2- Methyl-2-butene with hydrogen chloride

A-7. Give a mechanism describing the elementary steps in the reaction of 2-methyl-1-butene with hydrogen chloride. Use curved arrows to show the flow of electrons. 

A-10. The reaction of 3-methyl-l-butene with hydrogen chloride gives two alkyl halide products one is a secondary alkyl chloride and the other is tertiary. Write the structures of the products, and provide a mechanism explaining their formation. 

In Section 9.2, we looked at the reaction of symmetrically substituted 2,3-dimethyl-2-butene with hydrogen chloride (Fig. 9.2). In the formation of the carbocation, there was no choice to be made—only one cation could be formed. When the less symmetrical alkene 2-methyl-l-butene reacts with hydrogen... 

Thexylchloroborane-Dimethyl sulHde (1). This boranc can be prepared by treating thexylborane-dimethyl sulfide with hydrogen chloride or by hydroboration of 2,3-di-methyl-2-butene with monochloroborane-dimethyl sulfide. 

For example, 3,3-dimethyl-l-butene adds hydrogen chloride to give not only 2-chloro-3,3-dimethylbutane but also 2-chloro-2,3-dimethylbutane. The new, rearranged compound results from an alkyl shift—in this case the migration of a methyl group with its pair of electrons (Fig. 9.47). Why is this reaction called an alkyl shift, when by analogy to hydride shift it should be alkide shift We don t know, but it is ... 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

Bromination of isoprene using Br2 at —5 ° C in chloroform yields only /n j -l,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with one-third excess of isoprene at —15 ° C to form the 1,2-addition product, 2-chloro-2-methyl-3-butene (60). When an equimolar amount of HCl is used, the principal product is the 1,4-addition product, l-chloro-3-methyl-2-butene (61). The mechanism of addition is essentially all 1,2 with a subsequent isomerization step which is catalyzed by HCl and is responsible for the formation of the 1,4-product (60). The 3,4-product, 3-bromo-2-methyl-1-butene, is obtained by the reaction of isoprene with 50% HBr in the presence of cuprous bromide (59). Isoprene reacts with the reactive halogen of 3-chlorocyclopentene (62). 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

SAMPLE SOLUTION (a) Hydrogen chloride adds to the double bond of 2-methyl-2-butene in accordance with Markovnikov s rule. The proton adds to the carbon that has one attached hydrogen, chlorine to the carbon that has none. 

Bromination of isoprene using at —5 0 C in chloroform yields only 1,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with... 

Addition of hydrogen chloride to three typical alkenes is outlined below, with the two steps of the mechanism shown. In accord with Markovnikov s rule, propylene yields isopropyl chloride, isobutylene yields icrr-butyl chloride, and 2-methyl-2-butene yields rer/-pentyl chloride. 

The reaction of formaldehyde with alkenes is of industrial interest and has been extensively studied. Reaction of excess formaldehyde as formalin with an alkene and aqueous acid gives 1,3-dioxanes (2) in 40-90% yield. Reaction of 2-butenes with paraformaldehyde and hydrogen chloride at -65 C gives a mixture of diastereomeric y-chloro alcohols rich in the isomer formed by trans addition to the alkenyl double bond. For example frans-2-butene gives an 85 15 mixture of erythro- and fAreo-3-chloro-2-methyl-1-butanol (equation 2). Reaction of 1-alkenes under similar conditions gives 3-alkyl-4-chlorotet-rahydropyrans (5) in 50-80% yield (Scheme 2). Initial reaction occurs via addition of formaldehyde to the terminal carbon of the double bond, followed by loss of a proton to give the 3-alken-l-ol. Reaction of... 

LAPIS INFERNALIS (7761-88-8) A powerful oxidizer. Forms friction- and shock-sensitive compounds with many materials, including acetylene, anhydrous ammonia (produces compounds that are explosive when dry), 1,3-butadiyne, buten-3-yne, calcium carbide, dicopper acetylide. Contact with hydrogen peroxide causes violent decomposition to oxygen gas. Violent reaction with chlorine trifluoride, metal powders, nitrous acid, phosphonium iodide, red or yellow phosphorus, sulfur. Incompatible with acetylides, acrylonitrile, alcohols, alkalis, ammonium hydroxide, arsenic, arsenites, bromides, carbonates, carbon materials, chlorides, chlorosulfonic acid, cocaine chloride, hypophosphites, iodides, iodoform, magnesium, methyl acetylene, phosphates, phosphine, salts of antimony or iron, sodium salicylate, tannic acid, tartrates, thiocyanates. Attacks chemically active metals and some plastics, rubber, and coatings. 

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