Problem Set 3 – Aromatic
Systems
- Which
of the following structures represent aromatic systems?
- Sondheimer
synthesized a series of conjugated cyclic polyalkenes which are known as
[n]-annulenes where n indicates the size of the ring.
a.
[18]-annulene, shown below, is
aromatic. Explain.
[18]-annulene is cyclic and
planar with 18 p electrons delocalized over
the whole ring. 18 is a
Hϋckel number.
b.
[14]-annulene is not aromatic. It reacts with bromine at ambiant
temperature. Explain.
[14]-annulene is cyclic and has 14 p electrons delocalised over the ring which satisfies
the Hϋckel rule. However the ring is
not planar due to the crowding of the hydrogens at the centre of the ring.
- Azulene
is readily attacked by electrophiles at C-1 and by nucleophiles at C-4.
a.
Azulene,
is aromatic. Justify.
The molecule
is planar, cyclic with 10 p
electrons delocalized.
b.
One resonance structure of the cation
formed by attack at carbon-1 is shown below.
Justify electrophilic attack at this carbon.
The seven-membered ring is aromatic.
c.
One resonance structure of the anion
formed by attack at carbon-4 is shown below.
Justify nucleophilic attack at this carbon.
The five-membered ring is aromatic.