Page 24 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
4
+ + + 2+
H , H 3 O, Li , Mg ,
BF , AlCl 3 , TlCl 3 ,
3
+
+
RX, R , RCO, R SiX, R 3 SnX, Pb(OAc) 4 ,
3
PCl 5 , Ph BiCl ,
3
2
SF 4 , SO 3 , SeCl 2 , SeO 2 ,
, , XeF
X 2 BrF 3 2
Some common electrophiles; X is a halogen. The electrophilic atoms are indicated in
Figure 1.3
green.

halides, the Si atom in silyl halides, molecular halogens, and even the fluorine atoms in
xenon difluoride (XeF ).
2
The ease with which a given S 2 displacement occurs depends on multiple factors, such
N
as the nucleophilicity of the incoming nucleophile (which depends on both its electronic
and steric character), steric hindrance at the electrophilic carbon center, the effectiveness of
the leaving group, and the solvent and other environmental effects. By defining a standard
substrate and standard reaction conditions, the reactivity of different nucleophiles may be
quantified. One such measure of nucleophilicity is the Swain–Scott nucleophilicity con-
stant n, for which methyl iodide is chosen as the standard substrate and reaction rates are
∘
measured in methanol at 25 C:
k Nu
n CH 3 I = log
k CH 3 OH

where k Nu is the rate constant for the nucleophile of interest (Nu) and k CH 3 OH is the rate
constant for methanol itself as the nucleophile. Table 1.1 lists n CH 3 I values for a number of
representative nucleophiles, along with the pK values of their conjugate acids (i.e., a mea-
a
sure of the basicity of the nucleophiles). Observe that there is only a very rough correlation
between n CH 3 I and the conjugate acid pK ; we’ll return to this point in the next section.
a
Table 1.2 presents a more qualitative characterization of some common nucleophiles,
classifying them from strong to very weak.
Table 1.1 shows that, for a given electrophile (CH I) and standard conditions, the rate
3
9
constants for common nucleophiles vary by a factor of well over a billion (10 ). This
tremendous variation of reactivity of the different nucleophiles might pose a conundrum
−
in relation to their synthetic utility. Note (from either Table 1.1 or 1.2) that alkoxide (RO )
4
3
anions are some 10 –10 times more nucleophilic than neutral alcohols, and the rates for
−
carboxylate anions (RCO ), relative to the un-ionized carboxylic acids, differ by even
2
5
6
more: 10 –10 . With such low rates, are alcohols and carboxylic acids, in their un-ionized
forms, at all useful as nucleophiles? The answer is a clear yes. In acidic media, many
of the anionic nucleophiles simply don’t exist; they are entirely protonated. Under such
conditions, weak nucleophiles such as alcohols and carboxylic acids react effectively with
cationic electrophiles such as carbocations. Second, although weaker nucleophiles may not
react at a useful rate with alkyl halides, many of them do react at perfectly acceptable rates