8.7 Nucleophiles and Nucleophilicity TABLE 8.4 Nucleophilicity of Some Common Nucleophiles Reactivity class Nucleophile Relative reactivity* Very good nucleophiles Good nucleophiles Br, Ho, RO,CN, N3 Fair nucleophiles NH3, CI, F, RCO Weak nucleophile H2O, ROH Very weak nucleophile RCoH Relative reactivity is nucleophile)k(methanol) for typical SN2 reactions and is approximate Data ethanol as the solvent RO is more nucleophilic than ROH RCO is more nucleophilic than RCOH Carboxy late ion As long as the nucleophilic atom is the same, the more basic the nucleophile, the more reactive it is. An alkoxide ion(Ro is more basic and more nucleophilic than a carboxylate ion(RCO 2) is more nucleophilic than RO Weaker base Conjugate acid is ROH: Conjugate acid is RCO2H e connection between basicity and nucleophilicity holds when comparing atoms in the same row of the periodic table. Thus, HO is more basic and more nucleophilic than F, and H3 N is more basic and more nucleophilic than H2O. It does not hold when proceeding down a column in the periodic table. For example, I is the least basic of the halide ions but is the most nucleophilic. F is the most basic halide ion but the least nucleophilic. The seems most responsible for the inverse relationship between basicity and nucleophilicity among the halide ions is the degree to which they are sol- vated by hydrogen bonds of the type illustrated in Figure 8.4. Smaller anions, because of their high charge-to-size ratio, are more strongly solvated than larger ones to act as a nucleophile, the halide must shed some of the solvent molecules that it. Among the halide anions, F forms the strongest hydrogen bonds to water and alco- hols, and I the weakest. Thus, the nucleophilicity of F is suppressed more than that of CI, Cl more than Br and Br more than I. Similarly, Ho is smaller, more sol vated, and less nucleophilic than HS Nucleophilicity is also related to polarizability, or the ease of distortion of the elec- A descriptive term applied to tron"cloud"surrounding the nucleophile. The partial bond between the nucleophile and a highly polarizable species the alkyl halide that characterizes the SN2 transition state is more fully developed at a longer distance when the nucleophile is very polarizable than when it is not. An increased nucleophile. Conversely degree of bonding to the nucleophile lowers the energy of the transition state and izable and is said to be hard nucleophile Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.7 Nucleophiles and Nucleophilicity 313 As long as the nucleophilic atom is the same, the more basic the nucleophile, the more reactive it is. An alkoxide ion (RO) is more basic and more nucleophilic than a carboxylate ion (RCO2 ). The connection between basicity and nucleophilicity holds when comparing atoms in the same row of the periodic table. Thus, HO is more basic and more nucleophilic than F, and H3N is more basic and more nucleophilic than H2O. It does not hold when proceeding down a column in the periodic table. For example, I is the least basic of the halide ions but is the most nucleophilic. F is the most basic halide ion but the least nucleophilic. The factor that seems most responsible for the inverse relationship between basicity and nucleophilicity among the halide ions is the degree to which they are sol￾vated by hydrogen bonds of the type illustrated in Figure 8.4. Smaller anions, because of their high charge-to-size ratio, are more strongly solvated than larger ones. In order to act as a nucleophile, the halide must shed some of the solvent molecules that surround it. Among the halide anions, F forms the strongest hydrogen bonds to water and alco￾hols, and I the weakest. Thus, the nucleophilicity of F is suppressed more than that of Cl, Cl more than Br, and Br more than I. Similarly, HO is smaller, more sol￾vated, and less nucleophilic than HS. Nucleophilicity is also related to polarizability, or the ease of distortion of the elec￾tron “cloud” surrounding the nucleophile. The partial bond between the nucleophile and the alkyl halide that characterizes the SN2 transition state is more fully developed at a longer distance when the nucleophile is very polarizable than when it is not. An increased degree of bonding to the nucleophile lowers the energy of the transition state and RO is more nucleophilic than Stronger base Conjugate acid is ROH: Ka  1016 (pKa  16) RCO O X Weaker base Conjugate acid is RCO2H: Ka  105 (pKa  5) RO is more nucleophilic than Alkoxide ion ROH Alcohol RCO is more nucleophilic than O X Carboxylate ion RCOH O X Carboxylic acid TABLE 8.4 Nucleophilicity of Some Common Nucleophiles Reactivity class Very good nucleophiles Good nucleophiles Fair nucleophiles Weak nucleophiles Very weak nucleophiles I , HS, RS Br, HO, RO, CN, N3 NH3, Cl, F, RCO2 H2O, ROH RCO2H Nucleophile 105 104 103 1 102 Relative reactivity* *Relative reactivity is k(nucleophile)/k(methanol) for typical SN2 reactions and is approximate. Data pertain to methanol as the solvent. A descriptive term applied to a highly polarizable species is soft. Iodide is a very soft nucleophile. Conversely, fluoride ion is not very polar￾izable and is said to be a hard nucleophile. 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