_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main Groups IV and V
achieved e.g.
MeSi(OMe),
PhMgBr
PhMeSi(OMe) 2
NpMgBr
( ± )-NpPhMeSi(OMe)
1- )MenOH
( ± )NpPhMeSiO(- )Men
racemic mixture diastereoisomers
of enantiomers
(- )MenOH = (- )menthol: Np = 1-naphthyl.
In this way a racemic mixture of the two enantiomers of NpPhMeSi(OMe) was
obtained. This molecule is chiral on account of its having four different groups
arranged tetrahedrally around the silicon atom. The diastereoisomeric menthoxides
were separated by fractional crystallization at low temperature. Compounds
ofthis type ha ve been used to study mechanisms of substitution at silicon centres.
In industry, organosilicon compounds are made by the 'direct synthesis'
(p. 109) or by hydrosilation (p. 24). Alloy methods (p. 19) are used in the
manufacture of the antiknock compounds Me 4 Pb and Et 4 Pb. Antiknock compounds
are blended into gasoline (petrol) to resist uneven combustion or
detonation in spark ignition engines. Aromatic and branched aliphatic hydrocarbons
are more resistant to 'knock' than straight chain paraffins. In the early part
of this century it was impracticable to increase the content of knock resistant
hydrocarbons in petrol by refining. In 19 2 O Midgley discovered that the addition
of small amounts of Et 4 Pb effected a similar improvement. It was soon found that
it was also necessary to add dichloro- and dibromoethane as scavengers to
convert lead oxides formed in combustion into the more volatile dihalides.
Tetramethyllead, which has a higher vapour pressure than the tetraethyl.
becomes more evenly distributed between cylinders in multi-cylinder engines. It
is therefore more suitable for use with low grade petrols which contain
components with a wide range of octane numbers.
In the manufacture of Me 4 Pb or Et 4 Pb, lead-sodium alloy reacts with the
chloroalkane
4PbNa + 4RCI ------> R 4 Pb + 3Pb + 4NaCl
The organolead product is separated by steam distillation and the metallic lead
residue refined in a furnace for reuse. The alloy is prepared by mixing weighed
quantities ofmolten lead and sodium under a blanket ofnitrogen. The hot molten
alloy is transferred to a flaking machine where it is cooled and stripped into solid
flakes. The process provides a good example of industrial practice where as many
components as possible are produced on one site, which is placed next to an oi!
refinery. Electrolysis ofmolten salt yields sodium metal and chlorine (Downs cel!).
Hydrogen and chlorine are produced by electrolysis ofbrine in a mercury cel! and
then burned together to yield hydrogen chloride. Caustic soda is sold as a byproduct.
Ethylene from the refinery is converted into ethyl chloride and
dichloroethane by the action of HCl and chlorine respectively.
In recent years there bas been growing awareness of the hazards from lead in
the environment. The permitted amount of lead in petrol is gradually being
reduced, although the UK is lagging behind some other countries such as
Germany and Japan in this regard.
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