The butylenes

For general reactions see olefins. The butylenes are used to prepare 2-butanol. I-Butene and isobutene are formed into widely used polymers. 

Table 2 presents other important physical properties for the butylenes (3). Thermodynamic and transport properties can also be obtained from other sources (4). 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Electrophile Addition. The addition of electrophilic (acidic) reagents HZ involves two steps the slow transfer of hydrogen ion from Z to the butylene to form a carbocation and, a rapid combination of the carbocation with the base Z. 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

Sulfuric acid is about one thousand times more reactive with isobutylene than with the 1- and 2-butenes, and is thereby very useful in separating isobutylene as tert-huty alcohol from the other butenes. The reaction is simply carried out by bubbling or stirring the butylenes into 45—60% H2SO4. This results in the formation of tert-huty hydrogen sulfate. Dilution with water followed by heat hydrolyzes the sulfate to form tert-huty alcohol and sulfuric acid. The Markovnikov addition implies that isobutyl alcohol is not formed. The hydration of butylenes is most important for isobutylene, either directiy or via the butyl hydrogen sulfate. 

Isomerization. Isomerization of any of the butylene isomers to increase supply of another isomer is not practiced commercially. However, their isomerization has been studied extensively because formation and isomerization accompany many refinery processes maximization of 2-butene content maximizes octane number when isobutane is alkylated with butene streams using HF as catalyst and isomerization of high concentrations of 1-butene to 2-butene in mixtures with isobutylene could simplify subsequent separations (22). One plant (Phillips) is now being operated for this latter purpose (23,24). The general topic of isomerization has been covered in detail (25—27). Isomer distribution at thermodynamic equiUbrium in the range 300—1000 Kis summarized in Table 4 (25). 

Table 6 compares the total production of butylenes in the United States, Western Europe, andjapan. Included in this table are relative amounts of productions from different processes. In the United States, about 92% of the butylene production comes from refinery sources, whereas only about 45% in Western Europe andjapan are from this source. This difference arises because the latter cracks mostiy petroleum distillates in the steam crackers that produce larger amounts of butylenes than the light feedstocks cracked in the United States. 

Among the butylenes, isobutylene has become one of the important starting materials for the manufacture of polymers and chemicals. There are ... 

The Cy and Cg paraffias comprise about 90% of the alkylate Cg accounts for over 60%. Over 70% of the commercial alkylation processes employ sulfuric acid as the catalyst. Among the butylenes, 2-butene is superior to 1-butene. The C —fraction from the catalytic crackers is considered to be a superior feedstock to the alkylation unit. 

The price of butanes and butylenes fluctuates seasonally depending on the demand for gasoline ia the United States. Siace much chemical-product usage is determiaed by price—performance basis, a shift to development of butylene-based technology may occur. Among the butylenes, demand for isobutylene is likely to iacrease (and so its price) as more derivatives such as methyl methacrylate and methacrylic acid are produced from isobutylene iastead of the coaveatioaal acetoae cyanohydria process. 

Polymers account for about 3—4% of the total butylene consumption and about 30% of nonfuels use. Homopolymerization of butylene isomers is relatively unimportant commercially. Only stereoregular poly(l-butene) [9003-29-6] and a small volume of polyisobutylene [25038-49-7] are produced in this manner. High molecular weight polyisobutylenes have found limited use because they cannot be vulcanized. To overcome this deficiency a butyl mbber copolymer of isobutylene with isoprene has been developed. Low molecular weight viscous Hquid polymers of isobutylene are not manufactured because of the high price of purified isobutylene. Copolymerization from relatively inexpensive refinery butane—butylene fractions containing all the butylene isomers yields a range of viscous polymers that satisfy most commercial needs (see Olefin polymers Elastomers, synthetic-butylrubber). 

Unsaturated Hydrocarbons. Olefins from ethylene through octene have been converted into esters via acid-catalyzed nucleophilic addition. With ethylene and propjiene, only a single ester is produced using acetic acid, ethyl acetate and isopropyl acetate, respectively. With the butylenes, two products are possible j -butyl esters result from 1- and 2-butylenes, whereas tert-huty esters are obtained from isobutjiene. The C5 olefins give rise to three j iC-amyl esters and one /-amyl ester. As the carbon chain is lengthened, the reactivity of the olefin with organic acids increases. 

The most important olefins used for the production of petrochemicals are ethylene, propylene, the butylenes, and isoprene. These olefins are usually coproduced with ethylene by steam cracking ethane, LPG, liquid petroleum fractions, and residues. Olefins are characterized by their higher reactivities compared to paraffinic hydrocarbons. They can easily react with inexpensive reagents such as water, oxygen, hydrochloric acid, and chlorine to form valuable chemicals. Olefins can even add to themselves to produce important polymers such as polyethylene and polypropylene. Ethylene is the most important olefin for producing petrochemicals, and therefore, many sources have been sought for its production. The following discusses briefly, the properties of these olefmic intermediates. 

The butylene feed stream is spht and fed into each of a series of perfectly mixed tanks (usually in one large vessel). This stepwise addition of butylene and the large excess of isobutane that is used both help to prevent undesirable reaction of butylene molecules with each other to form high-boiling, low octane polymers. Low temperature (40°F) also favors the desired iCJC reaction. 

The butylene isomers shown in Figure 1—10 add another degree of complexity because of the double bond. It is an easy mistake to go overboard in drawing isomers that have the same formula but appear to look different. But be careful, because molecules don t know left from right or front from back. What may look different on paper may be identical when rolled over in space. That s why the isobutylene in Figure 1—10 is drawn the way it is. If you try to attach that =CH2 group to some other carbon in the molecule, the whole thing becomes a normal butylene. 

Like the butane isomers, the butylenes each have their own properties that rnake them unique and of individual appeal to the petrochemical industry. 

Isobutylene is the most chemically reactive of the butylene isopiers. If the objective is just to get the isobutylene out of the C4 stream, it can be removed by reaction with methanol (CH3OH) to make MTBE (methyl tertiary butyl ether), by reaction with water to make TBA (tertiary butyl alcohol), by polymerization, or by solvent extraction. After that, butene-1 can be removed by selective adsorption or by distillation. That leaves the butene-2 components, together with iso- and normal butane, which are generally used as feed to an alkylation plant. 

Extractive distillation is used to remove butadiene from a C4 stream fractionation can be used to separate out butene-1 adsorption is also sometimes used to separate out butene-1 polymerization is sometimes used to pull out the isobutylene dehydrogenation can be used to convert some of the butylenes and normal butane to butadiene and alkylation is used to convert the butylenes to alkylate. 

BEA (Si/Al2 = 30), FAU (Si/Al2 = 8.6) and EMT (Si/Afi = 8.6) framework types were compared for i-butane/2-butylene alkylation. During the lifetime of the catalyst the butylene turnover number (TON) was approximately the same for each of the three zeolites and the acid sites were equivalent from the standpoint of stability in each case. With EMT the lowered selectivity to consecutive reaction products 2,2,4-TMP -I- 2,3,4-TMP relative to 2,2,3-TMP -i- 2,3,3-TMP and the lowered selectivity to heavies relative to BEA was interpreted as higher hydride transfer activity. 

The olefins—ethylene, propylene, and the butylenes—are derived from natural gas and petroleum. Methane is the major constituent in natural gas. The aromatics— benzene, toluene, and the xylenes— are derived from petroleum. About 90% by weight of the organic chemicals in the world comes from natural gas and petroleum. But actually only 3% of this crude oil and 6% of refinery output in the U.S. is processed into chemicals, with the rest going as various fuels. Although we are a small user of the petroleum industry, this 3-6% going to petrochemical feedstock is important to us ... 

Phosphoric acid may be used for the polymerization of all the gaseous olefins. Ethylene is converted to ethyl phosphoric acid at temperatures below 250°. At higher temperatures, the ester decomposes to yield conjunct polymer including isobutane. Propylene Undergoes either conjunct or true polymerization depending on whether the reaction temperature is above or below 300°. The butylenes undergo true polymerization chiefly. 

Table 2 presents other important physical properties for the butylenes other sources (4). 

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