An organic chemist compiles a list of anionic species to precisely evaluate their leaving group abilities in nucleophilic substitution reactions: (i) (ii) (iii) (iv) (v) (acetate) (vi) (vii) (triflate

Question

An organic chemist compiles a list of anionic species to precisely evaluate their leaving group abilities in $S_N2$ nucleophilic substitution reactions:
(i) $F^-$ 
(ii) $Cl^-$ 
(iii) $Br^-$ 
(iv) $I^-$ 
(v) $CH_3COO^-$ (acetate)
(vi) $OH^-$ 
(vii) $CF_3SO_3^-$ (triflate)
How many of these anions are considered strictly better leaving groups than the bromide ion ($Br^-$) under standard polar aprotic conditions?

Prashant Kotian · Accepted Answer

Correct answer: 2. A good leaving group is a weak, stable base (the conjugate base of a strong acid). The order of basicity among halide ions is . Therefore, the leaving group ability is the reverse: . Among the halides, only is a better leaving group than . Looking at the oxygen-based leaving groups, and are relatively strong bases and poor leaving groups. However, the triflate ion () is exceptionally stable due to extensive resonance across three oxygen atoms and the powerful effect of the group. It is the conjugate base of triflic acid (a superacid), making it a much better leaving group than even the halides. Thus, the two species better than are and .

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