Alkyne Acidity Formation of Acetylide Anions

The most striking difference between alkencs and alkynes is that terminal alkynes are weakly acidic. When a terminal alkyne is treated with a strong base, such as sodium amide, Na NH the terminal hydrogen is removed and an acetylide anion is formed  

Since a stronger acid donates its proton to the anion of a weaker acid in an acid-base reaction, a rank-ordered hst tells which bases are needed to deproto-nate which acids. For example, since acetic acid (pAg = 4.75) is a stronger acid than ethanol (pK = 16), we know that the anion of ethanol (ethoxide ion, CH3CH2O ) will remove a proton from acetic acid. Similarly, amide ion ( NH2), the anion of ammonia = 35), will remove a proton from ethanoUpKa = 16). 

Where do hydrocarbons lie on the acidity scale As the data in Table 8.1 indicate, both methane (pifg 60) and ethylene (pX, = 44) are very 

A comparison of methyl, vinylic, and acetylide anions. The acetylide anion, with sp hybridization, has more s character and is more stable. Electrostatic potential maps show that placing the negative charge closer to a carbon nucleus makes carbon appear less negative (red). 

Where do hydrocarbons lie on the acidity scale As the data in Table 8.1 show, both methane (pKg == 60) and ethylene (pX = 44) are very weak acids and thus do not react with any of the common bases. Acetylene, however, has pK = 25 and can be deprotonated by the conjugate base of any acid whose pK is greater than 25. Amide ion (NH2 ), foi example, the conjugate base of ammonia (pKj. = 35), is often used to deprotonate terminal alkynes. 

I he preseiTice of a negative charge and an unsl ared electron pair on carbon makes acetylide anions strongly nucleophilic As a result, they react with many different kinds of electrophiles. 

The of acetone, C143COCH3, is 19.3. Which of the following bases is strong enough to deprotonate acetone  

Thomson Click Organic interactive tu use a web-based palette to predict products for alkyne alkylation reactions. 

Why are terminal aikynes more acidic than alkenes or alkanes In other 

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Alkyne Acidity Formation of Acetylide Anions Alkylation of Acetylide Anions 289... 

Carbon nucleophiles are very useful species because their reactions with carbon electrophiles result in the formation of carbon—carbon bonds. Section 10.8 introduced acetylide anions as nucleophiles that could be used in Sm2 reactions. These nucleophiles are prepared by reacting 1-alkynes with a strong base such as sodium amide. The relatively acidic hydrogen on the. vp-hybridized carbon is removed in this acid-base reaction ... 

In this equilibrium, acetylene is the stronger acid and sodium amide is the stronger base, and the position of equilibrium lies considerably toward the right and favors formation of the acetylide anion and ammonia (Section 2.4). Table 4.1 gives pA values for an alkane, alkene, and an alkyne hydrogen. Also given for comparison is the value for water. 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles... 

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