Isobutylene 2-Methylpropene

In an early electron diffraction investigation of the structure of 2-methylpropene (isobutylene), Bartell and Bonham (1960) found that the three terminal carbon atoms are arranged in an almost perfect equilateral triangle around the central carbon despite the considerable difference in the single and double bond lengths (Figure 5.4). This result led Bartell to suggest that the terminal carbon atoms are close-packed around the central carbon atom. He then... 

Isobutene (2-methylpropene, isobutylene) [115-11-7] M 56.1, b -6.6 /760mm. Dry isobutene by passage through anhydrous CaS04 at 0°. Purify it further by freeze-pump-thaw cycles and trap-to-trap distillation. [Beilstein 1IV 796.]... 

Rubber cement contains a polymer of 2-methylpropene (isobutylene) called polyisobutylene. Write an equation for the polymerization reaction. 

Polyisobutylene is the butyl in butyl rubber, one of the first synthetic rubber substitutes. Most inner tubes are a copolymer of 2-methylpropene (isobutylene) and 2-methyl-1,3-butadiene (isoprene). 

Starting with 2-methylpropene (isobutylene) and using any other needed reagents, outline a synthesis of each of the following (a) I (b) I (c) I (d) I... 

Isoamyl acetate see 3-Methylbutyl ethanoate Isoamyl alcohol see 3-Methylbutan-l-ol heta.-Isoamylene see 2-Methylbut-2-ene Isohutane see 2-Methylpropane Isohutanoic acid see 2-Methylpropanoic acid Isohutanol see 2-Methylpropan-l-ol Isobutene see 2-Methylpropene Isobutyl acetate see 2-Methylpropyl ethanoate Isobutyl alcohol see 2-Methylpropan-l-ol Isobutylbenzene see (2-Methylpropyl)benzene Isobutyl chloride see l-Chloro-2-methylpropane Isobutylene see 2-Methylpropene Isobutylene oxide see 2,2-Dimethyloxirane Isobutyl ethanoate see 2-Methylpropyl ethanoate Isobutyl methyl ketone see 4-Methylpentan-2-one Isobutyraldehyde see 2-Methylpropanal Isobutyric acid see 2-Methylpropanoic acid Isocyanic acid butyl ester see Butyl isocyanate Isohexane see 2-Methylpentane Isooctane see 2,2,4-Tiimethylpentane Isopentane see 2-Methylbutane Isopentanol see 3-Methylbutan-l-ol Isopentyl alcohol see 3-Methylbutan-l-ol Isoprene see 2-Methylbuta-l,3-diene Isopropanol see Propan-2-ol Isopropenylbenzene... 

When the addition of hydrogen halides to alkenes was first studied systematically in the 1930s, chemists observed that the addition of HBr sometimes gave Markovnikov addition and sometimes gave non-Markovnikov addition. These two modes of addition of HBr are illustrated for 2-methylpropene (isobutylene). 

Write a mechanism for the polymerization of 2-methylpropene (isobutylene) initiated by treatment of 2-chloro-2-phenylpropane with SnCl. Label the initiation, propagation, and termination steps. 

Addition of HX to an unsymmetrical alkene could, in principle, give two different products. The ionic reaction between hydrogen bromide (HBr) and 2-methylpropene (isobutylene [(CH3)2CH=CH2]), where both l-bromo-2-methylpropane (isobutyl bromide [BrCH2CH(CH3)2]) and 2-bromo-2-methylpropane (r-butyl bromide [(CH3)3CBr]) could have formed is shown in Equation 6.26. 

To the extent that the picture (Scheme 6.29) represents what is actually happening in the reaction it should be clear that both hydrogen bromide (HBr) and 2-methylpropene [isobutylene, (CH3)2C=CH2] are present in the rate-determining transition state. Therefore, both of the reactants should be represented in the expression for the rate (Equation 6.33) and, experimentally, this can be determined by measuring the rate at some specific temperature at which product forms (or either starting material disappears) as a function of the concentrations of the individual reactants. For the Equation 6.33, et seq as usual, the expressions in brackets are concentration terms (mol L" ) ko is observed rate constant and the exponents n and m are experimentally determined values whose sum (n -i- m) is the order of the reaction. 

With substituted alkenes such as 2-methylpropene [isobutylene, (CH3)2C=CH2)] and a catalyst such as sulfuric acid under anhydrous conditions, long polymer chains can be formed. In this way 2-methylpropene [isobutylene, (CH3)2C=CH2] forms a sticky polymer (po/yisobutylene) with a very high molecular weight. 

Scheme 6.33. A representation of the path of the dimerization of 2-methylpropene [isobutylene (CH3)2C=CH2] to form a carbocation that loses a proton (H ) to give the two alkenes, (path a) 2,4,4-trimethyl-2-pentene [(CH3)3CH=C(CHj)2] and (path b) 2,4,4-trunethyl-l-pentene [(CH3)3CCH2C(CH3)=CH2].The catalytic (Pt.Ni.etc.) reduction (hydrogenation) of the alkene mixture yields 2,2,4-trimethylpentane [isooctane, (CH3)3CCH2CH(CH3)2].

Scheme 8.24. The reaction of 2-methylpropene (isobutylene [H2C=C(CH3)2]) with phosphoric acid to produce the f-butyl cation followed by the reaction of the latter with phenol (hydroxybenzene). Substitution occurs in the para-position. The resulting 4-(l-methylethyl) hydroxybenzene (4-f-butylphenol) is rednced with hydrogen (H2) in the presence of a platinum catalyst to produce ( )-(or trans-) r-butylcyclohexanol [( )- or (frons)-4-(l-methylethyl) cyclohexanol)].

---