Hydrogen bonding, carbon acidity-carbanion

Cleavage of a carbon-hydrogen bond to yield a carbanion and proton is a Bronsted acid-base reaction (Equations 5.24 and 5.25). The mechanism is not... 

An electronegative heteroatom, Y, can be replaced by an electronegative carbon atom, C-ewg. The electron-withdrawing group is extremely necessary, for without a means to stabilize the carbanion formed, the electron flow cannot occur. The enhancement of nucleophilicity by hydrogen bonding may occur in a protic solvent whenever the nucleophile bears a reasonably acidic hydrogen. 

Alpha hydrogens are hydrogens on carbons directly attached to a carbonyl group. They are weakly acidic and can be abstracted by base to form a carbanion. The carbanion is called an enolate ion and is resonance stabilized. Neutralization of the enolate ion results in an enol, a compound in which an alcohol group is directly bonded to a carbon involved in a carbon-carbon double bond. The enol is in equilibrium with the original aldehyde or ketone in an equilibrium referred to as keto-enol tautomerism. The equilibrium usually favors the keto form. 

As a result of its aromaticity, the cyclopentadienyl anion is an unusually stable carbanion. This is why cyclopentadiene has an unusually low pA a. In other words, it is the stability conveyed by the aromaticity of the cyclopentadienyl anion that makes the hydrogen much more acidic than hydrogens bonded to other sp carbons. 

The hydrogen bonded to the imine carbon of TPP is relatively acidic (pA a = 12.7) because the ylide carbanion formed when the proton is removed is stabilized by the adjacent positively charged nitrogen. The ylide carbanion is a good nucleophile. 

Now that the properties of carbon-carbon double bonds have been examined in detail, it s time to have a brief look at their relatives, carbon-carbon triple bonds. Not surprisingly, what you will find will be very similar to what you ve just. seen. Addition reactions will mtike up the main portion of triple bond chemistry. An iinporlani feature is the hydrogen attached to triply bonded carbon It is unusually acidic, allowing ready removal by strong bases to form a new and synthetically useful class of carbanions called alkynyl anions. 

Typically, decarboxylation of a carboxylic acid takes place through the carboxylate anion, from which the group R departs along with its bonding electron pair. Once formed, the carbanions react with a proton source to give a new carbon-hydrogen bond. Any factor that increases the stabihty of the carbanion should promote decarboxylation (Scheme 3.22). 

Reagents such as n-butyllithium, methyllithium, and phenyllithium manufactured by this process are commercially available. Carbanions can also be formed by an acid-base reaction involving heterolytic dissociation of a carbon-hydrogen bond by a strong base. An example is the deprotonation of limonene (86) by -butyllithium complexed with tetramethylethylenedia-mine (TMEDA), as shown in equation 5.61. Note that there is more than one kind of allylic proton in 86, but the deprotonation preferentially produces the least-substituted carbanion. ... 

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