Chemistry Reference
In-Depth Information
Table 1 Rate constants k
2
for the reaction of acenes with
maleic anhydride
10
6
k
2
/L mol
1
s
1
Acene
Benzene
No reaction
Naphthalene
No reaction
Anthracene
2,270
Tetracene
94,200
Pentacene
1,640,000
Hexacene
6,570,000
Data taken from [
6
]
1
2
3
5
4
6
7
8
9
Fig. 2 The higher acene molecules 6-9 discussed in this review
2 Hexacenes
2.1 Parent Hexacene
The synthesis of this hydrocarbon with the formula C
26
H
16
was first reported by
Clar in 1939 [
12
]. It was obtained by dehydrogenation of 5,16-dihydrohexacene 10
during sublimation over copper powder at 300-320
C (Fig.
3
). Clar described
hexacene as a deep green compound with steel-blue surface reflections that is
only poorly soluble in boiling xylene. This solution quickly loses its color due to
the extreme sensitivity towards air and light. Immediate decoloration was also
observed upon addition of maleic anhydride. Clar also noted that hexacene
decomposes slowly upon heating to 300
C and upon vacuum sublimation at higher
temperatures.
French chemist Charles Marschalk also synthesized hexacene in the late 1930s
and reported properties similar to those described above [
13
]. An interesting
correspondence between Clar and Marschalk is described in Clar's 1939 paper: in
a September 1, 1938 letter by Marschalk to Clar, Marschalk communicates that he
is on the way to the synthesis of heptacene. Clar described having finished and had
already communicated the results of his work on hexacene to the chemical industry
on April 2, 1938. Clar replied to Marschalk on September 11, 1938 that he had
