Other examples of aromatic molecules

Cyclopentadienyl anion (C₅H₅^–)

· 6 electrons in p system, aromatic with 4n+2 (n=1) p electrons.

· Delocalization energy =2.47b.

· So predict cyclopentadiene to lose a proton relatively easily.

Experimentally:

pK_a of cyclopentadiene =16 (not very different from water!)

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In contrast, cyclobutadiene (C₄H₄) is extremely difficult to make in the laboratory.

p-electron energy

= 2´(a+2b) +2a

= 4a +4b

Delocalization energy

= 4a +4b – (2´(2a+2b))

= 0

Cyclobutadiene is described as ‘anti-aromatic’

Lecture 3