C6 Aromaticity Flashcards
describe the 6 carbon-carbon bonds in benzene
- experimental evidence shows all bond lengths are the same
- intermediate length between single and double bond length
- all bonds are partial double bonds
why is the Lewis structure not sufficient to explain benzene’s bonding?
all bond lengths are partial double bonds and have a length between single and double
what is aromaticity?
- an observation that som unsaturated cyclic molecules exhibit unusual stability
- certain cyclic compounds having different properties than we would expect from naive theory (structure, reactivity, energy levels)
explain reactivity in benzene
- doesn’t react with many electrophiles that add to double bonds in alkenes (eg. halogenation with bromine)
- benzene undergoes substitution with electrophiles not addition (eg. halogenation with bromine using an electron carrier (eg. FeBr3))
what kind of reactions are hydrogenations?
- exothermic
why does a reaction involving the hydrogenation of 2 double bonds not have an exactly double enthalpy to one involving the hydrogenation of 1 double bond?
- adding a double bond adds stability by extending conjugated chain so hydrogenation is less exothermic
- the system is larger (as it is cyclic) and more stable so releases less energy upon hydrogenation
state 4 key properties of aromatic molecules (not the rules)
- less reactive than expected
- react with substitution, not addition
- have lower than expected enthalpy of hydrogenation
- have bond lengths intermediate between double and single
what is the orbital description of benzene?
- carbon atoms in benzene are sp2 hybridised
- the hexagonal sigma-bonded framework is formed of carbon sp2 orbitals
- unhybridised 2p orbitals are arranged perpendicular to the plane
what are the rules for if a molecule is aromatic or not?
- cyclic
- planar
- completely conjugated
- 4n+2 electrons where n is an integer
what is meant by non-aromatic?
if any of the rules below are not satisfied, a molecule is non-aromatic
cyclic
planar
completely conjugated
what is meant by anti-aromatic?
if all the below rules are satisfied…
cyclic
planar
completely conjugated
… but the molecule has 4n electrons instead of 4n+2
- very high in energy
- energy increase makes anti-aromatic molecules do strange things
describe the anti-aromaticity of cyclooctatetraene
- 8 pi electrons make cyclooctatetraene anti-aromatic
- ANTI-aromaticity destabilises the molecule and forces it to adopt a non-planar geometry (forces itself out of planar geometry to be NON-aromatic (lower energy)
how can we encourage aromaticity in cyclooctatetraene?
- treat it with a powerful oxidising agent (antimony fluoride)
- causes loss of 2 electrons so there is no 4n+2 electrons
- it is now aromatic and lower energy
what is the hybridisation state of the nitrogen in pyridine?
sp2
what is the hybridisation state of nitrogen in pyrrole?
‘sp3’
- it looks sp3 hybridised but it is actually sp2 hybridised
- lone pair is in the unhybridised p orbital so there are 4n+2 electrons (allows for aromaticity)
