neither aromatic nor antiaromatic. Having the "right" number of electrons is necessary for a species to be unusually stable by virtue of aromaticity.

There are three different ways that two groups can be attached to a benzene ring, so a prefix -

ortho, meta, or para - can be used to designate the relative position of the two substituents. Ortho, meta, and para are also abbreviated as o, m, and p, respectively.

A benzene substituent (C6H5-) is called a:

phenyl group, and it can be abbreviated as Ph-.

PAH

polycyclic aromatic hydrocarbon

Two or more six-membered rings with alternating double and single bonds can be fused together to form:

polycyclic aromatic hydrocarbons (PAHs). *Joining two benzene rings together forms naphthalene.

Compounds containing two or more benzene rings that share carbon-carbon bonds are called:

polycyclic aromatic hydrocarbons (PAHs). *Naphthalene, the simplest PAH, is present in mothballs.

A phenyl group (C6H5-) is formed by:

removing one hydrogen from benzene (C6H6). *Benzene, therefore, can be represented as PhH, and phenol would be PhOH.

Because each π bond has two electrons, benzene has:

six π electrons.

Each carbon atom in a benzene ring is surrounded by three atoms and no lone pairs of electrons, making it:

sp2 hybridized and trigonal planar with all bond angles 120°. Each carbon also has a p orbital with one electron that extends above and below the plane of the molecule.

Benzo[a]pyrene, a more complicated PAH, is formed by:

the incomplete combustion of organic materials. It is found in cigarette smoke, automobile exhaust, and the fumes from charcoal grills. When ingested or inhaled, bento[a]pyrene and other similar PAHs are oxidized to carcinogenic products.

MO theory describes bonds as:

the mathematical combination of atomic orbitals that form a new set of orbitals called molecular orbitals (MOs). A molecular orbital occupies a region of space in a molecule where electrons are likely to be found.

[10]-annulene has 10 π electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other, so:

the ring puckers to relieve this strain. Because [10]-annulene is not planar, the 10 π electrons can't delocalize over the entire ring, and it is not aromatic. -not planar -not aromatic

Benzene (C6H6) is:

the simplest aromatic hydrocarbon (or arene). *Since its isolation by Michael Faraday from the oily residue remaining in the illuminating gas lines in London in 1825, it has been recognized as an unusual compound.

Benzene and toluene, the simplest aromatic hydrocarbons obtained from petroleum refining, are:

useful starting materials for synthetic polymers. They are two components of the BTX mixture added to gasoline to boost octane ratings.

To predict whether a compound has π electrons completely filling bonding MOs, we must know how many bonding molecular orbitals and how many π electrons it has. It is possible to predict the relative energies of cyclic, completely conjugated compounds, without sophisticated math (or knowing what the resulting MOs look like) by:

using the inscribed polygon method. inscribed polygon = Frost circle

The number of electrons - not the size of the ring - determines:

whether a compound is aromatic.

How to use the inscribed polygon method to determine the relative energies of MOs for cyclic, completely conjugated compounds:

1. Draw the polygon in question inside a circle with its vertices touching the circle and one of the vertices pointing down. Mark the points at which the polygon intersects the circle. 2. Draw a line horizontally through the center of the circle and label MOs as bonding, nonbonding, or antibonding. →MOs below this line are bonding and lower in energy than the p orbitals from which they were formed. Benzene has three bonding MOs. →MOs at this line are nonbonding and equal in energy to the p orbitals from which they were formed. Benzene has no nonbonding MOs. →MOs above this line are antibonding and higher in energy than the p orbitals from which they were formed. Benzene has three antibonding MOs. 3. Add the electrons, beginning with the lowest-energy MO. →All the bonding MOs (and the HOMOs) are completely filled in aromatic compounds. No π electrons occupy antibonding MOs. →Benzene is aromatic because it has six π electrons that completely fill the bonding MOs.

Valence bond theory:

1. Hydrogen uses its 1s orbital to form σ bonds with other elements. 2. Second-row elements use hybrid orbitals (sp, sp2, or sp3) to form σ bonds. 3. Second-row elements use p orbitals to form π bonds. *In valence bond theory, a covalent bond is formed by the overlap of two atomic orbitals, and the electron pair in the resulting bond is shared by both atoms. Thus, a carbon-carbon double bond consists of a σ bond, formed by overlap of two sp2 hybrid orbitals, each containing one electron, and a π bond, formed by overlap of two p orbitals, each containing one electron.

For three or more substituents on a benzene ring (polysubstituted benzenes):

1. Number to give the lowest possible set of numbers around the ring. 2. Alphabetize the substituent names. 3. When substituents are part of common roots, name the molecule as a derivative of that monosubstituted benzene. The substituent that comprises the common root is located at C1.

The resonance hybrid of benzene explains why all C-C bond lengths are the same.

Each C-C bond is single in one resonance structure and double in the other, so the actual bond length (139 pm) is intermediate between a carbon-carbon single bond (153 pm) and a carbon-carbon double bond (134 pm). *The C-C bonds in benzene are equal and intermediate in length.

Many widely used drugs contain a benzene ring.

Examples include: Zoloft (generic name sertraline) → a psychotherapeutic drug for depression and panic disorders Viracept (generic name nelfinavir) → an antiviral drug used to treat HIV Novocain (generic name procaine) → a local anesthetic

Histamine produces a wide range of physiological effects in the body.

Excess histamine is responsible for the runny nose and watery eyes symptomatic of hay fever. It also stimulates the overproduction of stomach acid and contributes to the formation of hives. These effects result from the interaction of histamine with two different cellular receptors. *Antihistamines (like Benadryl) that block the action of histamine on the H1 histamine receptor are used to treat the runny nose and watery eyes of an allergic reaction.

Histamine

Has an aromatic heterocycle with two N atoms, one of which is similar to the N atom of pyridine and one of which is similar to the N atom of pyrrole.

Electrostatic potential maps for pyridine and pyrrole illustrate that the lone pair in pyridine is localized on N, whereas the lone pair in pyrrole is part of the delocalized π system.

In pyridine, the nonbonded electron pair is localized on the N atom in an sp2 hybridized orbital, as shown by a region of high electron density (in red) on N. In pyrrole, the nonbonded electron pair is in a p orbital and is delocalized over the ring, so the entire ring is electron rich (red).

To name an annulene:

Indicate the number of atoms in the ring in brackets and add the word annulene. *Thus, benzene is [6]-annulene. Both [14]-annulene and [18]-annulene are cyclic, planar, and completely conjugated molecules that follow Hückel's rule, so they are aromatic.

Pyrrole

Pyrrole contains a five-membered ring with two π bonds and one nitrogen atom. The N atom also has a lone pair of electrons. -cyclic -planar -total of four π electrons from the two π bonds -two π electrons from the lone pair -aromatic

The inscribed polygon method is consistent with Hückel's 4n + 2 rule:

That is, there is always one lowest-energy bonding MO that can hold two π electrons and the other bonding MOs come in degenerate pairs that can hold a total of four π electrons. For the compound to be aromatic, these MOs must be completely filled with electrons, so the "magic numbers" for aromaticity fit Hückel's 4n + 2 rule.

How is the nitrogen atom of the pyridine ring hybridized?

The N atom is surrounded by three groups (two atoms and a lone electron pair), making it sp2 hybridized, and leaving one unhybridized p orbital with one electron that overlaps with adjacent p orbitals. *The lone pair on N resides in an sp2 hybrid orbital that is perpendicular to the delocalized π electrons.

Histamine has a five-membered ring with two π bonds and two nitrogen atoms, each of which contains a lone pair of electrons.

The heterocycle has four π electrons from the two double bonds. The lone pair on the N (of the N-H bond) also occupies a p orbital, making the heterocycle completely conjugated and giving it a total of six π electrons. The lone pair on this N atom is thus delocalized over the five-membered ring, and the heterocycle is aromatic. The lone pair on the N (of the C=N bond) occupies an sp2 hybrid orbital perpendicular to the delocalized π electrons.

The huge difference between the hypothetical and observed heats of hydrogenation for benzene cannot be explained solely on the basis of resonance and conjugation.

The low heat of hydrogenation of benzene means that benzene is especially stable, even more so than the conjugated compounds introduced in Chapter 14. *This unusual stability is characteristic of aromatic compounds.

The resonance description of benzene matches the Kekulé description with one important exception:

The two Kekulé representations are not in equilibrium with each other. Instead, the true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the π bonds.

Why does the number of π electrons determine whether a compound is aromatic? Cyclobutadiene is cyclic, planar, and completely conjugated, just like benzene, but why is benzene aromatic and cyclobutadiene antiaromatic?

To understand this difference, we must utilize molecular orbital (MO) theory.

A π bonding MO is lower in energy than the two atomic p orbitals from which it is formed because:

a stable bonding interaction results when orbitals of similar phase combine. A bonding interaction holds the nuclei together.

Buckminsterfullerene (C60) is:

a third elemental form of carbon. Its structure consists of 20 hexagons and 12 pentagons of sp2 hybridized carbon atoms joined in a spherical arrangement. It is completely conjugated because each carbon atom has a p orbital with an electron in it. Buckminsterfullerene, or buckyball, was discovered by Smaller, Curl, and Kroto, who shared the 1996 Nobel Prize in Chemistry for their work. Its unusual name stems from its shape, which resembles the geodesic dome invented by R. Buckminster Fuller. The pattern of five- and six-membered rings also resembles the pattern of rings on a soccer ball.

When two p orbitals of opposite phase overlap side-by-side:

a π* antibonding molecular orbital results. Opposite phases interact → no electron density between the nuclei.

What are the characteristic 1-H NMR absorptions of benzene derivatives?

aryl H → 6.5 - 8 ppm (highly deshielded protons) benzylic H → 1.5 - 2.5 ppm (somewhat deshielded Csp3-H)

Substituents derived from benzene, as well as all other substituted aromatic rings, are collectively called:

aryl groups, abbreviated Ar-.

The cyclopentadienyl anion is aromatic because:

it is cyclic, planar, completely conjugated, and has six π electrons. We can draw five equivalent resonance structures for the cyclopentadienyl anion, delocalizing the negative charge over every carbon atom of the ring.

Cyclopentadiene is more acidic than many hydrocarbons because:

its conjugate base is aromatic.

When a heteroatom is not part of a double bond (as in the N of pyrrole):

its lone pair can be located in a p orbital and delocalized over a ring to make it aromatic.

When a heteroatom is already part of a double bond (as in the N of pyridine):

its lone pair cannot occupy a p orbital, so it cannot be delocalized over the ring.

Considering benzene as the hybrid of two resonance structures adequately explains its equal C-C bond lengths, but does not account for:

its unusual stability and lack of reactivity toward addition.

Pyridine has six π electrons, two from each π bond, thus:

satisfying Hückel's rule and making pyridine aromatic. The nitrogen atom of pyridine also has a non bonded electron pair, which is localized on the N atom, so it is not part of the delocalized π electron system of the aromatic ring.

If two atomic p orbitals each have one electron and then combine to form MOs:

the two electrons will occupy the lower-energy π bonding MO.

Although benzene is still drawn as a six-membered ring with three alternating π bonds, in reality:

there is no equilibrium between two different kinds of benzene molecules. *Instead, current descriptions of benzene are based on resonance and electron delocalization due to orbital overlap.

Benzene is the most common aromatic compound having a single ring. Completely conjugated rings larger than benzene are also aromatic if:

they are planar and have 4n + 2 π electrons.

Aromatic compounds resemble benzene;

they are unsaturated compounds that do not undergo the addition reactions characteristic of alkenes.

The Kekulé structures are not accurate because:

they fail to account for the fact that all the C-C bond lengths in benzene are equal. In the Kekulé structures, having three alternating π bonds would mean that benzene should have three short double bonds alternating with three longer single bonds. This is not the case!

Both negatively and positively charged ions can also be aromatic if:

they satisfy all the necessary criteria.