Pentenal pyrolysis

Figure 6.1.27. Result fora Py-GC/MS analysis of a high M sample of poly(4-methyl-1-pentene). Pyrolysis done on 0.4 mg material at 600° C in He, with the separation on a Carbowax type column.

The purpose of this paper is to describe, briefly, several interrelated variables that affect the course of methyl pentene pyrolysis Initially, mechanistic aspects are described for the decomposition of methyl substituted pentenes to isoprene. Next, the high temperature degradation of isoprene and related by-product dienes will be discussed. Finally, the heterogeneous effects associated with reactor metallurgy are detailed. 

Kinetic studies have shown that methyl pentene pyrolysis occurs via a free radical process having chain lengths in the range of 10 (1 ) As a consequence, product distribution depends on the types of propagating radicals in the system as well as the parent methyl pentene structiare. 

Inspection of 2-methyl-l-pentene pyrolysis products indicates that fragmentations res ilting from hydrogen atom addition are increased over those... 

Since isoprene was the principal desired product from methyl pentene pyrolysis it was essential to determine its stability under reaction conditions. Isoprene degradation proceeds via second-order rate processes. Isoprene dimer and related compounds formed during the pyrolysis are dependent on the isoprene partial pressure in the reaction system. 

Esters. The monoisobutyrate ester of 2,2,4-trimethyl-1,3-pentanediol is prepared from isobutyraldehyde ia a Tishchenko reaction (58,59). Diesters, such as trimethylpentane dipelargonate (2,2,4-trimethylpentane 1,3-dinonanoate), are prepared by the reaction of 2 mol of the monocarboxyhc acid with 1 mol of the glycol at 150—200°C (60,61). The lower aUphatic carboxyHc acid diesters of trimethylpentanediol undergo pyrolysis to the corresponding ester of 2,2,4-trimethyl-3-penten-l-ol (62). These unsaturated esters reportedly can be epoxidized by peroxyacetic acid (63). 

Propylene Dimer. The synthesis of isoprene from propjiene (109,110) is a three-step process. The propjiene is dimeri2ed to 2-methyl-1-pentene, which is then isomeri2ed to 2-methyl-2-pentene in the vapor phase over siUca alumina catalyst. The last step is the pyrolysis of 2-methyi-2-pentene in a cracking furnace in the presence of (NH 2 (111,112). Isoprene is recovered from the resulting mixture by conventional distillation. 

Pentadiene has been prepared by the interaction of allyl bromide and vinyl bromide in the presence of magnesium and by the pyrolysis of 1,5-pentanediol diacetate or 4-penten-l-ol acetate. The present procedure is essentially that of Shoemaker and Boord with some modifications. ... 

If all the pentane is removed before pyrolysis, the bicyclo [2.1.0]pentane shows no impurities on vapor phase chromatography with a 20% Dow 710 on 50/60 U Anaprep column. Analysis by n.m.r. also revealed the absence of any traces of cyclo-pentene in the spectrum consisting of three complex multiplets at 0.3-0.8, 1.1-1.7, and 1.9-2.4 p.p.m. (downfield from internal tetramethylsilane reference). 

Isoprene (melting point -146°C, boiling point 34°C, density 0.6810) may be produced by the dehydrogenation of iso-pentane in the same plant used for the production of butadiene. However, the presence of 1,3-pentadiene (for which there is very little market) requires a purification step. One method produces isoprene from propylene. Thus, dimerization of propylene to 2-methyl-1-pentene is followed by isomerization of the 2-methyl-1-pentene to 2-methyl-2-pentene, which upon pyrolysis gives isoprene and methane. 

In the case that the CH2=CH substituent of 4-chloro-1-butene and the chlorine were separated by one or two methylene groups, i.e. in 5-chloro-l-pentene and 6-chloro-l-hexene (Table 18), the olefinic double bond did not provide assistance to the pyrolysis. In this respect, the formation of a three-membered conformation appeared to favor participation of the double bond in the transition state. 

The residue from the first distillation is a mixture of acetic acid, 4-penten-l-ol acetate, and 1,5-pentanediol diacetate. Another 15-35 g. (6-15%) of 1,4-pentadiene can be obtained by passing the residue through the pyrolysis tube under the conditions described above. 

The failure to observe cyclo-C4Hg in the pyrolysis or photolysis of 4-pentenal is evidence against the possibility of the cycloalkanes being formed by the subsequent decomposition of the unsaturated aldehydes . ... 

---