Fused benzene rings

Members of a class of arenes called polycyclic aromatic hydrocarbons possess subslanlial resonance energies because each is a colleclion of benzene rings fused logelher... 

Firefly lucifenn is an example of an azole that contains a benzene ring fused to the five membered ring Such structures are fairly common Another example is benzimidazole present as a structural unit m vitamin B12 Some compounds related to benzimidazole include purine and its ammo substituted derivative adenine one of the so called het erocychc bases found m DNA and RNA (Chapter 28)... 

Polycyclic aromatic hydrocarbons, of which anthracene is an example contain two or more benzene rings fused together... 

Indole is an aromatic heterocycle that has a benzene ring fused to a pyrrole ring. Draw an orbital picture of indole. 

A benzene ring fused to a 7r-excessive heterocycle is less reactive toward electrophiles than the heteroring, and it frequently reduces the reactivity of the latter, and may modify the orientation of substitution. Where the... 

Fluoran (1) is the commonly used name for the spiro [isobenzofuran- 1,9 -xanthen]-3-one. Benzo[a]fluoran (2) has the benzene ring fused to the 1-and 2-positions of the xanthene moiety. Fusion at the 3- and 4-positions gives benzo[c]fluoran (3). Numbering of the atoms is employed as shown in 1-3. 

Rate constants for reaction of Ca2+aq with macrocycles and with cryptands (281,282,291) reflect the need for conformational changes, considerably more difficult for cryptands than for crown ethers, which may be considerably slower than formation of the first Ca2+-ligand bond. Ca2+aq reacts with crown ethers such as 18-crown-6 with rate constants of the order of 5 x 107M 1 s, with diaza crown ethers more slowly (286,326). The more demanding cryptands complex Ca2+ more slowly than crown ethers (kfslow reaction for cryptands with benzene rings fused to the macrocycle. The dominance of kA over kt in determining stability constants is well illustrated by the cryptates included in Table X. Whereas for formation of the [2,1,1], [2,2,1], and [2,2,2] cryptates kf values increase in order smoothly and gently, the k( sequence Ca[2,l,l]2+ Ca[2,2,l]2+ Ca[2,2,2]2+ determines the very marked preference of Ca2+ for the cryptand [2,2,1] (290). 

A striking demonstration of the reduced activity towards electrophiles for the pyridine ring compared with the benzene ring will be seen later when we consider the fused heterocycles quinoline and isoquinoline (see Section 11.8.1). These contain a benzene ring fused to a pyridine ring electrophilic substitution occurs exclusively in the benzene ring. 

Both purine and pteridine are parent heterocycles for nomenclature purposes. The systematic procedure for naming fused heterocycles is an extension of that we saw in Section 11.8 where we considered a benzene ring fused to a heterocycle. The main difference is that we have to identify bonds in two... 

A series of tranquilizers, drugs that relieve anxiety and nervous tension without impairing consciousness, have a benzene ring fused to a seven-membered ring containing two nitrogens. As a group they are called benzodiazepines. The two most successful are diazepam (Valium ) and chlordiazepoxide (Librium ) introduced in 1964 and 1960, respectively. Flurazepam (Dalmane ), first used in 1970, is a hypnotic. 

Benzannulation reactions of Fischer carbene complexes have proved a useful route for the synthesis of benzene rings fused to one furan ring and one additional heteroaromatic ring (Equation 96) <2005TL2211>. Thus, reaction of chromium carbene complex 168 with the conjugated dienyne systems produces benzofuran derivatives (Table 7). 

Two or more benzene rings fused together form a number of polycyclic benzenoid aromatic compounds, naphthalene, anthracene and phenanthrene, and their derivatives. All these hydrocarbons are obtained from coal tar. Naphthalene is the most abundant (5%) of all constituents of coal tar. 

Indole contains a benzene ring fused with a pyrrole ring at C-2/C-3, and can be described as benzopyrrole. Indole is a ten tt electron aromatic system achieved from the delocalization of the lone pair of electrons on the nitrogen atom. Benzofuran and benzothiaphene are very similar to benzopyrrole (indole), with different hetero-atoms, oxygen and sulphur respectively. 

Porphyrins are cyclic tetrapyrrole-containing systems phthalocya-nines are related systems containing benzene rings fused to the external bond of the pyrrole units (see Fig. 5). 

IUPAC/Chemical Abstracts System (IUPAC Rule B-3). The fusion operations described in Section II, A, 3, a are applied to trivial and/or Hantzsch-Widman names of heterocyclic (and, if present, car-bocyclic) components see examples 25-27. In addition a unit consisting of a benzene ring fused to a one-ring heterocycle can be considered as a single component. If this one-ring heterocycle contains 3-10 members it... 

The prefix benz- in benzimidazole (structure given in text) signifies that a benzene ring is fused to an imidazole ring. By analogy, benzoxazole has a benzene ring fused to oxazole. 

Similarly, benzothiazole has a benzene ring fused to thiazole. 

Many other compounds also exhibit aromatic characteristics. Some of the most common have two or more benzene rings fused together. Such compounds are called polycyclic benzenoid aromatic compounds. A typical example of this type of molecule is naphthalene, C10H8. Structurally, naphthalene looks like this ... 

Polynuclear aromatic hydrocarbons (PAH) are aromatic compounds that contain two or more benzene rings fused together. These substances may be analyzed by HPLC, GC, GC/MS and enzyme immunoassay techniques. The latter is a rapid screening method that may be applied for a qualitative or semiquanti-tative determination Test kits are commercially available for such screening. U.S. EPA (1995) has specified a method (Draft Method 4035) that detects a range of PAHs to different degrees and measures the composite of individual responses to determine the total PAHs in the sample. 

We will take a semiempirical approach using numerous molecules, models, assumptions, and estimates rather than doing new calorimetric experiments and/or quantum chemical calculations. Indeed, we will also test what is probably the simplest assumption - that (4n + 2) n electrons found within a conjugated ring species is expected to result in enhanced stability and that this compound is called aromatic. We will consider the dihydroindene (indane) skeleton composed of a benzene ring fused to a nonaromatic five-membered ring that lacks additional double bonds, and will use this carbocyclic hydrocarbon with X = Y = Z = CH2 as a paradigm for many heterocyclic derivatives for which the possible aromaticity is of relevance to the current chapter. Similarly we use indene with -X-Y- = -CH=CH-, Z = CH2 for a variety of unsaturated heterocycles of interest here. 

Consider the dihydroindene (indane) (XIV) skeleton composed of a benzene ring fused to a nonaromatic five-membered ring that lacks additional double bonds. We may consider this carbocyclic hydrocarbon with X=Y=Z=CH2 (XV) as a paradigm for a... 

Phenothiazines are tranquilizing drugs in wWch the basic structure consists of two benzene rings fused to a central six-membered ring containing a sulfur and a nitrogen. They are sometimes administered as the hydrochloride salt by quaternarization of the side chain nitrogen. 

Quinoline, 1-azanaphthalene and isoquinoline are aromatic nitrogen compounds characterized by a double-ring structure (as shown in Fig. 1) that contains a benzene ring fused to pyridine at two adjacent carbon atoms [1]. These compounds are stable. Quinoline is a high-boihng hquid and isoquinoline is a low-melting solid, each with sweetish odor [2]. 

These compounds consist of two or more benzene rings fused together, and the number of delocalized tt electrons conforms to the HUckel (An + 2) rule for aromaticity. Figure 14.2.2(a) shows the carbon skeletons of some typical examples. 

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