3- Chloro-2-methyl*2-butene

With acyclic and alicyclic ketones regiocheraical complications can arise from alternative modes of enolization. With acyclic compounds there is the additional complication of geometrical isomers. Direction of enolization is usually in accord with thermodynamic stability thus butan-2-one gives ( )-80 and (Z)-81 forms of 3-chloro-2-methyl-2 butenal (Eq. 75). Chain branching at the a or p carbon... 

Exercise 10-6 Addition of chlorine to frans-2-butene in ethanoic acid (acetic acid, CH3C02H) as solvent gives 74% meso-2,3-dichlorobutane, 1, 24% 2-chloro-1-methyl-propyl ethanoate, 2, and 2% 3-chloro-1-butene, 3. (Note 2 is formed as a d,l pair, although only one enantiomer is shown here.)... 

Silver ethanoate in ethanoic acid with 1-chloro-2-butene gives 65% 2-butenyl ethanoate and 35% 1-methyl-2-propenyl ethanoate. 

Ethanoate ion (1M) in ethanoic acid with 1-chloro-2-butene gives a mixture of about 85% 2-butenyl ethanoate and 15% 1-methyl-2-propenyl ethanoate. About 36% of the overall reaction product results from a process that is zero order in ethanoate ion, whereas 64% comes from a process that is first order in ethanoate ion. 

When 3-chloro-l-butene was pyrolyzed in a static system at temperatures 776-835°K, the primary product was methane and some higher molecular weight hydrocarbons37. This result was ascribed to the initial dissociation of the substrate into chloroalkyl and methyl radicals followed by H abstraction (equations 6-9). 

Problem 14.20 Describe simple chemical tests (if any) that would distinguish between (a) ethylene bromohydrin and ethylene bromide (b) 4-chloro-1-butene and / -butyl chloride (c) bromocyclohexane and bromobenzene (d) 1-chloro-2-methyl-2-propanol and l,2-dichloro-2-methylpropane. Tell exactly what you would do and see. 

It is interesting to compare the hydrolysis of butadiene oxide (10) with those of 1-chloro-2-butene and 3-chloro-l-butene.38 l-Chloro-2-butene (19a) solvolyzes in water to yield 45% of 2-butene-l-ol (22a) and 55% of 3-butene-2-ol (23a) (Scheme 7). 3-Chloro-l-butene (20a) also solvolyzes to yield alcohols 22a and 23a, but in a different ratio (34% 66%, respectively). The lifetime of the allylic carbocation intermediate 21a is therefore not sufficient to allow dissociation of chloride ion and equilibration of solvent about the carbocation. However, hydrolyses of 19b and 20b (R = CH3 instead of H) yield identical ratios of allylic alcohol products 22b and 23b (15% 85%, respectively).38 Allylic cation 21b, with a second methyl group to stabilize positive charge, must have a longer lifetime, which allows departure of the leaving group and equilibration of solvent about the carbocation. 

Nickel-catalyzed stereospecific cyclocarbonylation of cWo-cyclopentadiene dimer 3 with ( )-l -chloro-2-butene, carbon monoxide and tetracarbonylnickel in acetone/water at room temperature gives a-methyl-3-oxotetracyclo[5.5.1.02,6.08,12]tridec-9-ene-4-aceticacid (4)in 10% yield51. An X-ray investigation of the acid reveals a selective cis-exo attack at the strained double bond, while the other double bond does not react. The exo dimer of cyclopentadiene gives a different product, thus there is no isomerization, in contrast to reactions under acidic conditions. 

Hamill, Guarino, and Ronayne (II) gamma irradiated 0.18 mole % benzyl chloride in glassy 2-methyltetrahydrofuran (MTHF) at liquid nitrogen temperature and obtained a maximum ultraviolet absorption band at 320 m/x in agreement with Porter and Strachan, see Table I. They also irradiated 1.0 mole % allyl chloride, allyl bromide and allyl alcohol in 3-methylpentane (3-MP) and in all cases observed a maximum absorption band at 228 m/x which they attributed to the allyl free radical. They also irradiated 3-chloro-1-butene and 3-chlorocyclohexene in 3-MP and determined the wavelengths of the absorption band maxima of the 1-methyl allyl and 2-cyclohexen-l-yl free radicals given in Table I. 

A fundamentally similar situation is obtained with the initiation of styrene by alkylaluminum halides or trialkylaluminum compounds. In the case of this monomer the demarcation between poor and good coinitiators is perhaps even more evident than with isobutylene. Fig. 2 shows the essential data obtained in a series of experiments in which various alkyl chlorides were added to styrene/Al(C2H5)2Cl charges in methyl chloride 24). Alkyl halides with high R—Cl heterolytic bond dissociation energies are very poor coinitiators compared with those with lower dissociation energies. Thus 3-chloro-l-butene, crotyl chloride, fert-butyl chloride, 1-chloroethylbenzene and diphenyl chloromethane are efficient because the carbocations which arise from these chlorides... 

Butyn-l-ol Biacetyl Methyl acrylate l-Chloro-2-methyl-l-propene a-3-Chloro-2-buten-l-ol / -3-Chloro-2-buten-l-ol... 

A soln. of butyllithium in hexane injected by syringe at -78 under Ng into a stirred soln. of 7r-phenylsulfonyltricyclene in tetrahydrofuran-hexamethylphos-phoramide, allowed to warm to 0 during 0.5 hr., 3-methyl-l-chloro-2-butene added at -78 , and stirring continued 1 hr. while the temp, is allowed to rise... 

The ionization energies of the carbon-chlorine bond for (E)-l-chloro-2-butene and 3-chloro-2-methyl-l-propene in the gas phase are 672 and 706 kj mole", respectively. Using molecular orbital concepts, explain this energy difference based on the stability of the carbocations. 

Both compounds give allyl carbocations. One of the resonance contributors for the carbocation from (E)-l-chloro-2-butene has its positive charge at a secondary carbon atom. In terms of molecular orbital theory, the methyl group is at the end of an allyl system, where it affects the stability of the MO. The two resonance contributors for the carbocation from 3-chloro-2-methyl-l-propene are both primary. In terms of molecular orbital theory, the methyl group is bonded to the center carbon of an allyl system. There is a node at the C-2 of the Tt MO, and the methyl group cannot stabilize the carbocation. 

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... 

The first order rate constant for ethanolysis of the allylic chloride 3 chloro 3 methyl 1 butene is over 100 times greater than that of tert butyl chloride at the same temperature... 

Because the positive charge m an allylic carbocation is shared by two carbons there are two potential sites for attack by a nucleophile Thus hydrolysis of 3 chloro 3 methyl 1 butene gives a mixture of two allylic alcohols... 

The same two alcohols are formed m the hydrolysis of 1 chloro 3 methyl 2 butene... 

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