Page 430 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 4.4. Hardness and Softness of Some Common Ions and Molecules 411
Bases (Nucleophiles) Acids (Electrophiles) SECTION 4.2
Structural and Solvation
–
–
Soft RSH, RS , I , R 3 P I 2 , Br 2 , RS X, RSe X, RCH 2 X Effects on Reactivity
– C N, :C O , RCH CHR Cu(I), Ag(I), Pd(II), Pt(II), Hg(II)
+
–
zero-valent metal complexes
benzene
–
–
Intermediate Br , N 3 , ArNH 2 Cu(II), Zn (II), Sn,(II)
+
pyridine R 3 C , R 3 B
Hard + +
NH 3 , RNH 2 H X, Li , Na , R 3 Si X
–
–
–
H 2 O, HO , ROH, RO , RCO 2 , Cl – Mg(II), Ca(II), Al(III), Sn(IV), Ti(IV)
– – +
F , NO 3 H
reflects a high charge density and is associated with more electronegative elements.
The hard nucleophile–hard electrophile combination implies an early TS with electro-
static attraction being more important than bond formation. For hard bases, the reaction
pathway is chosen early on the reaction coordinate and primarily on the basis of charge
distribution. Examples of hard bases from Table 4.3 are F and CH O . Table 4.4
−
−
3
classifies some representative chemical species with respect to softness and hardness.
Numerical values of hardness were presented in Table 1.3.
Nucleophilicity is also correlated with oxidation potential for comparisons
between nucleophiles involving the same element. 29 Good nucleophilicity correlates
with ease of oxidation, as would be expected from the electron-donating function
of the nucleophile in S 2 reactions. HSAB considerations also suggest that nucle-
N
ophilicity would be associated with species having relatively high-energy electrons.
Remember that soft species have relatively high-lying HOMOs, which implies ease of
oxidation.

4.2.2. Effect of Solvation on Nucleophilicity
The nucleophilicity of anions is very dependent on the degree of solvation.
Many of the data that form the basis for quantitative measurement of nucleophilicity
are for reactions in hydroxylic solvents. In protic hydrogen-bonding solvents, anions
are subject to strong interactions with solvent. Hard nucleophiles are more strongly
solvated by protic solvents than soft nucleophiles, and this difference contributes to
the greater nucleophilicity of soft anions in such solvents. Nucleophilic substitution
reactions of anionic nucleophiles usually occur more rapidly in polar aprotic solvents
than they do in protic solvents, owing to the fact that anions are weakly solvated in
such solvents (see Section 3.8). Nucleophilicity is also affected by the solvation of the
cations in solution. Hard cations are strongly solvated in polar aprotic solvents such
as N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), hexamethylphos-
phoric triamide (HMPA), N-methylpyrrolidone (NMP), N,N-dimethylpropyleneurea

29
M. E. Niyazymbetov and D. H. Evans, J. Chem. Soc., Perkin Trans. 2, 1333 (1993); M. E. Niyazymbetov,
Z. Rongfeng, and D. H. Evans, J. Chem. Soc., Perkin Trans. 2, 1957 (1996).