Contributing resonance structures

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

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. 

We may consider now the effect of substitution at C-4 of testosterone (0=C—C=C system). If there is a hydrogen atom at C-4, the large electronegativity of the oxygen atom gives an acceptor character to the carbonyl group and a donor character to the unsaturated bond. Therefore, the resonance contributing structures depicted below are important ... 

Acetylene prefers a linear molecular arrangement with bond angles of 180 rather than 120 as assumed in the resonance contributing structures. 

Biphenyl, CeHs-CeHs has a conjugation energy of 71 kcal/mole. (a) Draw an atomic orbital picture of biphenyl, (b) What are its most important resonance contributing structures (c) Estimate the heat of hydrogenation of biphenyl. 

The relative stabilities of 3 > 2 > 1 are explained through hyperconjugation and by the number of resonance contributing structures, t-butyl cation has nine contributing structures 2-butyl cation has five contributing structures n-butyl cation has two contributing structures. 

We note that two of the resonance contributing structures give particular stability to the resonance hybrid in that the double bonds present in them are conjugated. Thus, because of stability of the subsequently formed carbanion, the reactant 2-methyl-2-butenal, will be deuterated at all positions y to the carbonyl group. 

More importantly, as seen from the two major resonance contributing structures, the geometry of the system is such that the free pair of electrons, and both double bonds are not all components of a continuously conjugated system. 

Molecular orbital calculations provide the following set of resonance-contributing structures for the furan... 

Figure 1.12 shows three resonance contributing structures for the carbonate ion. The three are equivalent, meaning that they have identical patterns of covalent bonding (each contributing structure has one double bond and two single bonds) and are of equal energy. 

Notice in Figure 1.12 that the only change from resonance contributing structure (a) to (b) and then from (b) to (c) is a redistribution of valence electrons. To show how this redistribution of valence electrons occurs, chemists use a symbol called a curved arrow, which shows the repositioning of an electron pair from its origin (the tail of the arrow) to its destination (the head of the arrow). The repositioning may be from an atom to an adjacent bond or from a bond to an adjacent atom. 

Rules for Writing Acceptable Resonance Contributing Structures... 

You must follow these four rules in writing acceptable resonance contributing structures ... 

Draw the resonance contributing structure indicated by the curved arrows. Be certain to show all valence electrons and all formal charges. 

According to the theory of resonance, a molecule or ion for which no single Lewis structure is adequate is best described by writing two or more resonance contributing structures and considering the real molecule or ion to be a hybrid of the various contributing structures. 

Resonance contributing structures are interconnected by double-headed arrows. 

All bond angles in sets of resonance contributing structures must be the same. (1.5)... 

Draw acceptable resonance contributing structure(s) for each of the compounds shown. (See Example 1.11)... 

Resonance contributing structures Representations of a molecule or ion that differ only in the distribution of valence electrons. 

Part Three. The benzyl (and allyl) halides are a special case they have resonance. To see how the charge is delocalized in the benzyl carbocation, request two plots the electrostatic potential mapped onto a density surface and the LUMO mapped onto a density surface. Submit these for calculation at the AMI semiempirical level. On a piece of paper, draw the resonance-contributing structures for the benzyl cation. Do the computational results agree with the conclusions you draw from your resonance hybrid ... 

A symbol used to show that structures on either side are resonance contributing structures. 

A symbol used to show the redistribution of valence electrons in resonance contributing structures or reaction mechanisms, symbolizing movement of two electrons. 

To emphasize, do not confuse resonance contributing structures with equilibration among different species. A molecule described as a resonance hybrid is not equilibrating among individual electron configurations of the contributing structures. Rather, the molecule has only one structure, which is best described as a hybrid of its various contributing structures. 

Most examples of molecules described by more than one resonance contributing structure have charge and/or electron delocalization, a stabilizing effect in which charge and/or electrons are spread over more than two atoms. 

If atoms are taking part in delocalization as described by resonance contributing structures, they must have 2p orbitals so they must be sp hybridized or, in rare cases, sp hybridized. 

Protonation on the hydroxyl oxygen gives cation B, for which we can write two resonance contributing structures. 

Delocalizing the negative charge as described by resonance contributing structures. 

Delocalization of charge in the anion, usually described by resonance contributing structures because greater delocalization of charge is stabilizing. 

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