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Carbomer

A carbomer in organic chemistry is an expanded molecule obtained by insertion of a C₂ unit in a given molecule ^[1]. Carbomers differ from their templates in size but not in symmetry when each C–C single bond is replaced by at least one alkyne bond and when a double bond is replaced by an allene bond. The size of the carbomer will continue to increase when more alkyne bonds are introduced and for this reason carbomers are also called carboⁿ-molecules with n the number of acetylene or allene groups in a n expansion unit. This concept, devised by Remi Chauvin in 1995 is aimed at introducing new chemical properties for existing chemical motifs.

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Two representations exist for carbo-benzene (written as an [18]annulene derivative: 1,2,4,5,7,8,I0,11,13,14,16,17-dodecadehydro[18]annulene), one has the aromatic core of benzene expanded and one with the hydrogen substituents expanded. The substituted benzene derivative hexaethynylbenzene is a known compound,^[2] and the core-expanded molecule also exists although with the hydrogen atoms replaced by phenyl groups ^[3]. The final step in its organic synthesis is organic oxidation of the triol with stannous chloride and hydrochloric acid in diethyl ether.

The proton NMR spectrum for this compound shows that the phenyl protons are shifted downfield compared to a proton position in benzene itself (chemical shift position for the ortho proton is 9.49 ppm) suggesting the presence of a diamagnetic ring current and thus aromaticity.

The total carbomer of benzene with core and periphery expanded (C₃₀H₆) only exists in silico ^[4].

Calculations predict a planar D_6h structure with bond lengths similar to the other two carbobenzenes. Its non-planar isomer is called hexaethynyl-carbo-[6]trannulene after a wordplay on the all-cis annulenes and resembling a cyclohexane ring. This hypothetical molecule is more energetic by 65 kcal/mol.

Other carbomer meanings

Carbomer is also a tradename for a synthetic polymer of acrylic acid. Carbomers are used as emulsion stabilizers or thickening agents in cosmetic products. They may be homopolymers of acrylic acid, crosslinked with an allyl ether pentaerythritol, allyl ether of sucrose, or allyl ether of propylene.

References

^ Carbomers. I. A general concept of expanded molecules Remi Chauvin Tetrahedron Letters Volume 36, Issue 3 , 16 January 1995, Pages 397-400 doi:10.1016/0040-4039(94)02275-G
^ Hexaethynylbenzene Rainer Diercks, James C. Armstrong, Roland Boese, K. Peter C. Vollhardt. Hexaethynylbenzene. Angewandte Chemie International. 1986; 25(3):268-269. doi:10.1002/anie.198602681
^ Synthesis of the first 3,6,9,15,18,18-hexa-substituted-1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulenes with D6h-symmetry Yoshiyuki Kuwatani, Naoto Watanabe and Ikuo Ueda Tetrahedron Letters Volume 36, Issue 1 , 2 January 1995, Pages 119-122 doi:10.1016/0040-4039(94)02181-A
^ Total carbo-Mer of Benzene, Its carbo-Trannulene Form, and the Zigzag Nanotube Thereof Christine Lepetit, Chunhai Zou, and Remi Chauvin J. Org. Chem.; 2006; 71(17) pp 6317 - 6324; (Article) doi:10.1021/jo052551j

Categories: Cosmetic chemicals | Polyacrylates | Alkynes

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Carbomer". A list of authors is available in Wikipedia.