Inorganic chemistry, acarbo-mer (oftencarbo-mer orcarbomer) is an expandedmolecule obtained byinsertion of C2 units into a given molecule.[1]Carbo-mers differ from their templates in size but not in symmetry when each C–C single bond is replaced by analkyne bond C-C≡C-C, each C=C double bond is replaced by anallene bond C=C=C=C, and each C≡C triple bond is replaced by C≡C-C≡C. The size of thecarbo-mer continues to increase when more C2 units are inserted, socarbo-mers are also calledcarbon-molecules, where "n" is the number of acetylene or allene groups in an n-expansion unit. This concept, devised by Rémi Chauvin in 1995, is aimed at introducing new chemical properties for existing chemical motifs.
Two distinct expansions ofbenzene can be calledcarbo-benzene (C18H6):
One (above right) expands each C-H bond to C-C≡C-H, making hexaethynylbenzene, a substituted benzene derivative.[2]
One (above left) expands each C=C and C≡C bond of the benzene core, making 1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulene. An analog of this molecule, with the hydrogen atomsreplaced byphenyl groups, 3,6,9,12,15,18-hexaphenyl-1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulene, is stable.[3] Itsproton NMR spectrum shows that the phenyl protons are shifted downfield compared to a proton position in benzene itself (chemical shift position for theortho proton is 9.49 ppm), suggesting the presence of adiamagnetic ring current and thusaromaticity. The final step in itsorganic synthesis is reaction of the triol withstannous chloride andhydrochloric acid indiethyl ether:
With both core and periphery expanded, thetotalcarbo-mer of benzene (C30H6) only existsin silico (computer simulation).[4]
Calculations predict a planar D6h structure withbond lengths similar to the other two carbobenzenes. Its non-planarisomer is called "hexaethynyl-carbo-[6]trannulene" - a pun on the all-cisannulenes - and resembles acyclohexane ring. This hypothetical molecule is predicted to be more energetic by 65kcal/mol.