Benzene and Aromaticity | Organic Chemistry 2

Benzene and aromaticity are studied in this chapter: nomenclature of benzene derivatives, properties of benzene, criteria of aromaticity and Hückel’s rule, aromatic, antiaromatic, nonaromatic systems, reduction of aromatic compounds

Nomenclature of Benzene Derivatives

Monosubstituted benzene: add the name of the substituent in front of 'benzene'.

Polysubstituted benzene:

  1. Identify and name the parent (e.g., benzene, phenol).
  2. Identify the substituents (e.g., chloro, bromo, methyl).
  3. Assign positions: use 1,2-, 1,3-, and 1,4- (or ortho-, meta-, and para-) to indicate the positions.
  4. Arrange substituents alphabetically with locants.


Benzene as substituent:

  • Phenyl group: it is used when benzene acts as a substituent and can be abbreviated in a structure as Ph-.
  • Benzyl group: refers to the structure derived from benzene when one hydrogen atom is replaced by a methyl group (CH2). It is typically attached to another atom or group via the CH2 unit.

Properties of Benzene


Benzene is a planar, cyclic hydrocarbon with six carbon atoms connected by alternating single and double bonds, forming a hexagonal ring. Because each π bond has 2 electrons, benzene has 6 π electrons. Each carbon atom in a benzene ring is sp2 hybridized and trigonal planar wih all bond angles 120o.


Benzene is particularly stable ⇒ not the same reactivities as alkenes.
Each atom in the ring has a p orbital to delocalize electron density ⇒ benzene is stabilized by delocalization of its π electrons. The electrons of the p orbitals form a π cloud above and below the plane of the ring.

sp2 orbitals in the plane
p orb. perpendicular to the plane


Bond lengths and strength:

  • All carbon-carbon bonds in benzene are equal in length due to resonance, with bond lengths intermediate between single and double bonds.
  • The equalization of bond lengths reflects the intermediate bond strength between single and double bonds.

Spectroscopic properties:

  • 1H NMR: δ ~ 6.5-8 ppm (highly deshielded protons)
    Benzene and aromatic rings are characteristic in 1H NMR spectroscopy: they have highly deshielded protons due to the ring current effect of the circulating π electrons that reinforce the applied magnetic field.
  • 13C NMR: δ ~ 120-150 ppm
  • IR absorptions: 3150-3000 cm-1 (C-H); 1600, 1500 cm-1 (C=C)

Criteria for Aromaticity

Hückel's Rule:

According to Hückel's Rule, a compound is aromatic if it has 4n + 2 π electrons, where "n" is an integer (n = 0, 1, 2, ...). This formula ensures an even number of π electrons, making the compound more stable.

Criteria for Aromaticity

Four criteria must be satisfied for a compound to be aromatic:

  • The molecule must be cyclic.
  • The molecule must be planar.
  • The molecule must be completely conjugated.
  • The molecule must contain 4n + 2 π electrons (Hückel's Rule).

Aromatic, Antiaromatic, Nonaromatic Compounds

Aromatic system: 

A cyclic, planar, and fully conjugated system of 4n + 2 π electrons.
Aromatic compounds are exceptionally stable due to resonance and fully occupied, delocalized π molecular orbitals.

All of the molecules below have 6 π electrons (4n + 2 with n = 1) and are planar:


Antiaromatic system: 

A cyclic, planar, and fully conjugated system of 4n π electrons.
Antiaromatic compounds are often unstable and have higher energy levels compared to aromatic or nonaromatic compounds ⇒ this system wants to gain or lose 2 π electrons.

The molecules below have 4n π electrons (n = 1 and n = 3):


Nonaromatic system:

A non-planar or non-cyclically delocalized system.

The first molecule is non-planar and the π electrons of the other two are non-cyclically delocalized ⇒ these are nonaromatic systems:

Examples of Aromatic Compounds


Annulenes are cyclic hydrocarbons with a single ring structure composed of alternating single and double bonds ⇒ fully conjugated π sytem. For annulenes to exhibit aromaticity, the number of π electrons in the cyclic system must follow Hückel's Rule, which dictates 4n + 2 electrons for aromaticity.

Polycyclic aromatic hydrocarbons (PAHs):

PAHs consist of fused aromatic rings, forming a planar structure. The π electrons are delocalized across the entire fused ring system.


Aromatic heterocycles:

Heterocycles are cyclic compounds containing heteroatoms, such as S, N, and O.

If a heteroatom is already part of a double bond, its lone pair cannot occupy a p orbital and thus cannot be delocalized over the ring. Conversely, if a heteroatom is not part of a double bond, its lone pair can be located in a p orbital and delocalized over the ring to make it aromatic.

Examples of aromatic nitrogen-containing heterocycles:

  • Pyridine: a six-membered ring containing five carbon atoms and one nitrogen atom.
    6 π electrons: 2 from each π bond ⇒ the lone pair in pyridine is localized and does not participate in resonance.

  • Pyrrole:  a five-membered ring containing four carbon atoms and one nitrogen atom.
    6 π electrons: 4 from the π bonds and 2 from the lone pair ⇒ the lone pair in pyrrole is delocalized and participates in aromaticity.


Charged aromatic compounds:

  • Cyclopentadienyl anion: a five-membered ring containing five carbon atoms and one negative charge, forming a planar, cyclic structure.
    6 π electrons: 4 from the π bonds and 2 from the lone pair.

  • Tropylium cation: a seven-membered ring containing seven carbon atoms and one positive charge, forming a planar, cyclic structure.
    6 π electrons: 2 from each π bond, that are delocalized across the entire ring.

Reduction of Aromatic Compounds

Catalytic hydrogenation:

It requires a catalyst (often nickel), elevated temperatures (around 150-300°C) and high pressures (typically several atmospheres) to overcome the stability of the benzene ring.

Under certain conditions, a vinyl group can be selectively hydrogenated in the presence of an aromatic ring:


Birch reduction:


The aromatic moiety is reduced to give a nonconjugated diene.

  1. Na reacts with liquid ammonia (NH3) to produce a solvated electron that is transferred to the aromatic ring.
  2. Methanol donates a proton to the phenyl radical anion, forming a radical intermediate.
  3. A second single electron is transferred from Na to the radical intermediate, generating an anion.
  4. Methanol donates a proton to form a diene.


  • The two reduced carbon atoms are opposite each other ⇒ the product is a nonconjugated diene.
  • The carbon atom connected to an alkyl group is not reduced, while the carbon atom connected to an electron-withdrawing group is reduced.

Check your knowledge about this Chapter

The general structure of benzene is a six-carbon ring with alternating double bonds, represented by a hexagonal shape with a circle in the middle. This symbolizes the delocalized electrons that are shared across the entire ring structure, a phenomenon known as aromaticity. Benzene is unique due to its high degree of symmetry, stability, and the fact that its electron structure cannot be represented by a single Lewis structure but rather a resonance hybrid of multiple structures.

To name a benzene derivative with a single substituent, you begin with the name of the substituent, followed by the word 'benzene'.

  • If the substituent is a methyl group, the compound is named methylbenzene (commonly known as toluene).
  • If the substituent is a nitro group, the compound is named nitrobenzene.

When benzene has two or more substituents, the positions of the substituents are indicated by numbering the carbon atoms in the ring. The numbering starts from one substituent and proceeds around the ring to give the lowest possible numbers. For two substituents, the terms ortho (1,2-), meta (1,3-), and para (1,4-) are often used to describe their relative positions. If there are more than two substituents, number the ring such that substituents receive the lowest set of numbers and list the substituents in alphabetical order when naming the compound.

Benzene is unique among hydrocarbons due to its distinct ring structure and resonance stability. It has a planar hexagonal ring of six carbon atoms with alternating single and double bonds. This ring structure imparts significant stability due to delocalization of π electrons across the ring, making benzene less reactive than typical alkenes.

Additionally, benzene exhibits a phenomenon known as aromaticity, which includes a set of criteria such as a planar structure, a fully conjugated π electron system, and a matching number of π electrons to the Huckel's rule (4n+2 π electrons, where n is a non-negative integer).

The delocalized electrons in benzene are significant because they provide extra stability to the molecule through resonance. This delocalization results in the unique chemical reactivity of benzene, making it less reactive towards addition reactions that would disrupt the aromatic system, while favoring substitution reactions that preserve the aromatic electron configuration.

Benzene is less reactive than alkenes towards addition reactions due to the stability of its aromatic system. The delocalization of electrons in benzene's π-cloud above and below the plane of the carbon atoms contributes to significant stabilization. This delocalization forms a more stable structure compared to the localized π bonds in alkenes. Disrupting the aromaticity by adding atoms to benzene would mean losing this stability, which is why benzene tends to undergo substitution reactions that preserve its aromatic character rather than addition reactions that would destroy it.

The criteria for a compound to be considered aromatic are based on Hückel's rule, which states that a molecule must have:

  • A cyclic ring of sp2 hybridized atoms.
  • Planarity, allowing for overlapping p-orbitals.
  • A continuous π electron cloud above and below the plane of the molecule (system fully conjugated).
  • A total number of 4n + 2 π electrons (where n is a non-negative integer) in the ring system.

This combination of features allows for exceptional stability due to the delocalization of the π electrons, characteristic of aromatic compounds.

Hückel's rule, also known as the (4n + 2) π electron rule, is a criterion for determining if a planar ring molecule will have aromatic properties. According to this rule, a molecule must have a certain number of π electrons to be considered aromatic: a fully conjugated cyclic molecule with (4n + 2) π electrons, where n is a non-negative integer (n = 0, 1, 2, ...), will exhibit aromaticity. This denotes an extra stability due to the delocalization of π electrons across the ring structure, often leading to lower reactivity in aromatic compounds compared to non-aromatic compounds.

  • Aromatic compounds are cyclic, planar structures with a conjugated π-electron system that follows Hückel's rule, possessing 4n + 2 π electrons, which gives them added stability.
  • Antiaromatic compounds also have cyclic, conjugated π-electron systems but have 4n π electrons that result in instability and reactivity due to the lack of delocalization energy.
  • Nonaromatic compounds either do not fulfill the criteria for aromaticity, such as not being cyclic or planar, or they do not have a π-conjugated system ⇒ they are neither especially stable nor particularly unstable simply because of their electron configuration.

Benzene is the simplest aromatic compound, with a six-membered ring and alternating double bonds. Other examples include toluene, which is benzene with a methyl group attached, and naphthalene, which consists of two fused benzene rings. Polycyclic aromatic hydrocarbons (PAHs) such as anthracene and phenanthrene have multiple fused benzene rings. Aromaticity is not limited to hydrocarbons ⇒ heterocyclic compounds like pyridine and furan also exhibit aromatic characteristics, containing nitrogen and oxygen in the ring, respectively.

Catalytic hydrogenation in the reduction of aromatic compounds involves the addition of hydrogen in the presence of a metal catalyst. Unlike non-aromatic substrates, the process typically requires higher temperatures and pressures due to the stability of the aromatic ring.

Reaction conditions for Birch reduction typically involve the use of alkali metals (e.g., sodium) in the presence of liquid ammonia or an alcohol. Low temperatures and careful control of reagent stoichiometry are critical for successful outcomes.

Birch reduction is an organic reaction that results in the formation of a nonconjugated diene. The process begins with sodium (Na) reacting with liquid ammonia (NH3) to generate a solvated electron that is subsequently transferred to the aromatic ring. The aromatic ring is protonated, forming a radical intermediate. A second single electron is then transferred from sodium to the radical intermediate, forming an anion. The final step involves the protonation of the anion, ultimately leading to the formation of the nonconjugated diene.