Factsheet

• Word Type: Noun
• Pronunciation: /ˈan.jəˌliːn/ or /ˈænjəˌliːn/
• Field: Chemistry
• First Known Use: 1962
• General Formula: ( C_nH_n ) (even ( n )) or ( C_nH_{n+1} ) (odd ( n ))

Etymology

The term “annulene” is derived from the Latin word annulus, meaning “ring,” combined with the suffix -ene, which indicates an unsaturated hydrocarbon. The name was proposed in 1962 by chemists F. Sondheimer and R. Wolovsky in their research on fully conjugated monocyclic hydrocarbons.

Meaning & Use

In chemistry, annulene refers to:

1. Definition:
   Any monocyclic hydrocarbon that is fully conjugated. This means the molecule consists of a single closed loop (ring) of carbon atoms with alternating single and double bonds, allowing for maximum electron delocalization.
2. Key Features:

• Annulenes are described as fully conjugated systems, meaning every carbon atom in the ring participates in the conjugation.
• These compounds can be represented by general formulas:
  • ( C_nH_n ) (if ( n ) is even)
  • ( C_nH_{n+1} ) (if ( n ) is odd)
• Common examples include benzene ([6]annulene) and cyclooctatetraene ([8]annulene).

Examples

• [6]Annulene: Benzene ((C_6H_6))
  The smallest and most well-known annulene, with six carbon atoms forming a planar hexagonal ring.
• [8]Annulene: Cyclooctatetraene ((C_8H_8))
  This annulene adopts a tub-shaped conformation to minimize angle strain.
• [10]Annulene: A larger annulene that exhibits planar or distorted geometries depending on its specific structure.

Quotations in Context

1. 1962:
   “The systematic naming of these substances is lengthy and clumsy. We therefore propose the trivial name ‘annulene’ (annulus = ring, Latin) for a monocyclic fully conjugated polyene.”
   — F. Sondheimer & R. Wolovsky, Journal of American Chemical Society
2. 1974:
   “The cyclizations occur at the termini of the trans, cis, trans-hexatriene units of the [16]-annulene structure.”
   — G. B. Gill & M. R. Willis, Pericyclic Reactions
3. 2006:
   “The general name annulene is sometimes given to rings that contain alternating single and double bonds in a single Lewis structure.”
   — J. M. Hornback, Organic Chemistry

Pronunciation

Annulene is pronounced as:

• /ˈan.jəˌliːn/
• /ˈænjəˌliːn/

The stress is placed on the first syllable, and the second syllable contains a long “ee” sound.

Frequency

The use of “annulene” is primarily confined to academic and professional contexts in chemistry, particularly organic chemistry and molecular structure studies.

Synonyms

• Conjugated monocyclic hydrocarbon
• Fully conjugated ring compound
• Polyene ring

Conclusion

Annulene is a technical term used in chemistry to describe a class of monocyclic hydrocarbons that exhibit full conjugation. With its roots in Latin, the term aptly captures the essence of its structure—rings of carbon atoms with alternating bonds. Annulenes are vital in understanding the electronic properties and reactivity of aromatic and anti-aromatic compounds, making them a cornerstone of organic chemistry.