CHEMISTRY

Corresponding Member of the Academy of Sciences of the USSR Yu. G. MAMEDALIEV and M. R. MUSAEV

PREPARATION OF METHYLPENTADIENES BY DEHYDRATION OF OXIDATION PRODUCTS OF THE PROPYLENE DIMER

This communication presents experimental data on the preparation of hexadienes with conjugated bonds by dehydration of propylene dimer oxidates. At the same time, as our studies show, the proposed synthesis of alkadienes is of a more general character and may be regarded as a universal method for obtaining dienes from olefin hydrocarbons included in liquid and gaseous petroleum-processing products.

The polymerization reaction of propylene over solid phosphoric acid has wide industrial application in the production of alkylarylsulfonate-type detergents. In the polymerization of propylene, along with the formation of propylene trimers and tetramers, a considerable amount of propylene dimer is also formed. Operating practice at industrial propylene-polymerization units shows that the polymerization products contain up to 20% propylene dimer. By changing the polymerization conditions, the reaction can be directed toward the maximum formation of dimer.

The structure of the propylene dimer has been the subject of investigation by a number of authors (¹–⁵), who established the presence in it of hexenes of various structures. Without going into the details of disagreements among individual authors concerning the structure of the dimer, we shall point out that, according to the most reliable studies, the dimer consists mainly of 2-methylpentene-2, 3-methylpentene-2, and 4-methylpentene-2.

Hydrocarbons containing a double bond are readily oxidized at moderate temperatures in the liquid phase with formation of hydroperoxides (⁶–¹⁰); moreover, the most vulnerable site of oxidation is the carbon atoms located in the α-position to the double bond. In the case of the presence in the α-position of primary, secondary, and tertiary carbon atoms, as was to be expected, oxidation proceeds selectively in the following order: tertiary > secondary > primary.

In the presence of warm alkali, hydroperoxides rapidly decompose with liberation of oxygen and formation of unsaturated alcohols in yields up to 80% (⁸,⁹). Therefore, oxidation of olefins in the presence of alkali makes it possible partially to combine the reaction of hydroperoxide formation with the reaction of their decomposition, yielding unsaturated alcohols.

In the oxidation of unsaturated hydrocarbons in the presence of alkaline reagents, it has been established that other side reactions also occur, leading to the formation of oxides and small amounts of glycols, carbonyl compounds, and other oxygen-containing products.

The oxidation products of propylene dimers obtained by the above method were used by us for the first time as starting material for the synthesis of diene-type rubber monomers.

Experimental Part

The starting product for obtaining hexadienes was the oxidate obtained by oxidation of the propylene dimer with atmospheric oxygen at a temperature of 70–80°, a pressure of 15–20 atm, in the presence of a NaHCO₃ solution and small amounts of catalyst—manganese resinate—and an initiator—cumene hydroperoxide. According to the data of N. N. Vorozhtsov, B. D. Kruzhalov, L. A. Ivanova, and others (unpublished), who carried out the oxidation of the dimer for the purpose of the subsequent synthesis of hexyl and heptyl alcohols, under the optimum oxidation conditions the conversion of the hexenes is about 28–32%.

After decomposition of the peroxide compounds contained in it with alkali, the oxidate had the following characteristics: $d_4^{20}$ 0.8484; $n_D^{20}$ 1.4280, molecular weight 130; Engler distillation: initial b.p. 78°, 10% 89°, 20% 98°, 30% 105°, 40% 114°, 50% 125°, 60% 135°, 70% 149°, 80% 171°, 90% 225°, end of boiling 234°.

In accordance with the boiling ranges of the hexene oxides, unsaturated alcohols, and glycols, the oxidate was separated by rectification into hexene oxide fractions, hexenol fractions, and glycol fractions (Table 1).

Table 1

The fractions corresponding to the oxides and to the unsaturated alcohols, both separately and together, were subjected to dehydration over alumina. At temperatures of 350–400° and a rate of 0.5–2 volumes of oxidate per volume of catalyst per hour, 80–82% of catalyzate is formed, which corresponds to the theoretical yield of dienes from the unsaturated alcohols and oxides. The results of individual experiments carried out at a rate of 0.5 l per 1 l of catalyst per hour are summarized in Table 2.

Table 2

As is evident from the data in Table 2, the content of the 60–80° fraction in the catalyzate fluctuates within the range 92–95%. The analyses carried out showed that the fractions boiling within the range 60–80° contain 70–80% diene hydrocarbons.

The formation of dienes from 2-methylpentene-2, 3-methylpentene-3, and 4-methylpentene-2, which are the principal constituents of propylene dimer, through stages involving the formation both of hydroperoxides and unsaturated alcohols, and of hexene oxides, may be represented in the form of the following two schemes:

1. Formation of hexadienes through stages involving the formation of hydroperoxides and unsaturated alcohols

[
\begin{array}{ccc}
\ce{CH3-C( CH3 )=CH-CH2-CH3} &
\ce{CH3-CH=C( CH3 )-CH2-CH3} &
\ce{CH3-CH=CH-C( CH3 )H-CH3} \
\downarrow & \text{Oxidation of hexenes with formation of hydroperoxides} & \downarrow \
\ce{CH3-C( CH3 )=CH-CH(OOH)-CH3} &
\ce{CH3-CH=C( CH3 )-CH(OOH)-CH3} &
\ce{CH3-CH=CH-C(OOH)(CH3)-CH3} \
\downarrow & \text{Decomposition of hydroperoxides with formation of hexenols} & \downarrow \
\ce{CH3-C( CH3 )=CH-CH(OH)-CH3} &
\ce{CH3-CH=C( CH3 )-CH(OH)-CH3} &
\ce{CH3-CH=CH-C(OH)(CH3)-CH3} \
\downarrow & \text{Dehydration of hexenols} & \downarrow \
\ce{CH3-C( CH3 )=CH-CH=CH2} &
\ce{CH3-CH=C( CH3 )-CH=CH2} &
\ce{CH3-CH=CH-C( CH3 )=CH2}
\end{array}
]

2. Formation of hexadienes through stages involving the formation of oxides and unsaturated alcohols

[
\begin{array}{ccc}
\ce{CH3-C( CH3 )=CH-CH2CH3} &
\ce{CH3-CH=C( CH3 )-CH2-CH3} &
\ce{CH3CH=CH-CH( CH3 )-CH3} \
\downarrow & \text{Oxidation of hexenes with formation of oxides} & \downarrow \
\ce{CH3-C( CH3 )-CH-CH2-CH3} &
\ce{CH3-CH-C( CH3 )-CH2-CH3} &
\ce{CH3-CH-CH-CH( CH3 )-CH3} \
\multicolumn{1}{c}{\ce{\ \ \backslash O /}} &
\multicolumn{1}{c}{\ce{\ \ \backslash O /}} &
\multicolumn{1}{c}{\ce{\ \ \backslash O /}} \
\downarrow & \text{Isomeric transformation of oxides into hexenols} & \downarrow \
\ce{CH2=C( CH3 )-CH(OH)-CH2-CH3} &
\ce{CH2=CH-C(OH)(CH3)-CH2-CH3} &
\ce{CH2=CH-CH(OH)-CH( CH3 )-CH3} \
\downarrow & \text{Dehydration of hexenols with formation of hexadienes} & \downarrow \
\ce{CH2=C( CH3 )-CH=CH-CH3} &
\ce{CH2=CH-C( CH3 )=CH-CH3} &
\ce{CH2=CH-CH=C( CH3 )-CH3}
\end{array}
]

As is evident from the schemes presented, the successive conversion of the hexenes that make up the propylene dimer, both in the first and in the second case, leads to the formation of three methylpentadienes with a conjugated bond system, namely: 2-methylpentadiene-1,3, 3-methylpentadiene-1,3, and 4-methylpentadiene-1,3.

The catalyst obtained by us was subjected, in the laboratory of the Scientific Research Institute of Synthetic Rubber, with the kind assistance of B. A. Dolgoplosk, to emulsion copolymerization with butadiene and styrene, yielding latex and rubber of the required technical properties.

Thus, we have developed a new process for producing alkadienes with conjugated double bonds from unsaturated hydrocarbons.

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