Page 22 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
2
d-Block
p-Block
s-Block
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Lr Rf Db Sg Bh Hs Mt Ds Rg Cn

Figure 1.1 The periodic table: group numbers and the s, p, and d blocks.

is, how you might approach a given mechanistic problem. In the course of our mechanis-
tic survey, we’ll encounter a number of so-called hypervalent p-block compounds, which
you may not have encountered until now. These call for a brief discussion of the bonding
involved, which we will present in Sections 1.24–1.27. That said, we will not cover some
of the more elementary aspects of structure and bonding theory, including the very useful
VSEPR (valence shell electron pair repulsion) model; feel free to go back to your general
or organic chemistry text for a quick refresher.

1.1 NUCLEOPHILES AND ELECTROPHILES: THE S 2 PARADIGM
N
In this book, we will be overwhelmingly concerned with polar or ionic mechanisms. These
involve the movement of electron pairs, unlike radical reactions which involve unpaired
electrons. The components of a polar mechanism can generally be classified as nucleo-
philes or electrophiles. A nucleophile (“nucleus-lover”) is typically an anion or a neutral
molecule that uses an electron pair to attack another atom, ion, or molecule. The species
being attacked is called an electrophile (“electron-lover”). The terms “nucleophile” and
“electrophile” often refer to the classic S 2 reaction of organic chemistry. In the example
N
below (which happens to be a Williamson ether synthesis), the methoxide anion is the
nucleophile, methyl iodide is the electrophile, and iodide is the leaving group.

H H H H
− −
C O C I H C C H + I (1.1)
H H H O H
H H

A key feature of the S 2 reaction is that the nucleophile attacks from the “back side”
N
relative to the leaving group, leading to an umbrella-like inversion of the carbon undergoing