Fundamentals 4: Aromaticity Flashcards
what is hydrogenation?
addition of H to molecules, presence of a catalyst and high heat.
Benzene:
highly stable, aromatic compound,
stable due to aromaticity, which arises from delocalisation of π electrons around the ring.
results in lower energy state, increased stability, thus less reactive.
more resistant to hydrogenation compared to alkenes because it would disrupt the aromaticity of the ring.
what is aromatic stabilisation energy?
measure of extra stability that arises from delocalisation of π electrons in an aromatic ring.
ASE is the difference in energy between actual aromatic compound and hypothetical structure with localised π electrons.
what are Huckels rules?
used to define aromaticity.
it states a continuous system of π orbitals will be aromatic if the molecule :
-is planar
-is monocyclic
-contains (4n + 2)π electrons.
what is anti-aromatic and non-aromatic?
anti = only (4n)π electrons.
non = not a cyclic molecule or has other numbers of π electrons.
what is antiaromaticity?
refers to a situation where a cyclic compound with a certain number of pi electrons is actually less stable that it would be if it were not cyclic.
must be cyclic and planar.
continuous ring of pi electrons, not interrupted by sp3.
exactly 4n π electrons.
highly reactive and unstable.
tell me about H NMR of aromatic systems:
the signals in H are additionally deshielded by the induced magnetic field from the circulation of π electrons.
in essence:
circulation of π electrons gives rise to ring currents.
induced magnetic field due to ring current.
proton deshielding due to induced magnetic field.
what role does a negative charge play in Huckels rules and determining whether the molecule is aromatic or not?
negative charge acts as extra π electron.
what are heterocyclic aromatic compounds?
ring compounds typically with N, O or S.
what is unique about N in heterocyclic compounds?
N optimises its hybridisation state to afford lowest energy state for the molecule.
pyridine the LP is in sp², not delocalised around the ring.
pyrrole the LP is in unhybridised p orbital, delocalised around the ring.
what is unique about oxygen containing 5-membered heterocycles?
one lone pair will sit above the ring in order to contribute to the π electron count towards aromaticity.
furan is eg.
1 LP in p orbital delocalised around ring.
1 LP in sp² perpendicular to the ring.
