Isobutyl rearrangement

One drawback to Fnedel-Crafts alkylation is that rearrangements can occur espe cially when primary alkyl halides are used For example Friedel-Crafts alkylation of benzene with isobutyl chloride (a primary alkyl halide) yields only tert butylbenzene... 

In radical reactions not involving bromine or chlorine on the substrate, rearrangements are much rarer One example is the fluorination of di-tert butyl ketone which produces perfluormated / rt-buty isobutyl ketone [J5] Although isolated yields are poor only the rearranged ketone could be isolated This is perhaps only the second example of a 1,2-acyl shift Low fluorine substrate ratios show that this rearrangement occurs after monofluorination... 

Carbocation rearrangements occur in the reactions of some secondary alco hols with DAST, thus isobutyl alcohol gives a mixture of isobutyl fluoride and tert-hxAy] fluonde [95] (Table 6), and both bomeol and isoborneol rearrange to the same 3-fluoro-2 2,3-tnraethylbicyclo[2 2 IJheptane (72-74%) accompanied by camphene [95]... 

The isobutyl cation spontaneously rearranges to the tart-butyl cation by a hydride shift. Is the rearrangement exergonic or endergonic Draw what you think the transition state for the hydride shift might look like according to the Hammond postulate. 

A loss of 55 is possibly the loss of C4H7 from esters (double hydrogen rearrangement). The loss suggests a butyl or isobutyl group, especially when m/z 56 is also present. 

For some tertiary substrates, the rate of SnI reactions is greatly increased by the relief of B strain in the formation of the carbocation (see p. 366). Except where B strain is involved, P branching has little effect on the SnI mechanism, except that carbocations with P branching undergo rearrangements readily. Of course, isobutyl and neopentyl are primary substrates, and for this reason they react very slowly by the SnI mechanism, but not more slowly than the corresponding ethyl or propyl compounds. 

It was shown that no rearrangement of isobutyl radical to tert-butyl radical (which would involve the formation of a more stable radical by a hydrogen shift) took place during the chlorination of isobutane. 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

The behaviour of the butyl system provides important information on the nature of the intermediate formed during the rearrangement of the isobutyl to the 2-butyl cation. Thus, from the observation that isobutyl chloride yields n-butane which has exchanged one proton with the acid, while the solvolysis of 2-butyl chloride in the same acid (2% HjO), yields unexchanged n-butane one might deduce that an intermediate was formed during the former s solvolysis which exchanged one proton with the acid before it converted to a secondary butyl ion. A reasonable mechanism is shown in Scheme 1. 

Because salicylic acid contains the deactivating meta-directing carboxyl group, Friedel-Crafts reactions are generally inhibited. This effect is somewhat offset by the presence of the activating hydroxyl group. Salicylic acid reacts with isobutyl or /-butyl alcohol in 80 wt % sulfuric acid at 75°C to yield 5-/-butylsalicylic acid [16094-31-8], In the case of isobutyl alcohol, the intermediate carbonium ion rearranges to (CH3)3C+. 

Refluxing a mixture of (lS, 4S, 7RS, 8R, 9R )-7-hydroxy-8-isobutyl-9-methyl-6-azatricy-clo[6.1.1.04-9]decane-6-carbaldehyde (5) and 95% formic acid for 12 hours gave a 1 1 mixture of (E)- and (Z)-(l/ , 4S,, 7S, 8/ )-9-isobutylidene-8-methyl-2-azatricyclo[5.2.1.04-8]decaiie-2-carbaldehyde (6) and the secondary formate (lR, 4S, 77 , 8R, 10S )-10-isobutyl-8-formy-loxy-7-methyl-2-azatricyclo[5.2.1.04,1°]decane-2-carbaldehyde (7) in 24% and 55% yield, respectively.87 The rearrangement is believed to involve the cyclobutyliminium ion, as shown.87... 

Alternatively a direct rearrangement of the isobutyl peroxyl radical may account for the products ... 

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

A set of rapidly equilibrating carbenium ions might account for the rearrangements and the label scrambling but this cannot be the correct explanation, for cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl systems all solvolyze much more rapidly than would be expected from model compounds. Thus, for example, the rate of solvolysis of cyclopropylcarbinyl tosylate is 10 times that of the solvent-assisted solvolysis of isobutyl tosylate.77 Cyclobutyl tosylate solvolyzes 11 times... 

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