Benzene And Aromatic Compounds Pdf

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The quantitative description of aromaticity is based on energetic, geometric e. The substituent effect can be described using either traditional Hammett-type substituent constants or characteristics based on quantum-chemistry. For this purpose, the energies of properly designed homodesmotic reactions and electron density distribution are used.

Aromatic compounds, originally named because of their fragrant properties, are unsaturated hydrocarbon ring structures that exhibit special properties, including unusual stability, due to their aromaticity. They are often represented as resonance structures containing single and double bonds.

Aromatic Substitution Reactions. The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier section. The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram some comparable reactions of cyclohexene are shown in the green box.

15.3: Structure and Stability of Benzene

The quantitative description of aromaticity is based on energetic, geometric e. The substituent effect can be described using either traditional Hammett-type substituent constants or characteristics based on quantum-chemistry. For this purpose, the energies of properly designed homodesmotic reactions and electron density distribution are used.

In the first case, a descriptor named SESE energy stabilizing the substituent effect is obtained, while in the second case cSAR charge of the substituent active region , which is the sum of the charge of the ipso carbon atom and the charge of the substituent. Intramolecular charge transfer a resonance effect is privileged in cases where the number of bonds between the electron-attracting and electron-donating atoms is even.

For larger systems, a long-distance substituent effect has been found. Aromaticity and substituent effects are among the most important and useful terms in organic chemistry and related fields. Both aromaticity and substituent effect concepts are an old story, but still alive, fascinating, and inspiring. For the first time, the chemical idea of aromaticity appeared as a structural concept: Kekule addressed the term to compounds containing the benzene ring [ 2 ].

A year later, Erlenmayer [ 3 ] named as aromatic the compounds having similar properties as benzene derivatives. The most important aspects of the development of the concept of aromaticity are presented in Table 2 of the review paper by Schleyer and coworkers [ 4 ].

There has been some kind of dichotomy since then: how to understand the aromatic character, using a chemical structure or chemical properties? To date, most of the works on aromaticity have been devoted to relationships between the structure and the properties of so-called aromatic compounds. The RE concept has undergone many modifications concerning both the models of reference molecules and the level of theory used to estimate energies.

RE values can be estimated from experimental thermochemical data [ 9 , 10 ] or by the use of quantum chemistry computation. Dewar et al. Schematically, the idea of KK index is presented in Fig. Reprinted adapted from Tetrahedron Lett [ 20 ]. Copyright , with permission from Elsevier. The higher the KK value, i. This definition of aromaticity is evidently related to the old chemical issue that aromatic molecules prefer a substitution reaction rather than an addition reaction [ 21 ].

A similar graph but only for annulenes was presented earlier by Dewar [ 23 ] and Figeys [ 24 ], for review and generalization see [ 25 ]. Reprinted adapted from Tetrahedron Lett [ 22 ]. The first quantitative characteristic of aromaticity based on molecular geometry was introduced by Julg and Francoise [ 26 ]. It was defined as a function of the normalized variance of the perimeter bond lengths.

Next year, the bond lengths were replaced by the HMO bond orders and the difference between the mean bond order and the bond orders of all bonds of a molecule, taken in modulo and normalized, gave a numerical descriptor of aromaticity [ 27 ].

In the next step, the average value of bond lengths was replaced by an empiric concept of optimal bond length [ 28 , 29 ]. Then, differences in the length of bonds, d i , in a given molecule from the optimal bond length, d opt , were used as the basis for estimating aromaticity index named HOMA Harmonic Oscillator Model of Aromaticity :. The approach has been modified many times [ 34 , 35 , 36 ] but the basic idea has not been changed. A few years later, Bird introduced aromaticity index I 6 for six-membered rings [ 37 ] and I 5 for five-membered ones [ 38 ] using the bond orders calculated directly from bond lengths via the formula suggested by Gordy [ 39 ].

Some help in understanding the aromatic character can come from the harmonic oscillator stabilization energy HOSE [ 40 , 41 ]. This approach is related to the well-known way in organic chemistry of presenting the chemical properties of molecules using their resonance structures [ 42 ]. Among the many applications of the HOSE model, two of them show their advantages. The obtained HOSE values [ 41 ] were found to be in a good correlation with the RE values obtained by Hess and Schaad for alternant hydrocarbons [ 15 ] and non-alternant species [ 17 ], with correlation coefficients, cc , 0.

A detailed overview of the geometry-based aromaticity indices can be seen in the review [ 45 ]. One of the first descriptors of this type is the diamagnetic susceptibility exaltation. It is important to mention that magnetic susceptibility is a property of a whole molecule and can be obtained both experimentally and by quantum chemistry computations. Some kind of revolution was introduction in by Schleyer of the concept of nucleus independent chemical shift NICS [ 49 ].

It was defined as the negative value of the absolute shielding calculated in the geometric center of the ring system. Now it is also calculated at other points inside [ 50 ] or around molecules [ 4 ]. It should be mentioned that all NICS values describe only local aromaticity, i.

In recent decades, there have been characteristics of aromaticity based clearly on electron structure and electron delocalization. For this purpose, electron structure descriptors based on AIM theory [ 52 , 53 , 54 ] were used: charges, Laplacian, energy, and its components such as kinetic and potential energies, estimated at the bond or ring critical points [ 55 , 56 ]. Other approaches based on the electronic structure are associated with characteristics of electron delocalization.

In , Sola et al. It has been documented that above-mentioned indices are well correlated with HOMA and NICS for benzenoids as well as non-benzenoid hydrocarbons, and even nitrogen analogs and some unsaturated cyclic systems. For review see [ 60 ]. In front of so many possible criteria of aromaticity an important question arises: how far so different approaches lead to equivalent conclusions? This problem was the subject of many papers [ 61 , 62 , 63 , 64 , 65 ]. In general, the overall trend is broadly met and there are correlations between the aromaticity indexes, but in many specific situations, they may lead to inconsistent results.

However, the use of any of the well-accepted aromaticity descriptors for structurally similar molecular systems should lead to reliable conclusions [ 67 ].

This condition is met for substituted derivatives of a given molecule. Recently, an approach based on quantum chemistry has been introduced — named as the electron density of delocalized bonds, EDDB [ 69 ], and successfully applied as an aromaticity criterion [ 70 ] as well as used for description of aromaticity of acenes [ 71 ]. The EDDB method revealed that the local aromaticity of a particular ring in a polycyclic benzenoid hydrocarbon may be significantly affected by long-range exchange corrections in the description of electron delocalization [ 72 ].

Substituent effect SE is another term in the title which requires a substantial comment. It is well recognized that benzene is a toxic and dangerous carcinogen, its substituted derivative — benzoic acid — is applied in the preparation of commonly used preservatives, in a form of its sodium or calcium salts.

Subsequent substitution by acetylic group leads to a drug, well known under the name of aspirin [ 73 ]. However, still, a significant problem exists: how to describe the substituent effect quantitatively. The first quantitative approach in describing the substituent effect was proposed by Louis Plack Hammett [ 74 , 75 ]. For chemical processes, rate, or equilibrium constants k or K , respectively , the use of the substituent constants leads to the Hammett equation, Eq.

The value of const in Eq. In principle, the Hammett equation is a typical similarity model [ 76 ], changes in physicochemical properties P X follow the general equation:. The Hammett equation or its modifications have found countless applications. It has been widely documented that substituent constants may well serve to describe impact of the substituent on most of the physicochemical and even biochemical properties of molecules [ 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 ].

Already in the first three decades, since the original idea was introduced, over 20 different modifications of the original Hammett substituent constants appeared [ 85 ].

They have been designated for various specific types of intramolecular interactions. However, in general, they have caused some disappointment in understanding how these empirical modeling of substituent effect can really work. Thus, some clarification of the topic is important for the advantageous use of the terms of the substituent effect and substituent constants. A very interesting description of electronic properties of the substituent results from the statistical analysis of the geometry patterns of monosubstituted benzene rings [ 90 , 91 ].

The benzene ring deformations are associated with the old concept of group electronegativity [ 92 , 93 ] and with the one recently modified by the Domenicano research group [ 94 , 95 ]. The dynamic development of quantum chemistry methods and computer-aided applications [ 96 ] has created a very convenient atmosphere for research in the field of SE.

To define one of the first SE descriptors based on quantum chemistry, a homodesmotic reaction [ 97 , 98 ] was used:. Then, the energy of this reaction, according to Eq. Most often, its values are well correlated with the Hammett constants [ 99 ]. In addition, the MESP approach allowed to appraise the through bond and through space interactions [ ].

Molecular electrostatic potential has also been used for the quantitative assessment of the inductive effect [ ] and finally to the classification of the substituent effect [ ]. The first electronic interpretation of the substituent effect was proposed by Hammett [ 75 ]. However, the direct application of the substituents atomic charges, q X , does not correlate with the Hammett substituent constants.

Such correlation works well when instead q X the charge of the substituent active region approach, abbreviated cSAR X , introduced by Sadlej-Sosnowska [ , ], is applied. It is defined as a sum of atomic charges of all atoms of the substituent X and the ipso carbon atom:.

In addition, in the disubstituted benzene derivatives X-Ph-Y, the cSAR values allowed to estimate the magnitude of the charge transferred from X to Y, or vice versa [ ]. This has been documented for 12 para-substituted derivatives of nitrobenzene. Figure 3 presents linear regressions between cSAR X values calculated by the use of different methods of atomic charge assessment. Even if the correlation for AIM data is weaker, however, when the cSAR NO 2 values are estimated using these different methods, all mutual correlations are excellent, as presented in Fig.

Reused from [ ], this work is licensed under the Creative Commons Attribution 4. Recent studies on disubstituted benzene and cyclohexa-1,3-diene derivatives have provided support for use of quantum chemistry—based substituent characteristics.

In addition, the use of both traditional and quantum chemistry—based descriptors of SE allows us to answer the question of how far the substituent effect in disubstituted cyclohexa-1,3-diene derivatives differ from those in bicyclo[2,2,2]octane and benzene [ ].

The use of quantum chemistry—based descriptors has allowed to study dependence of the solvent on SE [ ] and also provided a physical interpretation of the inductive and resonance effects [ ]. There are two types of studies related to the substituent effect. In this report, we will review the second type of the approach. There are a few possible classifications of how the substituent effect can be taken into account. These interactions are known as the reverse SE [ ]. One more aspect of SE is observed when properties of the R transmitting moiety are a subject of the influence of the substituent X or of both, X and Y , and finally when various properties of the Y fragments are mutually interrelated.

Model approach to the substituent effect. Another classification can be made when the SE is considered for mono-, di-, tri-, and multi-substituted species. The di- and multi-substituted systems are much more complex and problems with additivity or non-additivity of SE appear [ , ]. Hence, related papers are rarely found in literature.

Finally, some types of SE may be considered for polycyclic systems, where sometimes no simple rules work. This type of effects was already observed by Hammett [ 75 ] showing that the substituent constants for nitro group in para -NO 2 -phenol and in para -NO 2 -benzoic acids differ significantly: 1.

Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds

Benzene C 6 H 6 , simplest organic, aromatic hydrocarbon and parent compound of numerous important aromatic compounds. Benzene is a colourless liquid with a characteristic odour and is primarily used in the production of polystyrene. It is highly toxic and is a known carcinogen ; exposure to it may cause leukemia. As a result, there are strict controls on benzene emissions. Benzene was first discovered by the English scientist Michael Faraday in in illuminating gas. In German chemist Eilhardt Mitscherlich heated benzoic acid with lime and produced benzene. In German chemist A.

Aromatic compound , any of a large class of unsaturated chemical compounds characterized by one or more planar rings of atoms joined by covalent bonds of two different kinds. The unique stability of these compounds is referred to as aromaticity. Although the term aromatic originally concerned odour , today its use in chemistry is restricted to compounds that have particular electronic, structural, or chemical properties. Aromaticity is often reflected in smaller than expected heats of combustion and hydrogenation and is associated with low reactivity. Benzene C 6 H 6 is the best-known aromatic compound and the parent to which numerous other aromatic compounds are related. The six carbons of benzene are joined in a ring, having the planar geometry of a regular hexagon in which all of the C—C bond distances are equal.

Aromatic compound

Aromatic compounds can be drawn with alternating single and double bonds, each atom in the ring must have a p-orbital available. There are common benzene derivative names that are used by IUPAC such as toluene, phenol, benzoic acid and benzaldehyde. A benzene group that is named as a substituent is called phenyl. A benzene with a CH 2 as a substituent group is called benzyl. Aromatic molecules must have all ring atoms in the same plane to allow delocalization of the pi electrons.

Originally fragrant substances were called aromatic compounds. Today, however, the term Arene is reserved for the chemical substances whose molecules are flat planar and ring-shaped cyclic and have conjugated double bonds. And: aromaticity - an attribute of aromatic compounds - is a chemical property associated with the extra stability of certain types of pi-systems. Benzene is considered a model compound of aromatics, consisting of a ring of 6 carbon atoms and six hydrogen atoms.

In chemistry , aromaticity is a property of cyclic ring-shaped , planar flat structures with pi bonds in resonance those containing delocalized electrons that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds —they might be cyclic , but only aromatic rings have enhanced stability. Since the most common aromatic compounds are derivatives of benzene an aromatic hydrocarbon common in petroleum and its distillates , the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many non-benzene aromatic compounds exist.

Orbitals with the same energy are described as degenerate orbitals. Among the many distinctive features of benzene , its aromaticity is the major contributor to why it is so unreactive.

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Но Стратмор смотрел на молодого сотрудника лаборатории систем безопасности. Коммандер спускался по лестнице, ни на мгновение не сводя с него глаз. Он быстро подошел к ним и остановился в нескольких сантиметрах от дрожащего Чатрукьяна. - Вы что-то сказали.

 34-62-10, - ответили на другом конце провода. Ролдан нахмурился. Голос показался ему отдаленно знакомым.


Лампы зловеще гудели. На стене криво висело баскетбольное кольцо. Пол был уставлен десятками больничных коек.

После фиаско Попрыгунчика. Четыре года назад конгресс, стремясь создать новый стандарт шифрования, поручил лучшим математикам страны, иными словами - сотрудникам АНБ, написать новый супералгоритм. Конгресс собирался принять закон, объявляющий этот новый алгоритм национальным стандартом, что должно было решить проблему несовместимости, с которой сталкивались корпорации, использующие разные алгоритмы. Конечно, просить АН Б приложить руку к совершенствованию системы общего пользования - это все равно что предложить приговоренному к смертной казни самому сколотить себе гроб. ТРАНСТЕКСТ тогда еще не был создан, и принятие стандарта лишь облегчило бы процесс шифрования и значительно затруднило АНБ выполнение его и без того нелегкой задачи.

Беккер не мог оторвать глаз от ее руки. У него кружилась голова. Слова, которые он прочитал, были теми же, что произнес немец: ПРОВАЛИВАЙ И УМРИ. Девушка, заметно смутившись, посмотрела на свою руку.

Это придает правдоподобность его электронной переписке. - Тебе следовало бы работать в полиции, - улыбнулся Стратмор.  - Идея неплохая, но на каждое послание Танкадо, увы, поступает ответ.

Synthesis (7): Reaction Map of Benzene and Related Aromatic Compounds

 - Кроме того, ТРАНСТЕКСТ уже больше двадцати часов не может справиться с каким-то файлом. Фонтейн наморщил лоб.

 Я люблю тебя, - шептал коммандер.  - Я любил тебя. У нее свело желудок. - Останься со. В ее сознании замелькали страшные образы: светло-зеленые глаза Дэвида, закрывающиеся в последний раз; тело Грега Хейла, его сочащаяся кровь на ковре; обгорелый труп Фила Чатрукьяна на лопастях генератора.

 Я… я… прошу прощения, - заикаясь, сказал Беккер и застегнул молнию на брюках. Повернувшись, он увидел вошедшую в туалет девушку. Молоденькая, изысканной внешности, ну прямо сошла со страниц журнала Севентин. Довольно консервативные брюки в клетку, белая блузка без рукавов.

Он ждал, когда зазвонит прямой телефон, но звонка все не. Кто-то постучал в дверь. - Войдите, - буркнул Нуматака. Массажистка быстро убрала руки из-под полотенца.

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  1. AmГіs S. 30.04.2021 at 22:55

    When a benzene ring has two substituent groups, each exerts an influence on subsequent substitution reactions.

  2. Ganix T. 06.05.2021 at 12:11

    benzene, that exhibit special stability due to resonance delocalization of π-​electrons. Page 4. 4. 7. Naming aromatic compounds: (arenes) large number on.

  3. Thaiel T. 07.05.2021 at 06:26

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