Structure, condensed Kekule

Straight-chain alkane, 80 Strecker synthesis, 972 Structure, condensed, 22 electron-dot, 9 Kekule, 9 Lewis, 9 line-bond, 9 skeletal, 23... 

Kekule structure org chem A molecular structure of a cyclic conjugated system that is depicted with alternating single and double bonds. ka-k3,la. strak-char j ketal oRG CHEM 1. Former term for the =CO group, as in dimethyl ketal (acetone). 2. Any of the ketone acetates from condensation of alkyl orthoformates with ketones in the presence of alcohols. ke,tal ... 

Benzene was discovered in 1825 by Michael Faraday, who isolated it from the liquid that condensed from the gas that was burned in the street lamps of London. Although Faraday was able to deduce that the formula of benzene is C6Hb, it was not until 1866 that the correct structure was proposed by Kekule (see the Focus On box in Chapter 12 on page 469). 

Such posets can also be obtained by describing condensed aromatic compounds in terms of their Kekul structures (see Figure 3.8). 

Balaban, A.T. and Tomescu, 1. (1984) Chemical graphs. XL. Three relations between the Fibonacci sequence and the numbers of Kekule structures for non-branched cata-condensed polycyclic aromatic hydrocarbons. Croat. Chem. Acta, 57, 391-404. 

Fig. 8.17 The first three rows display some of the Kekule structures of one and the same benzenoid perifusene one and the same algebraic Kekule structure on the left corresponds to the pair of geometric Kekule structures on the right. The fourth row shows that a similar degeneracy occurs even when the moieties condensed to the left- and right-hand bonds of pyrene are different. The bottom line shows the generalized feature of degeneracy that holds even for nonaltemants...

We first consider the numeric Kekule structures of cata-condensed benzenoids. As an example, we consider dibenzo[b, ]phenanthrene. In Fig. 9.14 we give geometric Kekule stmctures of dibenzo[b,g]phenanthrene and in Fig. 9.15 their numeric counterparts. Kekule structures of dibenzo[b,g]phenanthrene can be generated in... 

From the numbers coding the numeric Kekule structures of cata-condensed benzenoids, a given Kekule structure can be easily recovered because the number of digits in the code determines the number of hexagons in the cata-condensed benzenoid, and the sum of digits gives the total rc-electron count in the benzenoid. 

It should be noted that the numeric code is a linear code in which the codes for peri-condensed benzenoids are built in the same way as the Wiswesser codes. For example, the numeric code for the Kekule structure VI (see Fig. 9.17) is 45335. 

However, it appears that there exist peri-condensed benzenoids with distinct geometric Kekule structures but with the identical numeric Kekule structures (Gutman et al. 2004 Vukicevic et al. 2004). The smallest example of such a case is pyrene. In Fig. 9.18, we give its six geometric Kekule stmctures and in Fig. 9.19 its six numeric Kekule structures. Note, stmctures denoted by B and C possess identical numeric codes. Therefore, there is no one-to-one correspondence between geometric and numeric Kekule stmctures for some peri-condensed benzenoids. In such cases the recovery of geometric stmctures requires the amendment of the numeric codes. 

In Fig. 9.21, we show how a pair of peri-condensed benzenoid with identical numeric codes can be recovered from their perimeter codes. As an illustrative example, we selected numeric code 5 5 2 2 5 5 which is identified to belong to a pair of Kekule structures of dibenzo[, op]naphthacene. 

Delete an (arbitrary) angularly annelated hexagon, say A, and denote the number of Kekule structures of the resulting (cata-condensed) chain by k. 

Bergan JL, Cyvin BN, Cyvin SJ (1987) The Fibonacci Numbers, and Kekule Structures of Some Corona-Condensed Benzenoids (Corannulenes). Acta Clum Hung 124 299... 

Bergan JL, Cyvin SJ, Cyvin BN (1986) Number of Kekule Structures of Single-Chain Corona—Condensed Benzenoids (Cycloarenes). Chem Phys Letters 125 218... 

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