Formate anion resonance

The racemization process involves removal of the a-hydrogen to form the enolate anion, which is favoured by both the enolate anion resonance plus additional conjugation with the aromatic ring. Since the a-protons in esters are not especially acidic, the additional conjugation is an important contributor to enolate anion formation. The proton may then be restored from either side of the planar system, giving a racemic product. 

The mechanism Favorskii envisioned involved the initial attack of the ethoxy anion on the triple bond to form a vinyl ether. The now accepted carbanionic mechanism assumes the formation of resonance-stabilized anions, allowing the stepwise interconversion of 1- and 2-alkynes, and allenes143 147 (Scheme 4.9). 

Let s consider the formate anion, shown in Figure 3.15. The first Lewis structure (a) does not accurately represent the structure of this covalent ion, because a second Lewis structure can be drawn (b) that is equivalent to the first. The actual structure is a resonance hybrid of these two structures. Experiments confirm that the two CO bonds are identical, with a bond length between that of a single and a double bond, and that the charge on each oxygen is the same, approximately —y. 

Figure 3.15c is an attempt to show how the AOs might overlap to form localized MOs in the formate anion. In this localized MO picture, a p orbital on the carbon overlaps with a p orbital on the upper oxygen to form a pi bond, corresponding to the Lewis structure of Figure 3.15a. In this structure, the lower oxygen has three unshared pairs of electrons. Whenever an atom with an unshared pair of electrons is adjacent to a pi bond, as occurs here, that atom usually assumes a hybridization that places an unshared pair in a p orbital because the overlap of this p orbital with the p orbital of the pi bond on the adjacent atom is stabilizing. It is this overlap that allows resonance to occur. In this case the p orbital with the unshared pair on the lower oxygen overlaps equally well with the p orbital on the carbon so that the pi bond could also be shown using these two orbitals with an unshared pair of electrons in the p orbital on the upper oxygen. This corresponds to the second Lewis structure (b).

Formate anion is an example of an ion for which two equivalent Lewis structures (0 and ) can be drawn.The actual structure is a resonance hybrid of these two structures. Remember to use the double-headed arrow only between resonance structures. Never use equilibrium arrows ( .) between resonance structures. 

This is an attempt to show an orbital picture for the formate anion. It corresponds to the Lewis structure in part . (The two unshared pairs of electrons on each oxygen that are not involved in resonance have been omitted for clarity.) The two red p orbitals overlap to form the pi bond. One unshared pair of electrons is in the blue p orbital on the other oxygen. This blue p orbital overlaps the red p orbital on the carbon just like the red p orbital on the other oxygen does. 

Formation of the Au-S bond in hexanethiol adsorption leads to a localized a resonance which broadens and shifts when the hexanethiol molecules, initially lying flat at low coverages, realign vertically at higher coverages. Extensive studies of the model system C61 v>/Cu(111) have shown the presence of a transient anionic resonance, and similar studies are now being carried out on other systems. 

The NPA electron distribution can be related to the VB concept of resonance structures. The orbitals corresponding to localized structures and those representing delocalization can be weighted. For example, Scheme 1.5 shows the relative weighting of the most important resonance structures for 1,3-butadiene, benzene, the benzyl cation, formamide, and the formate anion. These molecules are commonly used examples of the effect of conjugation and resonance on structure and reactivity. 

This analysis leads to a discussion of the important concept called resonance, which will be used in many places in this book. Reaction of formic acid (acetic acid) and a base gives a carboxylate anion (the formate anion, 79A) as the conjugate base. Note that the three atoms (0=C-0) are connected. One atom (O) has a negative charge, which can be viewed as a full p-orbital (a p-or-bital containing two electrons). In addition, there is an adjacent 7C-bond (C=0). In Figures 5.1-5.3, it is clear that two adjacent and parallel orbitals overlap and share electron density to form a 7C-bond. The concept that adjacent orbitals share electron density can be extended from two orbitals to three, four, or even more. All that is required is that the orbitals are on adjacent atoms and are parallel. Therefore, when three orbitals are on adjacent atoms and are parallel, electron density is shared between all three atoms. 

Structure 79B is a more accurate representation of the formate anion, where the charge is delocalized over three atoms. It is difficult to draw 79B, so the resonance delocalization is represented by two structures, as shown in 80 with a double-headed arrow to indicate resonance. The two structures labeled 80 represent one resonance-stabilized anion, rather than two different molecules. Resonance will be seen many times in connection with intermediates of various reactions and, in all cases, resonance leads to a more stable species. 

Chemists use an alternative method to show the equivalent resonance contributors, as with the formate anion 79 discussed in Section 5.9.3. If benzene is drawn with the n-bonds localized as in 87A, there is no indication of electron delocalization and this single structure does not adequately represent the structure of benzene. If the double bonds are moved from their position in 87A to give 87D, 87D is also inadequate because all the double bonds in 87A are single bonds in 87D and vice versa. The actual structure of benzene is not 87A or 87D, but it can be represented by both structures—called resonance contributors. Imagine the electrons shifting back and forth between 87A and 87D (the electrons are moving within the framework of 7t-orbitals) to represent the electron delocalization in benzene. 

Other Raman studies of aqueous solutions at elevated temperatures and pressures include two recent articles on the formate anion [180,181], where the inconsistency of the C2V local symmetry for the anion, which more likely would possess Q symmetry, and the effect of the temperature on the Fermi resonance between —H stretching) and 2ps... 

Enolate ion formation (Section 18.6) An a hydrogen of an aldehyde or a ketone is more acidic than most other protons bound to carbon. Aldehydes and ketones are weak acids, with pK s in the 16 to 20 range. Their enhanced acidity is due to the electron-withdrawing effect of the carbonyl group and the resonance stabilization of the enolate anion. 

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