2- Methyl-2-butene protonation

A C5 synthetic unit which has ylide functionality and which is likewise accessible from 2-hydroxy-2-methyl-but-3-enal-dimethylacetal (27) has proved suitable particularly for the synthesis of apocarotenals. The copper-catalyzed reaction of (27) with triphenylphosphine (15) in the presence of aqueous acid leads to 4-triphenyl-phosphonium-2-methyl-buten-2-al (33). The bifunctional C5 ylenal (34), which is important for carotenoid syntheses, is formed therefrom with proton acceptors. 

By contrast, the 10 protons of 2-methyl-2-butene are not all equivalent. There are three different kinds of methyl-group protons and one vinylic proton, leading to four different possible substitution products and four different signals in the H NMR spectrum (Figure 13.14)... 

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 ... 

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 ... 

Draw Lewis structures for the possible carbocations resulting from protonation of the double bond in 3-methyl-1-butene, and decide which is favored. (Check your result using available energy data for C5H11 carbocations.) What would be the product of bromide addition to the more stable cation Is this the observed product ... 

The 1H NMR spectrum shown is that of 3-methyl-3-buten-l-ol. Assign all the observed resonance peaks to specific protons, and account for the splitting... 

The addition of the anions of racemic cyclic allylic sulfoxides to various substituted 2-cyclopentenones gives y-l,4-adducts as single diastereomeric products22. The modest yields were due to competing proton-transfer reactions between the anion and enone. The stereochemical sense of these reactions is identical to that for the 1,4-addition reaction of (Z)-l-(/erf-butylsulfinyl)-2-methyl-2-butene to 2-cyclopentenone described earlier. 

The 300 MHz H NMR and 20 MHz 13C NMR spectra of poly(4-methyl-l-pentene) have been found to be more complex than the corresponding spectra of poly(3-methyl-l-butene) due to the presence of an additional isomer structure in the polymer. Investigation of the 20 MHz 13C NMR spectrum of the polymer has indicated that placement of units in different triad sequences is die cause of multiple methyl proton resonances which have been observed in the H NMR spectra of poly(3-methyl-l-butene) and poly(4-methyl-l-pentene). The use of a computer program for simulating and plotting spectra has enabled measurements of polymer composition to be made of poly(4-methyl-l-pentene) s prepared under a variety of synthesis conditions. 

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. 

The modern view of HX addition is that H+ is transferred from HX to the alkene to give a carbocation. The major product is the one derived from the more stable carbocation. Compare the energies of 1-propyl and 2-propyl cations (protonated propene), 2-methyl-1-propyl and 2-methyl-2-propyl cations (protonated 2-methylpropene), and 2-methyl-2-butyl and 3-methyl-2-butyl cations (protonated 2-methyl-2-butene). Identify the more stable cation in each pair. Is the product derived from this cation the same product predicted by Markovnikov s rule Is the more stable carbocation also the one for which the positive charge is more delocalized Compare atomic charges and electrostatic potential maps for one or more pairs of carbocations. 

A proton is transferred to the terminal carbon atom of 2-methyl-1-butene so as to produce a... 

The carbocation formed by protonation of the double bond of 3,3-dimethyl-l-butene is secondary. Methyl migration can occur to give a more stable tertiary carbocation. 

In the presence of sulfuric acid, the carbon-carbon double bond of 2-methyl-1-butene is protonated and a carbocation is formed. 

This carbocation can then lose a proton from its CH2 group to form 2-methyl-2-butene. 

The protons of a single methyl group are equivalent to one another, but all three methyl groups of 2-methyl-2-butene are nonequivalent. The vinyl proton is unique. 

The 2-methyl-2-butene can be protonated by sulfuric acid to make a tertiary... 

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