Ziegler syntheses

The Ziegler syntheses are chemical-technical processes in the field of organic chemistry . They are named after the German chemist Karl Ziegler (1898–1973). They can be divided into:

• the Ziegler direct method,
• the Ziegler-Alfen synthesis and
• the Ziegler-Alfol synthesis.

Important by-products of the synthesized compounds are, for example, detergents and personal care products.
In 1953, Ziegler noticed that trialkylaluminum compounds could be used to prepare even-numbered oligomers from alkenes. Today, polyolefins (e.g. polyethylene ) are mainly produced technically using this process.

Ziegler direct method

The Ziegler direct process is used to synthesize triethylaluminum. This is done using a cycle process in two reaction steps. In the first step, the desired product triethylaluminum is even used as a starting material, which reacts with aluminum and hydrogen to form dieethylaluminium hydride. This reaction is called augmentation, and there is a rearrangement of ethyl groups. The reaction equation is:

In the second step of the direct process, diethyl aluminum hydride is reacted with ethene and triethyl aluminum is formed again. This step is called attachment and is described by the following reaction equation:

If the two reactions are considered together, there is an increase in triethylaluminum and the gross equation of the process results as:

Ziegler build-up reaction

The following figure shows the mechanism of the Ziegler build-up reaction based on the insertion of ethene:

The reaction takes place as a controlled chain growth reaction in which several ethene units are "pushed" piece by piece into the alkyl radicals of the triethylaluminum ( 1 ). The chain growth continues until the displacement reaction begins. (see Ziegler-Alfen reaction) The chain length is between 10 and 20 carbon atoms. If either propene or butene is used instead of ethene, the displacement reaction starts after the dimerization.

Ziegler – Alfen reaction

The following mechanism shows the displacement reaction already mentioned in the above section in a two-stage variant:

The trialkylaluminum molecules ( 1 ) formed in the Ziegler synthesis reaction continue to be reacted with ethene, but at significantly higher temperatures. This creates the transition state ( 2 ). In the next step, a hydrogen atom is rearranged and the longer-chain alkyl radical is split off. This process is implemented in the industry in two different ways.
The first variant is a two-stage variant in which the reaction takes place in two different reactors. In this case, the catalyst can continue to be used by processing.
In another variant, the conversion takes place in one step. Only catalytic amounts of triethylaluminum are used here and the temperature is selected to be higher during the chain build-up. The yield of unbranched α-olefins is increased in this way. In this variant, the catalyst cannot be used again, but is destroyed.

Ziegler – Alfol synthesis

The mechanism below shows the reaction of the Ziegler-Alfol synthesis following the Ziegler structure reaction:

The first step in this synthesis is a conversion to aluminum alkoxides ( 2 ). The decisive factor here is that strongly dried air is used, as otherwise many by-products such as esters, ethers, acids and aldehydes appear and the yield drops. In a second step, saponification takes place with water, whereby the desired alcohols ( 3 ) and aluminum hydroxide are formed.

Individual evidence

1. ↑ Jürgen Falbe, Manfred Regitz (ed.): Römpp Chemie Lexikon, 9th edition, Georg Thieme Verlag, Stuttgart 1990, ISBN 3-13-735109-X , p. 5129.
2. ↑ ^{a b c d} Klaus Weissermel, Hans-Jürgen Arpe: Industrial Organic Chemistry, 5th Edition, Wiley-VCH Verlag GmbH, Weinheim 1998, ISBN 3-527-28856-2 , pp. 232-233.
3. ^ ^{A b} Klaus Weissermel, Hans-Jürgen Arpe: Industrial Organic Chemistry, 5th Edition, Wiley-VCH Verlag GmbH, Weinheim 1998, ISBN 3-527-28856-2 , pp. 83-85.