Internal Olefins, Organoaluminum Compounds Give Alpha Olefins
Forms Ethylene
146TH
ACS
NATIONAL
MEETING
Organic C h e m i s t r y
CYCLOTRON. Dr. H. Ache (left) and Dr. A. P. Wolf get set to mount a target in Brookhaven's 60-inch cyclotron. The target contains the compound that will be irradiated to produce in situ carbon-11 used in studying insertion reactions
ucts. Cyclopropane, cyclobutane, and cyclopentane yield very little ethylene ( 1 % or less ). In aliphatic compounds, the yield of ethylene is directly proportional to the relative number of primary carbon-hydrogen bonds. There are no primary carbon-hydrogen bonds in simple cycloalkanes. This may mean that neither of the reaction paths proposed for the impact of carbon atoms (carbon-hydrogen bond insertion or a replacement-type reaction) can account for ethylene formation. Either of these reaction paths might show a deuterium isotope effect. The deuterium isotope effect, ranging from 1.12 to 1.17, is found in the formation of acetylene. It is not found in the formation of ethylene. Furthermore, if an intermediate radical were a precursor of ethylene, then much greater sensitivity to the presence of oxygen would have been observed, Dr. Wolf says. Thus, he adds, the simplest explanation of the ethylene yield is the insertion of methyne into a primary carbon-hydrogen bond, followed by a decay of the excited intermediate. The methyne would be one of the molecular fragments formed by the recoiling carbon atom. Dr. Wolf suggests that the methyne has an energy so high that the difference in energy between breaking a carbon-hydrogen bond and a carbondeuterium bond is not important. Carbon-11. Dr. Wolf and Dr. Stocklin distinguish reaction products from starting product with C 11 . The
isotope is convenient to work with in spite of its short life; accompanying radiation damage to the organic system can be minimized. Analysis is carried out by gas-liquid chromatographic effluent counting techniques. The Brookhaven workers use the C 1 2 ( n , 2 n ) C n nuclear reaction and the C 1 2 ( p , p n ) C n reaction to produce C 1 1 in the hydrocarbon mixtures. The C 1 2 ( n , 2 n ) C u reaction is brought about by neutrons produced in a lithium hydride target bombarded with deuterons. The C 1 2 ( p , p n ) C u reaction is brought about by protons from the Brookhaven cosmotron. The N 1 4 n ( p , a ) C reaction can also be applied when the hydrocarbon is used in admixtures with nitrogen. The beams of high-energy particles can cause the production of carbon atoms in organic media. The carbon atoms, recoiling from the nuclear reaction, dissipate their high kinetic energy by colliding with surrounding atoms and molecules. Dr. Wolf and Dr. Stocklin used a variety of acyclic and alicyclic alkanes as the surrounding molecules. The major products obtained from Cx to C 5 alkanes are acetylene-C 11 , ethylene-C 11 , propylene-C 11 , other unsaturated hydrocarbons, and saturated hydrocarbons. The radiochemical yield of ethylene-C 11 varies from 17% (percentage of total radiocarbon) for ethane or neopentane to about 10% for n-pentane. Alicyclic hydrocarbons from C 8 to Cr; show negligible yields (equal to or less than 1%).
Alpha olefins and primary alcohols can be made from internal olefins via organoaluminum compounds, according to Dr. Guenter Bruno, now at Mobil Chemical, Metuchen, N.J. Isomerization of alkylaluminum compounds can be carried out when an organoaluminum is dissolved in an internal olefin. Earlier workers had thought alkylaluminum compounds are too unstable to be used for isomerization. For example, Dr. A. J. Rutkowski of Enjay Laboratories, Linden, N.J., had turned to organoboron compounds instead (C&EN, April 15, 1963, page 72). Dr. Bruno (in work he did at Henkel & Cie, G.m.b.H., Dusseldorf, West Germany, before joining Mobil Chemical) has found that there is an isomerization reaction in organoaluminum chemistry that's similar to the isomerization process previously found in secondary organoboron compounds. The value of these isomerizations lies in their ability to convert internal double bonds to terminal, or alpha, functional groups. Internal olefins are easier to make from paraffins, but alpha olefins are more valuable. Two Ways. Dr. Bruno uses a class of organoaluminum intermediates called di-seoalkylaluminum hydrides. These are synthesized two ways: One is by a displacement reaction of trialkylaluminum compounds or diisobutylaluminum hydride with various straight-chain, internal olefins; the second synthesis is the direct reaction of aluminum with hydrogen and internal olefins. The same reactions but conducted with alpha olefins as starting materials \vere discovered by Nobel Laureate Karl Ziegler and coworkers at Max Planck Institute of Coal Research, Mulheim, West Germany. The organoaluminum reactions were generally used with alpha olefins in JAN.
2 7, 1 9 6 4
C&EN
47
Disec-Tridecylaluminum Hydride Acts as Intermediate Between 6-Tridecene and 1-Tridecanol or «Tridecene ^^(CH^CH-CHCcHOsCH 6-Tridecene
Either direct synthesis with AI + 1V4-H., or displacement
S e C — C i * f-L/r
reaction with diisobutylalummum hydr.de
S β£, - f \ - U * * £ 7 internal olefins. The double bond can be anywhere in the chain, but 2alkenes were excluded from the study. In direct synthesis, the olefins and aluminum metal and hydrogen react. In displacement reactions, the olefins replace primary, lower-molecular-weight trialkyl groups on aluminum atoms. For example, 6-tridecene replaces isobutyl groups on triisobutylaluminum, and 13-heptacosene displaces propyl groups from tri-??-propylaluminum. These reactions form di-sec-alkylaluminum hydrides. They do not proceed to trialkylaluminum compounds. The di-sec-alkylaluminum hydrides either decompose (if unstabilized) or isomerize to n-alkylaluminum compounds when heated. Main Products. By stabilizing the .sec-alkyl compounds. Dr. Bruno is able to get tri-n-alkylaluminum compounds as the main products of the over-all reaction. During the isomerization, one mole equivalent of the internal olefin solvent reacts. It is converted to an n-alkyl attached to the aluminum. Dr. Bruno feels that the addition of olefin to the hydride does not take place until at least one of the two secondary groups that had been attached to the aluminum is isomerized to an n-alkyl group. The di-sec-alkylaluminum hydrides have been isolated and identified, however. Di-sec-tridecylaluminum hydride, for instance, is an oily, colorless liquid. Using the displacement
a-Tridecene
reaction, he obtains yields of about 95% from 6-tridecene. To make the intermediate organoaluminum compounds used to prepare primary alcohols or alpha olefins from internal olefins, Dr. Bruno uses a one-step process. It results in predominantly tri-n-alkylaluminum compounds in internal olefin solution. Both the displacement or direct syntheses proceed with isomerization in internal olefin solution between 150° and 240° C. Primary alcohols are produced when the highly reactive tri-η-alkylaluminum compounds are oxidized with air, and the resulting aluminum alcoholates hydrolyzed with dilute acid. Yields of 60 to 7 5 % of theoretical of primary alcohols can be obtained. Alpha olefins result when isomerized intermediate alkylaluminum com pounds are pyrolized. The alpha ole fins can be obtained 97 to 99% pure, although a crude reaction product (about 80 to 85% isomerized organo aluminum compounds) is used. Dr. Bruno uses a two-step pyrolysis. At an external temperature of 180° C. at 10~ 4 mm., the isomerized second ary alkylaluminum compounds yield a precut of a distillation product which contains about 50% alpha olefin. Isomerized di-seononylaluminum hy dride yields 54% α-nonene, for in stance. Isomerized di-sec-dodecylaluminum hydride yields 47% adodecene. In the second stage, at an external temperature of up to 240° C. at 10~4 to 0.3 mm., α-nonene of 99% and α-dodecene of 98% purity were ob tained from the same reaction mixtures.
