Resonance structures sigma

How can the substituent influence the resonance shown in Figure 7-15 The answer is that if the substituent can create another resonance structure, the sigma complex is further stabilized. This additional stabilization leads to a preference for a certain attack. 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms in the alpha positions therefore lie in a plane. The pi bond which lies over the plane of the atoms acts as a source of electrons in addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the allylic resonance structure. This structure can stabilize both positive and... 

The anomeric effect in terms of a stabilizing effect can be illustrated by the concept of "double-bond - no-bond resonance" (14, 15) shown by the resonance structures 4 and 2 or by the equivalent modern view (16, 17) that this electronic delocalization is due to the overlap of an electron pair orbital of an oxygen atom with the antibonding orbital of a C —OR sigma bond (12). 

Know the meaning of molecular formula, structural formula, structural (or constitutional) isomers, continuous and branched chain, formal charge, resonance, contributing structures, sigma (a) bond, sp3-hybrid orbitals, tetrahedral carbon. 

M. Tonelli, E. Ragg, A.M. Bianucci, K. Lesiak and T.L. James. Nuclear magnetic resonance structure of d(GCATATGATAG). d(CTATCATATGC) a consensus sequence for promoters recognized by sigma K RNA polymerase. Biochemistry 37... 

The stability of the sigma complex is the guiding principle when determining the site of electrophilic attack on any aromatic ring. The more approximately equal energy resonance structures the carbocation has, generally the more stable it is. Resonance forms that place the positive charge next to an ewg are poor and should not be counted. 

Naphthalene undergoes electrophilic aromatic substitution at the position next to the second ring. The sigma complex leading to the obtained product has more resonance structures, indicating a more delocalized charge than that of the alternative. 

Accordiug to valeuce boud theory, the C atom is described as sp hybridized, aud it forms oue sigma boud with each of the three O atoms. This leaves one unhybridized 2p atomic orbital on the C atom, say the 2p orbital. This orbital is capable of overlapping and mixing with the 2p orbital of any of the three O atoms. The sharing of two electrons in the resulting localized pi orbital would form a pi bond. Thus, three equivalent resonance structures can be drawn in valence bond terms (Figure 9-10b). We emphasize that there is no evidence for the existence of these separate resonance structures. 

Resonance structures are diagrammatic tools used predominately in organic chemistry to symbolize resonant bonds between atoms in molecules. The electron density of these bonds is spread over the molecule, also known as the delocalization of electrons. Resonance contributors for the same molecule all have the same chemical formula and same sigma framework, but the pi electrons will be distributed differently among the atoms. Because Lewis dot diagrams often cannot represent the tme electronic stmcture of a molecule, resonance stmctures are often employed to approximate the tme electronic stmcture. Resonance stmctures of the same molecule are connected with a double-headed arrow. While organic chemists use resonance stmctures frequently, they are used in inorganic stmctures, with nitrate as an example. 

All resonance structures for the same molecule must have the same sigma framework (w sigma bonds form from the head on overlap of hybridized orbitals). Furthermore, they must be correct w Lewis structures with the same number of electrons (and consequent charge) as well as the same number of unpaired electrons. Resonance structures with arbitrary separation of charge are unimportant, as are those with fewer covalent bonds. These unimportant resonance structures only contribute minimally (or not at all) to the overall bonding description however, they are important in some cases such as for a w carbonyl group. 

We can now state that each carbon-to-carbon linkage in benzene contains a sigma bond and a partial pi bond. The bond order between any two adjacent carbon atoms is therefore between 1 and 2. Thus molecular orbital theory offers an alternative to the resonance approach, which is based on valence bond theory. (The resonance structures of benzene are shown on p. 349.)... 

The central N is sp hybridized. We can probably ignore the third resonance structure on the basis of formal charge, c. sp hybrid orbitals from the center N overlap with atomic orbitals (or hybrid orbitals) from the other two atoms to form the two sigma bonds. The remaining p orbitals from the center N overlap with p orbitals from the other N to form the two tt bonds. 69. a. 116 kj/mol ... 

Two structures are resonance structures if the atomic connectivity in the two structures is identical (same sigma bonding) but the electron density distribution (bond order, lone pair electrons, and pi bonds) over the atomic nuclei is different. 

When benzene is treated with I2 in the presence of CUCI2, iodination of the ring is achieved with modest yieids. it is beiieved that CuCi2 interacts with i2 to generate i+, which is an excellent electrophile. The aromatic ring then reacts with 1+ in an electrophilic aromatic substitution reaction. Draw the mechanism of the reaction between benzene and 1. Make sure that your mechanism has two steps, and make sure to draw all of the resonance structures of the sigma complex. 

Draw the mechanism of the following reaction, and make sure to draw all three resonance structures of the sigma complex. 

Draw all resonance structures of the sigma complex formed when toluene undergoes chlorination at the para position. 

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