Page 26 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
6
whereas Brønsted basicity refers to the thermodynamic affinity for protons, nucleophilicity
in organic chemistry typically refers to the rate of attack on a carbon center. Moreover,
in this book, we will talk about nucleophilic attacks on pretty much any p-block element!
Understandably, therefore, you should not expect more than a rough correlation between
the nucleophilicity of a nucleophile and its basicity.
To better appreciate this point, let us go back to Table 1.1, which lists a number of
nucleophiles in increasing order of n , an arbitrarily defined measure of nucleophilic-
CH 3 I
ity. Observe that the basicities of the nucleophiles, as indicated by the pK values of their
a
conjugated acids, increase in a general but highly erratic way with the value of n .To
CH 3 I
illustrate, whereas tertiary phosphines are more nucleophilic than tertiary amines by about
two orders of magnitude, the former are less basic than the latter by roughly the same factor.
What factors then make for a good nucleophile? The following trends have been observed:

• Anions are better nucleophiles than the related neutral molecules. Thus:
–
–
RO > ROH; RS > R S; NH 2 – > NH 3
2
where R = H, alkyl, or aryl.
• For analogous species in a given period, nucleophilicity decreases as one progresses
to the right of the periodic table:

NH > H O; R P > R S
3 2 3 2
The more electronegative elements hold on to their electrons more tightly.
• Nucleophilicity increases as one goes down a given group of the periodic table. Thus,
for example
–
–
PR > NR ; PhSe > PhS > PhO –
3
3
The larger atoms are less electronegative and the anions derived from them are more
polarizable, which translates to increasing nucleophilicity as one goes down a group.
Given that electronegativity and size (atomic radius) are the two key determinants of
nucleophilicity, it’s useful to remind ourselves how the two atomic properties vary across
the periodic table. Figure 1.4 presents Pauling electronegativities and Figure 1.5 the atomic
radii of the s- and p-block elements. Note that electronegativity increases from left to right
along a given period, and decreases down a group. Atomic radii shrink from left to right in
a given period and increase down a group.
Against this backdrop, the relative nucleophilicities of the halide anions make for some-
what of a puzzle. The Swain–Scott nucleophilicities (Tables 1.1 and 1.2), based on methanol
as solvent, are in the order:
– – – –
I > Br > Cl > F
The same order is found in other protic solvents. This is also the order expected on the basis
of polarizability: the larger and more polarizable anions should be the most nucleophilic.
In polar aprotic solvents (e.g., DMSO, DMF, THF, etc.), however, the relative rates are
completely reversed:
– – – –
F ≫ Cl > Br > I