Organic Chemistry

Section 10.3 Factors Affecting SN 2 Reactions 371

ethoxide ion

tert-butoxide ion

Nucleophilicity Is Affected by Steric Effects
Base strength is relatively unaffected by steric effects because a base removes a
relatively unhindered proton. The strength of a base depends only on how well the
base shares its electrons with a proton. Thus,tert-butoxide ion is a stronger base
than ethoxide ion since tert-butanol is a weaker acid than ethanol

Steric effects, on the other hand, do affect nucleophilicity. A bulky nucleophile
cannot approach the back side of a carbon as easily as a less sterically hindered nucle-
ophile can. Thus, the bulky tert-butoxide ion, with its three methyl groups, is a poorer
nucleophile than ethoxide ion even though tert-butoxide ion is a stronger base.

PROBLEM 6 SOLVED

List the following species in order of decreasingnucleophilicity in an aqueous solution:

SOLUTION Let’s first divide the nucleophiles into groups. We have one nucleophile

with a negatively charged sulfur, three with negatively charged oxygens, and one with a

neutral oxygen. We know that in the polar aqueous solvent, the one with the negatively

charged sulfur is the most nucleophilic because sulfur is larger than oxygen. We also know

that the poorest nucleophile is the one with the neutral oxygen atom. So now, to complete

the problem, we need to rank the three nucleophiles with negatively charged oxygens in

order of the ’s of their conjugate acids. A carboxylic acid is a stronger acid than

phenol, which is a stronger acid than water (Section 7.10). Because water is the weakest

acid, its conjugate base is the strongest base and the best nucleophile. Thus, the relative

nucleophilicities are

PROBLEM 7

For each of the following pairs of reactions, indicate which reaction occurs faster:

a.

b.

c.

d. CH 3 CH 2 Cl + I− or CH 3 CH 2 Br + I−

(in ethanol)

CH 3 CH 2 Cl + CH 3 O− or CH 3 CH 2 Cl + CH 3 S−

CH 3 CHCH 2 Br + HO− CH 3 CH 2 CHBr

CH 3

or + HO−

CH 3

CH 3 CH 2 Br + H 2 O or CH 3 CH 2 Br + HO−

SN 2

CH 3 S− > HO− >>>O− CH 3 CO− CH 3 OH

O

pKa

O− CH 3 OH HO− CH 3 CO− CH 3 S−

O

ethoxide ion

better nucleophile

tert-butoxide ion

stronger base

CH 3 CH 2 O− CH 3 CO−

CH 3

CH 3

1 pKa=15.9 2.

1 pKa= 182