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Clar's rule

In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule to its aromaticity.

Version
v1 · 2026-09-28 · History
Domain-specific #
8470
Domain group
Natural Sciences
Origin domain
Chemistry & Materials Science
Subdomains
Physical Organic Chemistry, Aromaticity → Chemistry & Materials Science

Core Idea

Clar's rule is treated here as the recurring formal models and representations identity summarized by this source-grounded definition: In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule to its aromaticity. In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule to its aromaticity. It was introduced in 1972 by the Austrian organic chemist Erich Clar in his book The Aromatic Sextet.

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The Most-Circles Drawing

Some molecules are made of little six-sided rings stuck together, like a honeycomb. Chemists can draw these molecules in several different ways. Clar's rule says the most useful drawing is the one with the most rings that look like a special, extra-steady ring called benzene, and that drawing helps tell how steady the molecule is.

The Best-Drawing Ring Rule

Some molecules, called polycyclic aromatic hydrocarbons, are made of carbon rings joined side by side. Chemists can draw their bonds in several different ways, called resonance structures. Some rings are especially stable, like the famous benzene ring, and in drawings they're shown with a circle inside. Clar's rule says the drawing with the most of these circled rings is the most important one for understanding how stable the molecule is and how it behaves. It's a rule that came from observing molecules, not a strict law.

Maximum Aromatic Sextet Rule

Clar's rule is an empirical rule in organic and physical organic chemistry that links a molecule's chemical stability to its aromaticity. It was introduced in 1972 by the Austrian chemist Erich Clar in his book The Aromatic Sextet. It applies to polycyclic aromatic hydrocarbons, molecules made of fused carbon rings, which can usually be drawn with several different arrangements of double bonds, called Kekulé resonance structures. The rule says the most important structure for describing the molecule's properties is the one with the largest number of aromatic π-sextets, benzene-like rings drawn as circles. In a Clar structure, no ring is drawn with three double bonds, because such rings are shown as circles, and each ring either has a circle, one double bond, two double bonds, or none. Because it's empirical, it's a useful guide rather than an exact law.

 

Clar's rule is an empirical rule of organic and physical organic chemistry relating the chemical stability of a molecule to its aromaticity, introduced in 1972 by the Austrian organic chemist Erich Clar in The Aromatic Sextet. A polycyclic aromatic hydrocarbon generally admits more than one resonance structure, the Kekulé structures. The rule states that the resonance structure most important for characterizing the molecule's properties is the one with the largest number of aromatic π-sextets, that is, benzene-like units. In the resulting Clar structure, sextets are drawn as circles, so no ring is shown with three double bonds, and the number of circles is maximized. Consequently each ring can be in only one of four states: containing one double bond, two double bonds, a circle, or being empty with no double bonds. The rule is a heuristic grounded in observation, and its scope is specifically the link between maximal sextet count and stability, not aromaticity theory in general.

Scope of Application

  • The rule. In general, the chemical structure of a given polycyclic aromatic hydrocarbon allows more than one resonance structure: these are sometimes referred to as Kekulé resonance structures.

  • The migrating π-sextet of anthracene. The anthracene molecule allows three resonance structures, each with a circle in one ring and two sets of double bonds in the other two.

  • The role of angular rings. Phenanthrene can be thought of as a benzene moiety with two fused rings; a third ring can be fused to obtain triphenylene, with three aromatic π-sextets in its Clar structure.

  • Experimental evidence and applications. For instance, Clar's rule can be used to predict several properties of graphene nanoribbons.

  • The resonance structures of phenanthrene. According to the rules expressed above, the phenanthrene molecule allows two different resonance structures: one of them presents a single circle in the center of the molecule, with each of the.

Clarity

A clear use of Clar's rule names the carrier, the operative relation, and the conditions under which the source treats the identity as present. The minimal definition is In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule to its aromaticity.

Manages Complexity

Clar's rule compresses multiple formal models and representations details into a stable diagnostic relation. The source shows both the central mechanism—in general, the chemical structure of a given polycyclic aromatic hydrocarbon allows more than one resonance structure: these are sometimes referred to as Kekulé resonance structures.—and the practical consequence—the anthracene molecule allows three resonance structures, each with a circle in one ring and two sets of double.

Abstract Reasoning

  1. Type the carrier. Identify the formal models and representations entities to which the claim applies.
  2. State the relation. Use the source-grounded identity: In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule to its aromaticity.
  3. Check operation and conditions. An aromatic π-sextet can be represented by a circle, as in the case of the anthracene molecule (below).
  4. Demand recognition evidence. Above, each covalent bond between carbon atoms is represented by one or two segments.

Knowledge Transfer

Within the home domain. Knowledge about Clar's rule transfers literally when a new case preserves the same carrier type, relation, and recognition test. In general, the chemical structure of a given polycyclic aromatic hydrocarbon allows more than one resonance structure: these are sometimes referred to as Kekulé resonance structures. The anthracene molecule allows three resonance structures, each with a circle in one ring and two sets of double bonds in the other two. Beyond the home domain. No canonical parent is asserted for Clar's rule.

Neighborhood in Abstraction Space

Clar's rule sits in a sparse region of the domain-specific corpus (84th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Chemical Structure & Reactivity Concepts (22 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-08