Assertion (A): Sodium acetylide is formed from ethyne with NaNH2 but not appreciably with NaOC2H5 in ethanol. Reason (R): pK_a of NH3 (38) is much higher than that of ethyne (25), whereas pK_a of ethanol (15.9) is lower than that of ethyne.

Both A and R are true and R is the correct explanation of A

Both A and R are true but R is not the correct explanation of A

Both A and R are true and R is the correct explanation of A

JEE Advanced levelSingle correct optionDifficulty 3 of 3g2136

Assertion (A): Sodium acetylide is formed from ethyne with $NaNH X_{2}$ but not appreciably with $NaOC X_{2} H X_{5}$ in ethanol. Reason (R): $p K_{a}$ of $NH X_{3}$ (38) is much higher than that of ethyne (25), whereas $p K_{a}$ of ethanol (15.9) is lower than that of ethyne.

lo g K

with

NH X_{2} X^{-}

38 - 25 = 13

(complete), and with

C X_{2} H X_{5} O X^{-}

15.9 - 25 = - 9.1

(negligible). Both statements are true and R explains A, since the reaction needs a base whose conjugate acid is weaker than the alkyne.