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

Aluminium. gallium. indium and thallium
At low temperatures (90-120'C), using ethene. insertion is very much faster
than transalkylation. Each of the alkyl chains grows by successive insertions of
ethene. The rates of insertion are essentially independent of chain length. A
Poisson type distribution* of chain lengths is produced in which the average
length depends on the number of ethene molecules added per metal atom.
In the Alfol process, the mixture of aluminium alkyls is converted into linear
primary alcohols by controlled oxidation followed by hydrolysis. The main use of
C 6 -C 10 alcohols is to make plasticizers such as diaJkylphthalates. C 12 -C 16 alcohols
are precursors to surfactants and biodegradable detergents such as alkyl
sulphates and alkyl sulphonates. Hydroformylation of alkenes provides an
alternative route to these materials.
At about 180°C in the presence of excess ethene. alkene insertion and
transalkylation take place at comparable rates. Under these conditions ethene is
converted into a mixture of 1-alkenes, the aluminium alkyl acting as a catalyst. A
geometrica! distribution of oligomers is produced.t This tends to give too much
butene. The aluminium alkyls are therefore sometimes mixed with this butene at
300oC/20 atm with very short contact times of about O. 5 s. This converts them
into tri-i-butylaluminium ( + 1-alkenes) which then reacts with ethene in a
separate reactor. The grown aluminium alkyls (now C 6 -C 10 ) are then passed
back to the transalkylation stage when they release the desired 1-alkenes.
:::::AtCH,CH,R + H,C=CHC,H, ___,:::::: AICH,CH,C,H, + RCH=CH 2
In the Shell Higher Olefins Process (SHOP) a much more efficient catalyst based
on transition metal compounds (probably nickel) is used to effect the oligomerization.
Initially about 30% of the ethene is converted into the desired C 10 -C 18
olefins. The remainder (C 4 -C 10 and above C 20 ) are isomerized to interna! alkenes
and then converted into more useful fractions by metathesis.
The initial association between an alkene and an aluminium alkyl may involve
interaction between the n-orbitals of the olefin and the Lewis acidic metal centre.
Intramolecular association ofthis type is indicated by the lowering in the v(C=C)
stretching frequency in Bu~AlCH 2 CH 2 CH 2 CH=CH 2 compared with that in its
ether complex or in the free alkene.
(C=C)/cm-1 1657 1635 1658
*The mole fraction XI' of M(CH,CH ,ilH/~'H, is given by X 1 , = (n~'e~"l/p! where n C,H 4 are
added to )-AICH,CH,.
tXp= {3/( 1 +fi)" where fi= k,/k,: k, rate constant f(Jr insertion. k, for transalkylation.
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