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The model assigned E and C parameters to many Lewis acids and bases. Each acid is characterized by an E A and a C A. Each base is likewise characterized by its own E B and C B. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is
Nevertheless, one can generally examine acid dissociation constants to qualitatively predict or rationalize rate or reactivity trends relating to variation of the leaving group. Consistent with this picture, strong bases such as OH − , OR 2 and NR − 2 tend to make poor leaving groups, due their inability to stabilize a negative charge.
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Sodium methoxide is a routinely used base in organic chemistry, applicable to the synthesis of numerous compounds ranging from pharmaceuticals to agrichemicals. [4] As a base, it is employed in dehydrohalogenations and various condensations. [5] It is also a nucleophile for the production of methyl ethers. [6]
A major factor in determining the reactivity of acyl derivatives is leaving group ability, which is related to acidity. Weak bases are better leaving groups than strong bases; a species with a strong conjugate acid (e.g. hydrochloric acid) will be a better leaving group than a species with a weak conjugate acid (e.g. acetic acid).
An example of a weak base is ammonia. It does not contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions. [4] The position of equilibrium varies from base to base when a weak base reacts with water. The further to the left it is, the weaker the base. [5]
For example, acetic acid is a weak acid which has a = 1.75 x 10 −5. Its conjugate base is the acetate ion with K b = 10 −14 /K a = 5.7 x 10 −10 (from the relationship K a × K b = 10 −14), which certainly does not correspond to a strong base. The conjugate of a weak acid is often a weak base and vice versa.
In addition to the use of strong bases, enolates can be generated using a Lewis acid and a weak base ("soft conditions"): For deprotonation to occur, the stereoelectronic requirement is that the alpha-C-H sigma bond must be able to overlap with the pi* orbital of the carbonyl: Stereoelectronic deprotonation requirements