Benzenoids rules

Two-Dimensional Representation of Chemical Structures. The lUPAC standardization of organic nomenclature allows automatic translation of a chemical s name into its chemical stmcture, or, conversely, the naming of a compound based on its stmcture. The chemical formula for a compound can be translated into its stmcture once a set of semantic rules for representation are estabUshed (26). The semantic rules and their appHcation have been described (27,28). The inverse problem, generating correct names from chemical stmctures, has been addressed (28) and explored for the specific case of naming condensed benzenoid hydrocarbons (29,30). 

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

Rule A-56. Radicals for Non-benzenoid Ring Systems (Alternative in part to Rule A-55.1)... 

Rule 1. Whenever the excised internal structure has an eigenvalue of zero, then the nonbisanthrene-like strictly peri-condensed benzenoid structure also has an eigenvalue of zero. 

Rule 2. Whenever a benzenoid structure can have a succession of edges bisected with a straight line drawn from one side of the molecule to the other with the terminal rings being symmetrically convex relative to the line, then those rings intersected by the line can be embedded with a perpendicular succession of ethene substructures and the benzenoid structure as a whole will have at least one eigenvalue pair of plus and minus one. This straight line will be called a selective lineation. 

In the theory known nowadays as the Clar theory of the aromatic sextet [12] a benzenoid system is represented by a Clar structure which is obtained by drawing circles in some of the hexagons of the corresponding benzenoid graph. These circles represent the aromatic sextets in the hydrocarbon. We consider here only Clar structures containing the maximum number of circles which are some times referred to as proper (or correct) Clar formulas. The rules for constructing such Clar structures are as follows [13]. 

For fully benzenoid systems the following two rules were formulated [39]. 

Rule 1. The effect of empty rings on the total ir-electron energy of fully benzenoid hydrocarbons is nearly the same. The corresponding ef-values vary in the narrow interval (0.02, 0.03). 

Rules 1 and 2 are special cases of some more general regularities which seem to hold for all benzenoid hydrocarbons. 

The effect of the biphenyl rule is small and is usually screened by much stronger conjugation modes (e.g. those taken into account by resonance, conjug-ated-circuit and/or Clar-aromatic-sextet theories [64]), In some exceptional cases, however, the biphenyl rule can completely invert the conjugation pattern anticipated by the classical theories. This particularly occurs in benzenoid... 

The fundamental aufbau principle is illustrated in Fig. 4. Here the aufbau units which come into operation are exactly the elementary aufbau units. The corresponding attachments of (i) C4H2, (ii) C3H and (iii) C2 are illustrated in Fig. 5 and pertain to the first example in each row (i)—(iii) of Fig. 4. But notice that the three types (a)-(c) specified in the Principles above correspond to the additions (ii), (i) and (iii) in that order. The formulation was chosen to be that way for several reasons, mainly because the type (a) is often sufficient for the aufbau, and because of our concern about extreme-left benzenoids [8] (see below). The last paragraph of the Principles gives rise to some general rules as specified in the following. 

Equivalence" is also used as a synonym for "equality with respect to"- This phrase implies that two such equivalent objects are not equal in 1 rejects, but equal with respect to a property of interest-For example, the well known aromaticity 4n+2 rule classifies even-polycyclic conjugated benzenoids into two classes- Under such a relation molecules such as naphthalene and anthracene are considered equivalent-... 

C.d. data recorded for a series of 5a-cholestane-3/3,6a-diol di-(4-chloro-benzoates) with additional substituents at C-4 have indicated that the diterpenoids caryoptin, 3-epicaryoptin, and clerodin have the same absolute configuration. C.d. data are reported for four isomeric steroidal 3-spiro-isoxazolidine[2,3- i]oxadiazolines, together with X-ray data confirming the structure (26) for one of them. These compounds result from 1,3-dipolar cycloadditions between benzonitrile oxide and the 3-methylene-steroid. An interpretation of the c.d. data at ca. 250 nm attributes the Cotton effect to coupled benzenoid transitions. A new sector rule is proposed for the c.d. of 2,2-dialkyl-l,3-oxathiolans (ketone hemithioacetals), which are essentially of thio-ether type with c.d. bands in the regions of 240 and 220 nm. 

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