aromatic chemistry - conventions Flashcards
(15 cards)
where does the name aromatic come from?
many of these compounds smelled nice
what are the 3 limitations of kekules structure?
1- C-C and C=C lengths are different but all C-C in benzene are equal
2- benzene has a less exothermic enthalpy of hydrogenation that expected based on cyclohexene
3- benzene is less reactive that typical alkenes
what model explains all these limitations of kekules model?
the delocalised model - explains why benzene is so stable
aromatic definition
stabilised by a π ring system
what are huckels rules?
a compound must conform to these rules to be aromatic:
- must be cyclic
- must be planar
- must be fully conjugated
- must have 4n+2 π electrons
what is a mobius system?
a system which has 4n π electrons, and is therefore antiaromatic
- they are more unstable/higher in energy, opposite to aromatic compounds
how does molecular orbital theory explain the 4n+2 rule?
if π electrons are paired + in bonding/non-bonding orbitals then the molecule is aromatic (4n+2) but if π electrons are unpaired in non-bonding/antibonding orbitals then the molecule is antiaromatic
outline molecular orbital theory of benzene
the 6 carbon p-orbitals in benzene combine to give 6π molecular orbitals
there are various conformations of the orbitals:
- all 6 in-phase, this is the most stable conformation
- 1 node, running halfway through the molecule any way, forming 2 degenerate MOs
- 2 nodes, perpendicular to each other, all 3 MOs are still lower in energy than all p orbitals
- 3 nodes, separating every p orbital, the resulting MOs are higher in energy than the contributing atomic orbitals, this is the least stable arrangement, all orbitals are out-of-phase
what is the purpose of frost circles + how are they made?
used to determine aromaticity via π MOs for monocyclic, planar, conjugated hydrocarbons
- these are constructed by drawing the shape of the molecule with its apex facing down, then draw a line horizontally across the shape, halfway - each vertex is an MO, if its below the halfway line, it is bonding, if directly on the line, it is non-bonding, and if above the line, it is anti-bonding
- fill up orbitals with the correct number of p-electrons - if there are any electrons above or on the line, or if the electrons are not 4n+2 (same thing) then the molecule is not aromatic
what is a polycyclic aromatic hydrocarbon?
2D or 3D structures/molecules given when benzene rings are fused to other benzene rings
how are polycyclic aromatic hydrocarbons different to typical aromatic rings?
they do not follow huckels rules
in these structures, only one ring can be aromatic at a given time - this is explained by clar’s sextet theory
outline how clar’s sextet theory explains the degree of aromaticity in polycyclic aromatic hydrocarbons
- the resonance structure with the largest number of disjoint aromatic π-sextets is the most important (remember only 1 aromatic ring of 2 adjacent aromatic rings can be aromatic at a time, but 2 rings can be aromatic at the same time if separated by another ring)
- polycyclic aromatic hydrocarbons with more π sextets in a single resonance structure are more stable than those with less
- rings which have π sextets in more resonance structures of a compound are more stable than others
- compounds with resonance structures that have all π sextets and no double bonds are fully benzenoid, and have similar stabilities to that of benzene
which rule of clars sextet theory is most important for determining reactivity?
‘rings which have more π sextets in more resonance structures of a single compound are more stable’
acene definition
an aromatic functional group (polycyclic aromatic hydrocarbons) with a very linear structure, this means only 1 π sextet can exist in a resonance form at any one time
- this suggests that larger acenes are more unstable as the stable π sextet has to be shared over more rings
phenacene definition
an aromatic functional group (polycyclic aromatic hydrocarbons) with a zigzaging structure, this means that multiple π sextets can exist at one, therefore they are musch more stable than acenes - as they often have an isolated C=C bond in its most stable resonance form, this can act as an alkene
