Comportamiento en pastas

Hydrocarbons are compounds which contain carbon and hydrogen atoms which can

be generalised into acyclic (such as ethane and propylene) and cyclic molecules

(such as cyclohexane and benzene). An important group of cyclic hydrocarbons are

polyaromatic hydrocarbons (PAHs). These compounds are structurally arranged with

two or more fused aromatics, often benzene rings.1The Kekulé structure of a PAH is

a structural formula based on August Kekulé’s 1865 model of benzene, where every

π electron pair in a cyclic π system can form a π bond.2, 3 _{Archibold Couper and}

Josef Loschmidt proposed a near-correct model of the structure of benzene in 1861

containing multiple double bonds and a cyclic structure.4-6 Loschmidt, who

suggested the structures of a variety of compounds including triazene, benzidine and

benzene, had made a significant step in the true representation of benzene as a large

cyclic ring (Figure 1) but did not rationalise his structures.5-8 Nevertheless, Kekulé

had explained aromaticity in relation to the structure and included the alternating

double bonds therefore rationalising the support for his theory of benzene fully.3, 7, 9

Hückel’s ‘4n+2’ rule can be applied to systems such as benzene as they are

monocyclic, planar and the atoms all participate in delocalisation. This rule predicts

whether a monocyclic fully-conjugated planar hydrocarbon is aromatic and therefore

stable, or not. However, there is a limitation to this rule in PAHs, as it only works

with molecules which possess one ring. The stability of three or more ring PAHs

cannot necessarily be predicted using this rule. This is due to a third ring being

formed by either a one bond or two bond fusion, where a one bond fusion (cata-

fused) obeys Hückel’s rule and a two bond fusion (peri-fused) does not, as this forms

a non-Kekulé structure (Figure 2). Naphthalene 4 is the only two ring PAH formed

of benzenes, coincidentally it is a cata-fused compound, therefore it seems to obey

Hückel’s rule. In the case of pyrene8a 16-electron compound Hückel’s theory does

not work either as this is produced by either a two bond or three bond fusion (bay-

fused) and so these rules should only be applied to monocyclic systems.10, 11

Figure 2: Showing that the ‘4n+2’ rule does not consistently apply to polyaromatic hydrocarbons. Where a ‘tick’ signifies that it obeys Huckel’s rule and a ‘cross’ does not.

There are a number of ways to analyse compounds to see if it is a Kekulé structure or

not; one method is drawing the double bonds into the system one at a time, which

can be a laborious process, another similar method shown by Claret al.(Figure 3) is

to label each alternate carbon and compare the difference, and if there is a difference

then it is a non-Kekulé structure.11, 12For example, in Figure 3 anthanthrene9has 11

of each marked and non-marked carbons therefore the difference is zero, thus it is a

Kekulé structure. However, triangulene10has a difference of two, therefore this is a

non-Kekulé structure and in this instance a diradical. A non-Kekulé molecule may be

defined as one that contains enough atoms but not enough bonds to satisfy the

standard rules of valence.13

Figure 3: Clar's method for determining Kekulé and non-Kekulé structures.12

Non-Kekulé molecules are of particular interest due to their unique photochemical

One of the first non-Kekulé triplet ground state structures to be synthesised was in

1915 by Schlenk and Brauns.16-18 _{The work of O. Starket al.laid the foundation for}

the formation of the Schlenk-Braun hydrocarbon 11 as Stark and his co-worker’s

attempts to form the exocyclic tetraphenyl substituted m-xylylene were

unsuccessful.19-23 The two tertiary carbon sites help stabilise the radical but upon

exposure to air the compound formed various peroxide products within a day.19

Figure 4: The Schlenk-Brauns hydrocarbon 11.

Due to the potential worth of these non-Kekulé molecules, certain polybenzenoids

have been of particular interest. Polybenzenoids can be described as fused six-

membered rings and there are a number of polyaromatic hydrocarbons which have

been identified.2In particular, one benzenoid PAH, triangulene10(Figure 3), which

is the simplest even-carbon-numbered non-Kekulé poly-benzenoid hydrocarbon.

This compound has been of interest because the analysis of its instability and

observable spin states could be potentially used in electronic devices. However, like

most non-Kekulé PAHs it is very reactive and has not been isolated, as reported from

the early work of Claret al.24, 25

Even though the dihydrides of dihydro-triangulene 12 are readily oxidised, the

dianion of triangulene 13 has been synthesised by O. Hara et al. (Scheme 1).26-28

Nevertheless, the diradical form 10 has yet to be isolated. It is suggested that the

neutral diradical is more reactive then a single radical and spontaneously

Scheme 1: Illustrates the simplest even-carbon-numbered non-Kekulé benzenoid structure of dihydro-triangulene 12 and the dianion 13.

Unlike the Schlenk-Brauns hydrocarbon11, where the spin density is the highest on

the central carbons, the spin density of triangulene 10 is highest around the outside

of the molecule (Figure 5).29 Taking this in to account as well as the proposal that

triangulene is flat, a poly-benzenoid and possesses a lower number of isolated

benzene rings (see section 1.3: Clar sextets), the radicals are less stable.

Figure 5: Shows the calculated spin density of triangulene with the large circles indicating greater values as reported by Inoueet al.29