Aromatization effects

Because of the aromaticity effects, the electronic ground state of the phenyl cation C6H5+ (263) is singlet A, (67r-electronic structure) [88-... 

S)-Aziridine-2-carboxylic acid (1), also named azyline (Azy)J4L42] exhibits an electron-withdrawing or quasi-aromatic effect on the nitrogen and is therefore susceptible to an attack by nucleophiles. [41 43] Unprotected aziridine-2-carboxylic acid (1) is very labile, but the... 

The Hiickel anti-aromaticity versus Mobius aromaticity effects for the seven-membered systems 1 and 2 have been studied computationally using Gaussian98 at the closed shell B3LYP/6-31G(d) level <20030BC182>. It was shown that Mobius aromaticity was preferred for the respective perfluorinated derivatives 3 and 4. 

A study on how the difference in the aromaticity between (35) and (36) may affect the intrinsic barriers to proton transfer has been reported (Scheme 18).137 The intrinsic barriers for the deprotonation of the thiophene derivative by amines and OH- have been found to be somewhat higher than for the furan analogue. This result has been attributed to a combination of steric, inductive, and n -donor effects which overshadow the aromaticity effect. 

Immobilization or entrapment. To limit contact between certain parts of a system. If some ingredient must be separated, encapsulation of this ingredient and release only upon rupture of the microcapsules fills this objective. The entrapment of a flavor could create a sustained aromatic effect, or to control the release at a specific time (such as during cooking). Immobilization of batteries or enzymes allows continuous processing while avoiding washout. 

Recently, the X-ray analysis of 3,4-bis(methylthio)-l,2,5-thiadiazole 1-oxide demonstrated that the oxidized form of the ring is essentially non-aromatic and shows a pyramidal sulfoxide structure. Interaction between the sulfur lone pair of electrons and the diene is small, the C(3)—C(4) bond length lying closer to that of cyclopentadiene than of thiophene or (3). Theoretical calculations indicate that aromaticity effects lower the inversion barrier nearly equally in the thiophene and thiadiazole 1-oxides by stabilizing the planar transition state and destabilizing the pyramidal structure (82JA1375). 

Separating antiaromaticity effects for the (CH)2EH species from the aromaticity effects in (CH)4EH species is as difficult as it was to decouple strain and antiaromaticity in the analysis above. Again the analysis is complicated by the absence of thermochemical data for the group 15 metalloles. Indeed, the only case known to the authors of experimental heat of formation data for an entire series of such compounds is for the group 16 species furan, thiophene, and selenophene, (CH)40, (CH)4S, and (CH)4Se . 

Recalling our discussion of isodesmic reaction in the previous section, it is clear that Reactions 3.17-3.19 contain energetic consequences for other effects besides aromaticity, including changes in hybridization. In particular, delocalization effects are not conserved. It is important to distinguish delocalization effects from resonance effects from aromatic effects. The first refers to stabilization... 

Introduction of heteroatoms complicates the analysis of charge effects on fragmentation reactions. In addition to the consideration of aromaticity effects discussed above, heteroatoms have profound effects on the stabilities of ions which contain them. There are essentially three cases that merit consideration (1) odd electron ions (2) odd alternant (see footnote p. 98) even electron ions and (3) even alternant (see footnote on p. 98) even electron ions. 

The strong leaving group character of the 9-fluorenylmethyl (Fm) group, which derives from the aromaticity effect of a base-induced proton abstraction to form the dibenzocyclo-pentadienyl anion, is exploited in the 5-(9-fluorenylmethyl) groupf to obtain a cysteine derivative fully stable even to hydrogen fluoride, but cleavable under relatively weak basic conditions such as piperidine (10-50%) or DBU (2%) in DMF (Scheme 13) [154,155] qijjg S-protection is thus compatible with the Boc/Bzl chemistry and fully or-... 

Benzannelation results in an attenuation of aromaticity effects in 4n + 2rc-systems.255 The base-catalyzed hydrogen isotope exchange and isomerization reactions of isomeric dihydrobenzazocines 135 and 136 show these compounds have moderately enhanced kinetic acidity as compared with dihydroquinoline models and is attributable to a small degree of aromatic stabilization in the incipient 1()n electron benzazocinyl anions.255... 

Trimethylsilyl nitrate, (CHj)3Si0N02, is another interesting but little-studied nitrating agent [33]. It is prepared from chlorotrimethylsilane and silver nitrate and nitrates aromatics effectively with BF3 as catalyst... 

Enumeration of ju-electrons present in different aromatic molecules reveals a certain regularity 6 jr-electrons in benzene, 10 jc-electrons in naphthalene and 14 7c-electrons in anthracene. The numbers 6, 10 and 14 all result from the formula 4A +2, where N is integer. For 6 ju-electrons N= 1, for 10 jc-electrons N=2 and for 14 jT-electrons N=3. Compounds whose molecules possess 4A +2 delocalized electrons afford greater stability than corresponding conjugated alkenes with planar chains. This additional stability is called the aromatic effect. 

Increasing delocalization, as in the cyclohexadienyl cation, is further stabilizing. This structure is simply protonated benzene, and it serves as a model for the intermediate in electrophilic aromatic substitution (see Section 10.18). Similarly, benzyl ion is quite stable for a formally 1° ion. Aromaticity effects are clear, as in the much greater stability of the six it electron tropylium ion vs. the four tt electron cyclopentadienyl ion (see Section 2.4.1 for a discussion of aromaticity). 

The preceding sections on sensory characters of organics cover only the major organic groupings widely encountered in flavor work and is by no means exhaustive. Discussing each compound class in detail would require a book in itself. It is intended merely to draw attention to the enormous range of aromatic effects that can be derived from synthetic chemicals and in no way is a substitute for practical exposure to the odor and flavor characteristics of the chemicals permitted for use in foods. 

Personal records and practical experience of the odor and flavor profiles of permitted flavorants that are necessary to enable reported data to be interpreted in terms of practical aromatic effects. 

Zhu J, Fogarty HA, Mollerstedt H, Brink M, Ottosson H (2013) Aromaticity effects on the profiles of the lowest triplet-state potential-energy surfaces for rotation about the C=C bonds of olefins with five-membered ring substituents an example of the Impact of Baird s rule. Chem Eur J 19 10698-10707... 

Aromatics Effect Decreasing aromatics content from 45 to 20% reduced exhaust hydrocarbon by 6% for current vehicles, but led to about 10% increase in hydrocarbon emissions... 

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