Contributing structures benzene

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

The pyrylium cation is isoelectronic with pyridine it has the same number of electrons and, therefore, we also have aromaticity. Oxygen is normally divalent and carries two lone pairs. If we insert oxygen into the benzene ring structure, then it follows that, by having one electron in a p orbital contributing to the aromatic sextet, there is a lone pair in an sp orbital,... 

Sometimes it is not possible to assign a single electronic structure to a molecule that may account for all its properties. In such a case the molecule is represented by two or more electronic structures but none of these represents the actual structure of the molecule. The actual structure of the molecule lies somewhere in between these structures but can not be expressed on paper. Such a molecule is said to exhibit resonance. The various structures assigned to the molecule are called contributing structures or canonical structures where as the intermediate structures is called the resonance hybrid. For example benzene (C6H6) maybe assigned the following two structures ... 

Pauling s use of valence bond theory had a direct connection with the types of structures commonly used by organic chemists, and was relatively easy to understand, provided one did not delve too deeply into its details. The basic postulate was that compounds having n-electron systems that can be described by more than one structure will be stabilized by "resonance" and will have a lower energy than any of the contributing structures. Thus, for benzene one would write... 

Bond order A theoretical index of the degree of bonding between two atoms relative to that for a normal single bond, that is, one localized electron pair. In valence bond theory it is expressed by the weighted average of the bond numbers between the respective atoms in the contributing structures. By this criterion each C—C bond in benzene has a bond order of 1.5. 

In the diagram above there are two identical structures having opposite charge distributions and there is no net separation of charge. The importance of resonance structures to the composite structure increases with the stability of the individual structures, so structures B and C are less important than A, as they have separation of charge and only one rather than two tt bonds. By applying resonance criteria 3a and 3b, we conclude that these two structures contribute less stabilization to butadiene than the two equivalent benzene resonance structures. Therefore, we expect the enhancement of electron density between C(2) and C(3) to be small. 

Kekule structure /kay-koo-lay/ A structure of BENZENE in which there is a hexagonal ring with alternate double and single bonds. Two possible Kekule structures contribute to the RESONANCE hybrid of benzene. The structure is named for the German chemist Friedrich Augus Kekule von Stradonitz (1829-96). 

Many of the properties of phenols reflect the polarization implied by the contributing structures. The hydroxyl oxygen is less basic, and the hydroxyl proton more acidic, in phenols than in alcohols. Electrophilic aromatic substitution in phenols is much faster than in benzene, indicating that the ring, especially at the positions ortho and para to the hydroxyl group, is relatively electron-rich. ... 

Kekule structure A proposed structure of benzene in which the molecule has a hexagonal ring of carbon atoms linked by alternating double and single bonds. KekulS structures contribute to the resonance hybrid of benzene. The structure was suggested in 1865 by Friedrich August Kekule. 

Structures 1 and 2 are the Kekule structures that contribute to any benzene derivative. Structures 3—5, however, delocalize the unshared electron pair of the nitrogen over the ortho and para positions of the ring. This delocalization of the electron pair makes it less available to a proton, and delocalization of the electron pair stabilizes aniline. 

One of the postulates of resonance theory is that, if we can represent a molecule or ion by two or more contributing structures, then that molecule cannot be adequately represented by any single contributing structure. We represent benzene as a hybrid of two equivalent contributing structures, often referred to as KekuU structures ... 

Draw the intermediate that is formed initially from para attack of the electrophile. Then draw a contributing structure by moving electrons from the pi bond adjacent to the positive charge. Repeat for all contributing structures until all resonance possibilities have been exhausted. Note Be sure to look for resonance possibilities outside of the benzene ring. 

Benzene consists of two contributing structures that rapidly interconvert between each other. (9.1)... 

Kekules model for the structure of benzene is nearly, but not entirely, correct. Kekules two structures for benzene differ only in the arrangement of the electrons all of the atoms occupy the same positions in both structures. This is precisely the requirement for resonance (review Sec. 1.12). Kekule s formulas represent two identical contributing structures to a single resonance hybrid structure of benzene. Instead of writing an equilibrium symbol between them, as Kekul did, we now write the double-headed arrow (<->) used to indicate a resonance hybrid ... 

Benzene is a resonance hybrid of these two contributing structures. 

To express this model another way, all benzene molecules are identical, and their structure is not adequately represented by either of Kekule s contributing structures. Being a resonance hybrid, benzene is more stable than either of its contributing Kekule structures. There are no single or double bonds in benzene—only one type of carbon-carbon bond, which is of some intermediate type. Consequently, it is not surprising that benzene does not react chemically exactly like alkenes. 

We have asserted that a resonance hybrid is always more stable than any of its contributing structures. Fortunately, in the case of benzene, this assertion can be proved experimentally, and we can even measure how much more stable benzene is than the hypothetical molecule 1,3,5-cyclohexatriene (the lUPAC name for one Kekule structure). 

We define the stabilization energy, or resonance energy, of a substance as the difference between the actual energy of the real molecule (the resonance hybrid) and the calculated energy of the most stable contributing structure. For benzene, this value is about 36 kcal/mol. This is a substantial amount of energy. Consequently, as we will see, benzene and other aromatic compounds usually react in such a way as to preserve their aromatic structure and therefore retain their resonance energy. 

Hi) Unusual stability The unusual stability and peculiar behaviour of benzene is due to high resonance energy (151.2 kJ moH) of benzene. In general, a resonance hybrid is always more stable than any of the contributing structures. 

The molecular orbital and resonance theories are powerful tools with which chemists can understand the unusual stability of benzene and its derivatives. According to resonance theory, benzene is best represented as a hybrid of two equivalent contributing structures. By analogy, cyclobutadiene and cyclooctatetraene can also be represented as hybrids of two equivalent contributing structures. Is either of these compounds aromatic ... 

Benzene is best represented as a resonance hybrid composed of two resonance forms in which the locations of the double bonds are reversed. For simplicity, benzene is often represented as a single contributing structure or as a hexagon with a circle drawn on the inside. 

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