8.7 Nucleophiles and Nucleophilicity

1. 8.7Nucleophiles and Nucleophilicity

2. – – – .. – .. .. : : : : : N C HS HO CH3O .. .. .. Nucleophiles The nucleophiles described in Sections 8.1-8.6have been anions. etc.

3. – – – .. – .. .. : : : : : N C HS HO CH3O .. .. .. .. CH3OH HOH .. .. Nucleophiles The nucleophiles described in Sections 8.1-8.6have been anions. etc. Not all nucleophiles are anions. Many are neutral. .. : for example NH3

4. – – – .. – .. .. : : : : : N C HS HO CH3O .. .. .. .. CH3OH HOH .. .. Nucleophiles The nucleophiles described in Sections 8.1-8.6have been anions. etc. Not all nucleophiles are anions. Many are neutral. .. : for example NH3 All nucleophiles, however, are Lewis bases.

5. .. CH3OH HOH .. .. Nucleophiles Many of the solvents in which nucleophilic substitutions are carried out are themselvesnucleophiles. .. for example

6. Solvolysis The term solvolysis refers to a nucleophilic substitution in which the nucleophile is the solvent.

7. Solvolysis substitution by an anionic nucleophile • R—X + :Nu— • R—Nu + :X— solvolysis + R—Nu—H + :X— R—X + :Nu—H step in which nucleophilicsubstitution occurs

8. Solvolysis substitution by an anionic nucleophile • R—X + :Nu— • R—Nu + :X— solvolysis + R—Nu—H + :X— R—X + :Nu—H products of overall reaction R—Nu + HX

9. CH3 CH3 CH3 + : : R : R : O O O .. H H Example: Methanolysis Methanolysis is a nucleophilic substitution in which methanol acts as both the solvent andthe nucleophile. –H+ + R—X The product is a methyl ether.

10. O O O O Typical solvents in solvolysis solvent product from RX water (HOH) ROH methanol (CH3OH) ROCH3 ethanol (CH3CH2OH) ROCH2CH3 formic acid (HCOH) acetic acid (CH3COH) ROCH ROCCH3

11. Nucleophilicity is a measure of the reactivity of a nucleophile • Table 8.4 compares the relative rates of nucleophilic substitution of a variety of nucleophiles toward methyl iodide as the substrate. The standard of comparison is methanol, which is assigned a relativerate of 1.0.

12. Table 8.4 Nucleophilicity • Rank Nucleophile Relative rate • strong I-, HS-, RS- >105 • good Br-, HO-, 104 • RO-, CN-, N3- • fair NH3, Cl-, F-, RCO2- 103 • weak H2O, ROH 1 • very weak RCO2H 10-2

13. Major factors that control nucleophilicity • basicity • solvation • small negative ions are highly solvated in protic solvents • large negative ions are less solvated • polarizability

14. Table 8.4 Nucleophilicity • Rank Nucleophile Relative rate • good HO–, RO– 104 • fair RCO2– 103 • weak H2O, ROH 1 When the attacking atom is the same (oxygenin this case), nucleophilicity increases with increasing basicity.

15. Major factors that control nucleophilicity • basicity • solvation • small negative ions are highly solvated in protic solvents • large negative ions are less solvated • polarizability

16. Figure 8.4 Solvation of a chloride ion by ion-dipole attractiveforces with water. The negatively charged chlorideion interacts with the positively polarized hydrogensof water.

17. Table 8.4 Nucleophilicity • Rank Nucleophile Relative rate • strong I- >105 • good Br- 104 • fair Cl-, F- 103 A tight solvent shell around an ion makes itless reactive. Larger ions are less solvated thansmaller ones and are more nucleophilic.

18. Major factors that control nucleophilicity • basicity • solvation • small negative ions are highly solvated in protic solvents • large negative ions are less solvated • polarizability

19. Table 8.4 Nucleophilicity • Rank Nucleophile Relative reactivity • strong I- >105 • good Br- 104 • fair Cl-, F- 103 More polarizable ions are more nucleophilic thanless polarizable ones. Polarizability increases with increasing ionic size.