Here, the base chosen is such that its conjugate acid has a p K a of at least With sodium amide as the base, terminal alkynes form sodium acetylide and ammonia as the conjugate acid. Since the p K a of ammonia is greater than 35, the equilibrium favors the formation of sodium acetylide, making sodium amide a suitable base for the deprotonation reaction. In contrast, sodium hydroxide forms water as its conjugate acid.
Since the p K a of water is less than 35, the equilibrium favors the reactants. Therefore, terminal alkynes cannot be deprotonated using sodium hydroxide. The strength of an acid is commonly expressed in units of p K a — the lower the p K a , the stronger the acid. Among the hydrocarbons, terminal alkynes have lower p K a values and are, therefore, more acidic. For example, the p K a values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
With a p K a difference of 26 units, acetylene is 10 26 times more acidic than ethane. Similarly, a difference of 19 p K a units makes it 10 19 times stronger than ethene. The pronounced increase in the acidity of terminal alkynes relative to the other hydrocarbons can be explained by considering the stability of the corresponding carbanions formed by deprotonation.
Note that, in the nomenclature of organic compounds, the suffix "-ide" indicates that the molecule is a negatively charged ion. The stability of the carbanion depends on the nature of the hybridized orbital occupied by the lone pair of electrons.
As shown above, in ethane the lone pair resides in an sp 3 orbital, while in ethene it occupies the sp 2 orbital and an sp orbital in the case of acetylene. Since " s " orbitals are closer to the positively charged nucleus, a hybrid orbital with a higher " s " character will effectively stabilize the negative charge.
Thus, the acetylide ions will be the most stable and readily formed in the presence of a suitable base. In general, for a base to deprotonate an acid, the p K a of the base's conjugate acid must be at least 10 p K a units greater than that of the acid. Therefore, an appropriate base would be one where the conjugate acid has a p K a that is at least Recall that for an acid-base reaction, the equilibrium favors the formation of weaker acids and bases from stronger acids and bases.
Since the p K a of ammonia is greater than 25, the equilibrium favors the formation of sodium acetylide, making sodium amide a strong enough base for the deprotonation reaction. Apart from sodium amide, sodium hydride, butyllithium, and lithium diisopropylamide LDA are other commonly used bases to form acetylide ions.
In the presence of sodium hydroxide as the base, terminal alkynes form sodium acetylide and water as the conjugate acid.
However, since the p K a of water is less than 25, the equilibrium favors the reactants. Therefore, sodium hydroxide is not a suitable base to form acetylide ions. The relative acidity of terminal alkynes finds application in the synthesis of organometallic compounds when treated with Grignard or organolithium reagents. These are examples of transmetalation reactions involving the transfer of a metal atom from one carbon to another, thereby forming new metal—carbon bonds.
However, they can also be interpreted as acid—base reactions that favor the formation of weaker acids and bases.
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My Preferences My Reading List. Organic Chemistry I. Alkynes: Acidity. There is a strong correlation between s-character in the orbital containing the non-bonding electrons in the anion and the acidity of hydrocarbons. The enhanced acidity with greater s-character occurs despite the fact that the homolytic C-H BDE is larger. Consequently, acetylide anions can be readily formed by deprotonation using a sufficiently strong base. Steven Farmer Sonoma State University.
Learning Objectives explain why alkynes are more acidic than alkanes and alkenes predict the products and specify the reagents to generate nucleophilic acetylide ions and heavy metal acetylides.
Acidity of Terminal Alkynes and Acetylilde Ion Formation Terminal alkynes are much more acidic than most other hydrocarbons. Table 9. Exercise Given that the p K a of water is
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