Hexatrienes cyclic

We reach a similar conclusion when comparing benzene with the open chain con jugated tnene (Z) 13 5 hexatriene Here we compare two real molecules both conju gated tnenes but one is cyclic and the other is not The heat of hydrogenation of... 

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

The electron delocalizations in the linear and cross-conjugated hexatrienes serve as good models to show cyclic orbital interaction in non-cyclic conjugation (Schemes 2 and 3), to derive the orbital phase continuity conditions (Scheme 4), and to understand the relative stabilities (Scheme 5) [15]. 

There are assnmed to be three n bonds. A, B, and C, in benzene. Here we consider the electron delocalization from A to C. The electron delocalization via B is the same as that in the linear conjngate hexatriene (Schemes 2 and 3) used as a model of non-cyclic conjngate systems. The cyclic orbital interaction has been shown to be favored by the phase continnity (Scheme 5a). There is an additional path for the delocalization in cyclic geometry, which is the direct path from A to C or from a to c. The path gives rise to the cyclic a-b-c and a-b -c interactions. The cyclic orbital interactions satisfy the orbital phase continnity conditions... 

This class of compounds is defined to have some of the three conjugated double bonds found in the ring and others not. This class includes the isomeric 3,3 -bis(cyclohexenylidenes), 100 and 101. Roth shows us that the two isomers have the same enthalpy of formation within ca 1 kJmol-1, a difference somewhat smaller than the 4 kJmol-1 found for the totally acyclic 1,3,5-hexatrienes, 79 and 80 respectively. Naively these two sets of trienes should have the same (E)/(Z) enthalpy difference. Given experimental uncertainties, we will not attempt to explain the difference69. We may compare 100 and 101 with phenylcyclohexane, 102, an isomeric species which also has the same carbon skeleton. There is nearly a 110 kJ mol-1 enthalpy of formation difference between the semicyclic and cyclic trienes. We are not surprised, for the word cyclic is customarily replaced by aromatic when in the context of the previous sentence. 

We naturally exclude here the cyclopropenyl, cyclopentadienyl and cycloheptatrienyl radicals, all of which can also be recognized as cyclic C H species much as we did not include in our discussion the enthalpies of formation of allyl and pentadienyl radical as part of our analysis of polyenes such as butadiene and hexatriene. 

The linear and cyclic hexatrienes are compared in the table below. 

Although a few other acyclic examples of stereospecific isomerisation of hexatrienes are known, specially in the field of natural product like steroid chemistry, the commonest reactions of this type are in cyclic hexatrienes. Cyclooctatriene and cyclooctatetraene are systems in which the electrocyclic reaction goes very readily and they show an interesting trend. 

Even the reaction of diazomethane in the presence of cuprous chloride with cis-hexatriene at — 40° C 3delds this cyclic diene rather than the cyclopropane ... 

The reaction of n-hexene at 773 K and high dilution over H-ZSM5 produced almost exclusively cracked products propene. Under these conditions the formation of aromatics and paraffins were not observed. In contrast over Ga-HZSN-5 the main products were propene and benzene. The very rapid dehydrogenation of n-hexene over Ga-HZSM-5 into hexadiene and hexatriene which could easily form cyclic hydrocarbons by Intramolecular alkylation catalyzed by H will explain the different behaviour of H-ZSM-5 and Ga-HZSM-5 in the reaction of highly diluted n-hexene. These suggestions are consistent in view of the finding that Ga-HZSM-5 shows dehydrogenating properties. 

Diphenylsilene (19a), produced by photolysis of 1,1-diphenyl- or 1,1,2-triphenylsilacyclobutane (17a and 18, respectively equation 11), has been particularly well studied, and absolute rate constants have been reported for a wide variety of silene trapping reactions in various solvents at room temperature (see Table 3)40-46. Not all of these have been accompanied by product studies, unfortunately. A number of other transient silenes have been characterized as well with solution-phase kinetic data for a range of bimolecular silene trapping reactions, though much less extensively than 19a. These include the cyclic l,3,5-(l-sila)hexatriene derivatives 21a-c (formed by photolysis... 

Pericyclic reactions are the ones where the electrons rearrange through a closed loop of interacting orbitals, snch as in the electrocyclization of 1,3,5-hexatriene (88). Lemal pointed ont that a concerted reaction could also take place within a cyclic array, bnt where the orbitals involved do not form a closed loop. Rather, a disconnection occnrs at one or more atoms. At this disconnection, nonbonding and bonding orbitals exchange roles. Such a reaction has been termedpseudopericyclic. 

The easiest way to rationalize the stereospecificity in electrocyclic reactions is by examining the symmetry of the HOMO of the open (non-cyclic) molecule, regardless of whether it is the reactant or the product. For example, the HOMO of hexatriene is 3, in which orbital lobes (terminal) that overlap to make the new a-bond have the same phase (sign of the wave function). Thus, in this case, the new cr-bond between these two terminal orbital lobes can be formed only by the disrotation suprafacial overlap) (Fig. 8.45). If the terminal orbital lobes of HOMO of hexatriene were to close in a conrotatory antarafacial overlap) fashion, an antibonding interaction would result. 

A cyclic conjugated system containing An + 2)ji electrons has an extra stability over that of a comparable number of isolated double bonds. This extra stabilization, known as aromaticity, leads to a characteristic pattern of reactivity which distinguishes the reactions of benzene (1.8) from, for example, the linear hexatriene (1.9) or cyclooctatetraene (1.10) An electrons, n = 2). The aromatic sextet may arise not just from the overlap of three double bonds as in benzene (1.8) or pyridine (1.11) but also from the participation of the lone pair of electrons on a heteroatom. Thus pyrrole (1.12), with effectively six n-electrons, shows some aromatic character. In allene (1.13) the double bonds are at 90° to each other and conjugation does not occur. 

Systematic studies were carried out on copolymers of isobutene with 1,3,5-hexatriene (HTI) and 2,4,6-octatriene (OTI). The structure of the triene units present in the copolymer chains was determined by riKans of C-NMR by making reference to the cationic triene homopolymers and to the same copolymers previously hydrogenated. In the case of HTI there are only traces of repeat units descending from the 1,2- and 1,4-opening of the trienic system, the 1,6-enchainment being prevalent some cyclic structures are also present The cationic homopolymer of 2,4,6-octatriene results from the preval t (ca. S)%) 2,7-opening of the monomer, while the remainder unit results from 2,5-addition. A similar situation also seems to be present in OTI ... 

Under the influence of heat or light, a conjugated polyene can undergo isomerization to form a cyclic compound with a single bond between the terminal carbons of the original conjugated system one double bond disappears, and the remaining double bonds shift their positions. For example, 1,3,5-hexatrienes yield 1,3-cyclohexadienes ... 

Although we cannot as yet converge on a desired enthalpy of formation of gaseous [18]annulene, it is quite apparent that this last number is suspect in terms of at least two acyclic paradigms for aromaticity. Recall the Dewar-Breslow definition for aromaticity and antiaromaticity of an [njannulene in terms of the corresponding acyclic polyene with n/2 double bonds. There is no experimental measurement of the enthalpy of formation of all-( )-l,3,5,7,9,ll,13,15,17-octadecanonaene, species 151. However, we should be surprised if this value seriously differed from that of nine ethylenes and 8(5) kJmol , the 5 kJ mol being taken as the enthalpy of reaction 19 for unstrained olefins and dienes . The enthalpy of formation of 151 is thus ca 513 kJmol . This is somewhat less than the value for [18]annulene and so we would conclude that the cyclic species is essentially nonaromatic . Alternatively, consider the series of acyclic polyenes, ethylene, 1,3-butadiene, 1,3,5-hexatriene, The gas phase enthalpies of formation are respec-... 

The simplest case of an aromatic compound is benzene. Each of the C atoms in benzene is sp2-hybridized, so each has a p orbital pointing perpendicular to the plane of the ring. The six p orbitals make a cyclic array. Each C atom contributes one electron to its p orbital, so there is a total of three pairs of electrons in the system. Because three is odd, benzene is aromatic. In fact, benzene is about 30 kcal/mol lower in energy than 1,3,5-hexatriene, its acyclic analog. 

Diene isocyanates (273) generated by Curtius rearrangement of several dienoic acid azides (272) represent another type of aza -hexatrienes. They are of elusive existence and cyclize smoothly to 2-pyridones (274)l0S). One may suggest an electro-cyclic ring-closure to (275) followed by a 1,5 sigmatropic hydrogen shift to (274) ... 

Far higher yields of the [2-fl] cycloadducts are obtained when 1,1-disubstituted alkenes or cycloalkadienes are used in the trapping reaction (Table 2). Moreover, in the presence of cyclic dienes, such as cyclopen tadiene, a high selectivity for the formation of the entfo-product is observed. Hexatrienes are obtained as a side product arising from dimerization of the ring-opened cyclopropene. The amount of [2-1-2] homo-cyclodimer of the cyclopropene is normally below 5% in the case of different substituents in the 3-position, several isomers are formed. 

Three double bonds in a non-cyclic structure like hexatriene... 

After understanding the usefulness of unsaturated compound, or conjugated system, we hope to explore the unique structure of aromatic compounds, including why benzene should not be called 1,3,5-hexatriene because it is more stable than a typical triene, and seemingly unreactive. Called aromatic initially because of its fragrance, aromaticity now refers to the stability of compounds that are considered aromatic, not only benzene. Any cyclic compound with 4n+2 pi electrons in the system is aromatic. The stability of aromatic compounds arises because all bonding orbitals are filled and low in energy. 

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