Methyl anion structure

Draw Lewis structures for methyl anion, ammonia and hydronium cation. How many electrons are left over in each after all bonds have been made Display and compare electron density surfaces for methyl anion, ammonia and hydronium cation. Which is the smallest molecule Which is the largest Rationalize your observation. (Hint Compare the number of electrons in each molecule, and the nuclear charge on the central atom in each molecule.)... 

The methyl anion (CH3) has been observed in the gas phase and reported to have a pyramidal structure. If this is a general structure for carbanions, then any... 

Aldol addition and related reactions of enolates and enolate equivalents are the subject of the first part of Chapter 2. These reactions provide powerful methods for controlling the stereochemistry in reactions that form hydroxyl- and methyl-substituted structures, such as those found in many antibiotics. We will see how the choice of the nucleophile, the other reagents (such as Lewis acids), and adjustment of reaction conditions can be used to control stereochemistry. We discuss the role of open, cyclic, and chelated transition structures in determining stereochemistry, and will also see how chiral auxiliaries and chiral catalysts can control the enantiose-lectivity of these reactions. Intramolecular aldol reactions, including the Robinson annulation are discussed. Other reactions included in Chapter 2 include Mannich, carbon acylation, and olefination reactions. The reactivity of other carbon nucleophiles including phosphonium ylides, phosphonate carbanions, sulfone anions, sulfonium ylides, and sulfoxonium ylides are also considered. 

However, other reaction directions via the formation of the phosphorane structure with migration of the methyl anion from one phosphorus atom... 

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

This is the methyl anion, and carries one negative charge. Carbon has four valence electrons, and in this structure the number of assigned electrons is... 

The situation is different in the case of a substituted methyl group. Structure [10a] and Fig. 1 both show the rather severe steric repulsions caused by the substitution of the methine proton by CH3. The methylation (CD3I) of trimer anion [19], however, proceeded with better than 95% stereoselectivity, although the absolute stereochemistry of this reaction is not known with certainty. 

A carbanion is closely related to the corresponding derivative of nitrogen the methyl anion is isoelectronic with ammonia itself. By analogy with the configuration of amines, carbanions should also possess a pyramidal structure (I) in which the pair of electrons of the carbanion is placed in an sp3 orbital, rather than the alternative trigonal structure (II) with the pair of electrons in a p orbital. 

Furthermore, just as the pyramidal structure in amines is inverting rapidly, so it would be expected that free carbanions will also invert their stereochemical configuration rapidly (III). Theoretical calculations have yielded7 a value of about 11 kcal.mole-1 for the barrier to inversion of the methyl anion, and it may be expected that a carbanion formed free in solution will, on further reaction, yield products derived equally from the two possible configurations. Should the... 

Three reactive species, a methyl anion, methyl cation, and methyl radical, are shown in Figure 1.1. Ethane is composed of two methyl groups connected by a covalent bond and is a very stable compound. The methyl anion and methyl cation have an ionic bond mainly between carbons and counter ions, respectively, and are not particularly unstable, though there are some rather moisture-sensitive species. However, the methyl radical is an extremely unstable and reactive species, because its octet rule on the carbon is not filled. The carbon atom in the methyl cation adopts sp2 hybridization and the structure is triangular (120°) and planar. The carbon atom in the methyl anion adopts sp3 hybridization and the structure is tetrahedral (109.5°). However, the carbon atom in the methyl radical adopts a middle structure between the methyl cation and the methyl anion, and its pyramidal inversion rapidly occurs as shown in Figure 1.1, even at extremely low temperature. 

The hybridization and bond angles of a simple carbanion also resemble those of an amine. The carbon atom is sp3 hybridized and tetrahedral. One of the tetrahedral positions is occupied by an unshared lone pair of electrons. Figure 4-16 compares the orbital structures and geometry of ammonia and the methyl anion. 

Comparison of orbital structures of the methyl anion and ammonia. Both the methyl anion and ammonia have an sp3 hybridized central atom, with a nonbonding pair of electrons occupying one of the tetrahedral positions. 

The methyl anion, CH3, and hydronium ion, HjO4-, are both isoelectronic with ammonia so that all share the same pyramidal structure. Each is approximately tetrahedral with a lone pair in an sp3 orbital. These elements follow each other in the periodic table so the change in charge occurs because each nucleus has one more proton than the last. VSEPRT also gives this answer. 

In certain organic compounds, covalent bonds can be so strongly polarized that their structure and reactivity may be approximated by the corresponding ionic formulas. For example, the covalent bonds in methyl triflate 48 and methyllithium 49 are so strongly polarized that they behave as if they were fully ionized compounds, sources of the methyl cation and methyl anion respectively ... 

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