The anions below are the conjugate bases of acids with the following acid dissociation constants K_a at 298 K: NO3-: 24, Cl-: 1x 10^7, CH3SO3-: 79, F-: 0.00068, CH3COO-: 1.8x 10^-5, CF3COO-: 0.59, CN-: 6.2x 10^-10, H2PO4-: 0.0071. Counting convention: a species is a good leaving group if the pKa of its conjugate acid is below 0 (pKa = -log_10K_a). Give the number of good leaving groups as an integer.

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JEE Advanced levelNumerical answerDifficulty 3 of 3o1034

The anions below are the conjugate bases of acids with the following acid dissociation constants $K_{a}$ at 298 K: $NO X_{3} X^{-}$ : $24$ , $Cl X^{-}$ : $1 \times 1 0^{7}$ , $CH X_{3} SO X_{3} X^{-}$ : $79$ , $F X^{-}$ : $0.00068$ , $CH X_{3} COO X^{-}$ : $1.8 \times 1 0^{- 5}$ , $CF X_{3} COO X^{-}$ : $0.59$ , $CN X^{-}$ : $6.2 \times 1 0^{- 10}$ , $H X_{2} PO X_{4} X^{-}$ : $0.0071$ . Counting convention: a species is a good leaving group if the pKa of its conjugate acid is below 0 (pKa $= - lo g_{10} K_{a}$ ). Give the number of good leaving groups as an integer.

Numerical answer type, tolerance 0.5.

Answer: 3

Compute pKa for each: NO3- -1.38, Cl- -7.00, CH3SO3- -1.90, F- 3.17, CH3COO- 4.74, CF3COO- 0.23, CN- 9.21, H2PO4- 2.15 (acid listed under the anion formula). Only pKa values below 0 qualify, namely

NO X_{3} X^{-}

Cl X^{-}

and

CH X_{3} SO X_{3} X^{-}

; trifluoroacetic acid has pKa +0.23, so

CF X_{3} COO X^{-}

does not. The count is 3.

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Answer: 3

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