Examples Antiaromaticity

1 examples

1.1 cyclobutadiene
1.2 cyclopentadienyl cation
1.3 cyclooctatetraene

examples

while there multitudes of molecules in existence appear antiaromatic on paper, number of molecules antiaromatic in actuality considerably less. compounded fact 1 cannot typically make derivatives of antiaromatic molecules adding more antiaromatic hydrocarbon rings, etc. because molecule typically loses either planar nature or conjugated system of π-electrons , becomes nonaromatic. in section, examples of antiaromatic compounds non-disputable included.

hexadehydro-[12]annulene

pentalene antiaromatic compound has been well-studied both experimentally , computationally decades. dicyclic, planar , has 8 π-electrons, fulfilling iupac definition of antiaromaticity. pentalene’s dianionic , dicationic states aromatic, follow hückel’s 4n +2 π-electron rule.

like relative [12]annulene, hexadehydro[12]annulene antiaromatic. structure of hexadehydro[12]annulene has been studied computationally via ab initio , density functional theory calculations , confirmed antiaromatic.

cyclobutadiene

cyclobutadiene classic textbook example of antiaromatic compound. conventionally understood planar, cyclic, , have 4 π electrons (4n n=1) in conjugated system.

cyclobutadiene

however, has long been questioned if cyclobutadiene genuinely antiaromatic , recent discoveries have suggested may not be. cyclobutadiene particularly destabilized , attributed antiaromaticity. however, cyclobutadiene adopts more double bond character in 2 of parallel bonds others , π electrons not delocalized between 2 double-bond-like bonds, giving rectangular shape opposed regular square. behaving 2 discrete alkenes joined 2 single bonds, , therefore non-aromatic rather antiaromatic.

despite lack of π-antiaromatic destabilization effect, none of 4n π-electron relatives (cyclooctatetraene, etc.) had close destabilization, suggesting there more going on in case of cyclobutadiene. found combination of angle strain, torsional strain, , pauli repulsion leads extreme destabilization experienced in molecule.

this discovery awkward in contradicts basic teachings of antiaromaticity. @ point of time, presumed cyclobutadiene continue used introduce concept of antiaromaticity in textbooks matter of convenience, though classifying antiaromatic technically may not accurate.

cyclopentadienyl cation

the cyclopentadienyl cation textbook example of antiaromatic compound. conventionally understood planar, cyclic, , have 4 π electrons (4n n=1) in conjugated system.

however, has long been questioned if cyclopentadienyl cation genuinely antiaromatic , recent discoveries have suggested may not be. lowest-energy singlet state antiaromatic, lowest-energy triplet state aromatic due baird s rule, , research in 2007 showed triplet state ground state.

cyclooctatetraene

cyclooctateraene in tub conformation

another example of molecule not antiaromatic though might appear cyclooctatetraene. cyclooctatetraene assumes tub (i.e., boat-like) conformation. not planar, though has 4n π-electrons, these electrons not delocalized , conjugated. molecule therefore nonaromatic.