Alkenes 2-methylpropene

The first step of acid-catalyzed ether cleavage is protonation of the ether oxygen to give an intermediate oxonium ion, which collapses to form an alcohol and a tertiary carbocation. The carbocation then loses a proton to form an alkene, 2-methylpropene. This is an example of E elimination. The acid used for cleavage is often trifluoroacetic acid. 

The alkene 2-methylpropene (isobutene), CH2C(CH3)CH3 or CH2C(CH3)2, is used to make many other substances, including the gasoline additive MTBE and the antioxidant BHT (Figure 17.5). 

Chapter 5 describes elimination reactions and their mechanisms. In one example, heating tert- miy bromide in ethanol gives the alkene 2-methylpropene by a two-step mechanism ... 

Scheme 10.17. A representation of the thermal decomposition of a-chloro-A,A-dimethylamine with bond fragmentation to produce A,A-dimethylamine [(CH3)2NH], methanal (formaldehyde, H2C=0), and an alkene (2-methylpropene).The process is called the Hofinann-Lotfler-Freytag reaction (see Hofman, A. W. Chem. Ber., 1883,16, 558 and Loffler, K. Freytag, C. Chem. Ber., 1909, 42, 3427).

FIGURE 7.71 The elimination and SnI reactions compete. In pure ethyl alcohol, the ratio of alkene (2-methylpropene) to SnI product (/fr/-butyl ethyl ether) is 0.25. In the presence of the much stronger base sodium ethoxide, this ratio increases to about 13. A strong base (ethoxide) favors the elimination reaction. 

ANSWER Well, there isn t one (yet). At this point in your study of organic chemistry, you have no way to make this simple alcohol. If we start from the obvious alkene, 2-methylpropene, the hydration reaction we learned in this chapter (p. 380) can only give the product of Markovnikov addition, / r/-butyl alcohol.This nonanswer to such a simple question points out how limited our synthetic skills are so far. 

Alkenes resemble alkanes m most of their physical properties The lower molecular weight alkenes through 4 are gases at room temperature and atmospheric pressure The dipole moments of most alkenes are quite small Among the 4 isomers 1 butene cis 2 butene and 2 methylpropene have dipole moments m the 0 3-05 D range trans 2 butene has no dipole moment Nevertheless we can learn some things about alkenes by looking at the effect of substituents on dipole moments... 

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

Although 2 methylpropene undergoes acid catalyzed hydration m dilute sulfuric acid to form tert butyl alcohol (Section 6 10) a different reaction occurs m more concentrated solutions of sulfuric acid Rather than form the expected alkyl hydrogen sulfate (see Sec tion 6 9) 2 methylpropene is converted to a mixture of two isomeric C Hig alkenes... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

On the basis of the mechanism of cationic polymerization predict the alkenes of molecu lar formula C12H24 that can most reasonably be formed when 2 methylpropene [(CH3)2C=CH2] IS treated with sulfunc acid... 

The ending ene is adopted for straight-chain monounsaturated hydrocarbons. Thus, butenes refer to 1-butene and 2-butene. The en.6m. jlene denotes a monounsaturated hydrocarbon that consists of the same number of carbons as expressed by the name ie, butylenes are 1-butene, 2-butene, and isobutylene (methylpropene). The generic names alkenes and olefins refer to monounsaturated hydrocarbons. 

How does H initially add to the alkene Does it behave as an electrophile A nucleophile A neutral atom (radical) Step through the sequence of frames depicting addition of the H end of HCl to 2-methylpropene (CIH+ 2-methylpropene). Plot both the charge on H and on Cl (vertical axis) vs. frame number (horizontal axis). Do the charges change significantly during addition or remain constant Why ... 

Is the stable cation that formed as a result of protonation of the more electron-rich end of the alkene Examine electrostatic potential maps for propene, 2-methylpropene and 2-methyl-2-butene. For each, can you tell whether one end of the 7t bond is more electron rich than the other end If so, does protonation on the more electron-rich end lead to the more stable carbocation ... 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

Reaction of 2-methylpropene with CH3OH in the presence of H2SO4 catalyst yields methyl tert-butyl ether, CP OQCHT, by a mechanism analogous to that of acid-catalyzed alkene hydration. Write the mechanism, using curved arrows for each step. 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Self-Test 18.3A (a) Name the alkene (CH3),CHCH=CH2 and (b) write the condensed structural formula for 2-methylpropene. 

A similar reaction occurs between alkenes and acylium ions, as in the reaction between 2-methylpropene, and the acetylium ion leads regiospecifically to (3,y-enones.54 A concerted mechanism has been suggested to account for this regiochemical preference. 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Protonation of alkenes yields carbocations, as we have seen, and in the absence of other effective nucleophiles (e.g. HaO, p. 187) these ions can act as electrophiles towards as yet unprotonated alkene (c/. p. 108), e.g. with 2-methylpropene (41) ... 

The first-formed cation (42) can add to a second molecule of 2-methylpropene (41) to yield the new (dimeric) cation (43) this in turn can lose a proton to yield the C8 alkene (44) or, alternatively, add to a third molecule of alkene to yield the (trimeric) cation (45), and so on. 

Methylpropene can be made to continue the process to yield high polymers—cationic polymerisation—but most simple alkenes will go no further than di- or tri-meric structures. The main alkene monomers used on the large scale are 2-methyIpropene (— butyl rubber ), and vinyl ethers, ROCH=CH2 (— adhesives). Cationic polymerisation is often initiated by Lewis acid catalysts, e.g. BF3, plus a source of initial protons, the co-catalyst, e.g. traces of HzO etc. polymerisation occurs readily at low temperatures and is usually very rapid. Many more alkenes are polymerised by a radical induced pathway, however (p. 320). 

These studies proved that the reactions of 22 with butadienes and propenes take place both regioselectively and stereoselectively and are accelerated by electron-donating groups on propenes and butadienes (e.g., 2-methylpropene in relation to propene) and retarded by increasing bulkiness of substituents in 1,4- or 1,3-positions. As in the case of alkenes and silenes, the reactions of 22 occur in a concerted way and are HOMO (dienes or enes)-LUMO (dienophiles or enophiles) controlled.31 However, some small differences are observed between germene 22 and the analogous... 

Better results are obtained if the alkyl halide is primary, in case of secondary and tertiary alkyl halides, elimination competes over substitution, if a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CHsONa with (CHsJaC-Br gives exclusively 2-methylpropene. 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

In practice, short-chain alkanes and alkenes are normally used as feedstock for shape-selective catalytic formation of isooctanes at relatively low temperatures. Until the 1980s, lead alkyls (Section 18.1) were added to most automotive fuels to help suppress engine knock, but they have been phased out in North America because of the chronic toxicity of lead and lead compounds. The most commonly used nonlead antiknock additive is now methyl tert-butyl ether [MTBE CH30C(CH3)3], which is made by the reaction of methanol with 2-methylpropene, (CHs C—CH2 (see Section 7.4). The latter is obtained by catalytic cracking of petroleum fractions to give 1-butene, which is then shape-selectively isomerized on zeolitic catalysts. 

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