Page 78 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE s-BLOCK ELEMENTS: ALKALI AND ALKALINE EARTH METALS
58
We have emphasized the s-block metals’ overriding tendency to lose their valence elec-
trons. When the energetics is right, however, alkali metals can also accept an electron
−
to yield alkalide anions, M , with two electrons in the valence s orbitals. Once again, a
cryptand does the trick:

O O O O
+ −
2 Na + N N N Na N + Na (2.25)

O O O O
O O O O

Although we won’t comment on the point, we urge you to pause a moment and appreciate
−
−
the remarkable role of the cryptand. Sodide or natride (Na ), potasside or kalide (K ),
−
−
−
rubidide (Rb ), and ceside (Cs ) are all known species; lithide (Li ), however, is not.
REVIEW PROBLEM 2.4
Would you expect lithide to be significantly more unstable relative to the other
alkalides? Explain.

2.5 ORGANOLITHIUM AND ORGANOMAGNESIUM COMPOUNDS

Lithium and magnesium react with alkyl halides (RX, X ≠ F) to yield alkyllithium (RLi)
and alkylmagnesium (RMgX) compounds, respectively:

2Li + RX → LiR + LiX (2.26)
Mg + RX → RMgX (2.27)

Mechanistically, the same kind of process is believed to be involved for both metals; but
here we arbitrarily choose to discuss the magnesium case. In the first step, the metal is
believed to transfer a single electron to the alkyl halide, cleaving the latter into an alkyl
radical and a halide ion:
+ −
R Br (2.28)
Mg Mg + R + Br

A second electron then reduces the alkyl radical to the carbanion stage, that is, effectively
an organometallic compound:

+ +
Mg R Mg R
(2.29)
− +
Mg
Br Mg R Br Mg R