Ferf-Butyl bromide

A remarkably rigid molecule in which the ring rotation is locked up to 100 °C was found in the form of Co(Bu2Cp)(PMe3)2 (11) [36]. This complex was synthesized as a brown oil by electrophilic addition of ferf-butyl bromide to Co(BuCp)(PMe3)2 followed by deprotonation of similarly rigid [Co(Bu2Cp)-... 

Radical addition/cross-coupling products 61 were obtained in 60-91% yield when Ni(dppf)Cl2 was applied as a catalyst in reactions of alkyl halides 60 with 2,3-disubstituted dienes 59 and aryl Grignard or arylzinc reagents (Fig. 12). Competition experiments of n-, sec-, and ferf-butyl bromide with 2,3-dimethylbutadiene... 

It lies completely on the side of the hthio-aromatic compound and of n- or ferf-butyl bromide. This is because the C atom of the C—Li bond is sp2 hybridized in the hthio-aromatic compound and therefore more electronegative than the sp3-hybridized C atom of the C—Li bond in n- or ferf-BuLi. More electronegative C atoms stabilize C—Li bonds because these are very electron-rich. 

Figure 6-6 shows the SN2 reaction of hydroxide ion with ethyl bromide (1°), isopropyl bromide (2°), and ferf-butyl bromide (3°). The nucleophile can easily approach the electrophilic carbon atom of ethyl bromide. In isopropyl bromide, the approach is hindered, but still possible. In contrast, SN2 approach to the tertiary carbon of ferf-butyl bromide is impossible because of the steric hindrance of the three methyl groups. Make models of ethyl bromide, isopropyl bromide, and ferf-butyl bromide, and compare the ease of bringing in an atom for a back-side attack. 

When ferf-butyl bromide is placed in boiling methanol, methyl ferf-butyl ether can be isolated from the reaction mixture. Because this reaction takes place with the solvent acting as the nucleophile, it is called a solvolysis (solvo for solvent, plus lysis, meaning cleavage ). 

This solvolysis is a substitution because methoxide has replaced bromide on the tert-butyl group. It does not go through the SN2 mechanism, however. The SN2 requires a strong nucleophile and a substrate that is not too hindered. Methanol is a weak nucleophile, and ferf-butyl bromide is a hindered tertiary halide—a poor SN2 substrate. 

If this substitution cannot go by the SN2 mechanism, what kind of mechanism might be involved An important clue is kinetic Its rate does not depend on the concentration of methanol, the nucleophile. The rate depends only on the concentration of the substrate, ferf-butyl bromide. 

When ferf-butyl bromide is heated with an equal amount of ethanol in an inert solvent, one of the products is ethyl ferf-butyl ether. 

The E2 dehydrohalogenation gives excellent yields with bulky secondary and tertiary alkyl halides, such as ferf-butyl bromide in the preceding example. A strong base forces second-order elimination (E2) by abstracting a proton. The molecule s bulkiness hinders second-order substitution (SN2), and a relatively pure elimination product results. Tertiary halides are the best E2 substrates because they are prone to elimination and cannot undergo Sn2 substitution. 

If 14 or more carbons are present, the product may be diamantane or a substituted diamantane. " These reactions are successful because of the high thermodynamic stability of adamantane, diamantane, and similar diamond-like molecules. The most stable of a set of C H isomers (called the stabilomer) will be the end product if the reaction reaches equilibrium. Best yields are obtained by the use of sludge catalysts (i.e., a mixture of AIX3 and ferf-butyl bromide or iec-butyl bromide).Though it is certain that these adamantane-forming reactions take place by nucleophilic 1,2-shifts, the exact pathways are not easy to unravel... 

We have proposed that, under the conditions we have described, methyl bromide reacts with hydroxide ion by the Sn2 mechanism, and that ferf-butyl bromide reacts by the SnI mechanism. Since 5ec-alkyl bromides are intermediate in structure between these two halides, it is not surprising to find that they can react by either or both mechanisms. 

I PROBLEM 6.20 Write a series of equations describing a synthesis of 1-bromo-2- I methyl-2-propanol from ferf-butyl bromide. 

PROBLEM 8.8 Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of ferf-butyl bromide in methanol, and outline a reasonable mechanism for its formation. J... 

Bromine will also halogenate alkanes, and it does so much more selectively than chlorine. For example, the following reaction yields ferf-butyl bromide with less than 1% of the primary isomer. 

CH3 ferf-butyl bromide CH2CH3 fert-pentyl bromide. H3... 

Given our understanding of Sn2 reactions, we would expect the rate of reaction of tert-butyl bromide with water to be relatively slow because water is a poor nucleophile and ferf-butyl bromide is sterically hindered to attack by a nucleophile. It turns out, however, that the reaction is surprisingly fast. In fact, it is over one million times faster than the reaction of methyl bromide—a compound with no steric hindrance—with water (Table 10.4). Clearly, the reaction must be taking place by a mechanism different from that of an Sn2 reaction. 

As we have seen, a study of the kinetics of a reaction is one of the first steps undertaken when one is investigating the mechanism of a reaction. If we were to investigate the kinetics of the reaction of ferf-butyl bromide with water, we would find that doubling the concentration of the alkyl halide doubles the rate of the reaction. We would also find that changing the concentration of the nucleophile has no effect on the rate of the reaction. Knowing that the rate of this nucleophilic substitution reaction depends only on the concentration of the alkyl halide, we can write the following rate law for the reaction ... 

The rate law for the reaction of ferf-butyl bromide with water differs from the rate law for the reaction of methyl bromide with hydroxide ion (Section 10.2), so the two reactions must have different mechanisms. We have seen that the reaction between methyl bromide and hydroxide ion is an Sn2 reaction. The reaction between ferf-butyl... 

When the methyl groups of ferf-butyl bromide are successively replaced by hydrogens, the rate of the SnI reaction decreases progressively (Table 10.4). This is opposite to the order of reactivity exhibited by alkyl halides in Sn2 reactions (Table 10.1). 

Arrange the following alkyl bromides in order of decreasing reactivity in an S l reaction isopropyl bromide, propyl bromide, ferf-butyl bromide, methyl bromide. 

The second kind of elimination reaction that alkyl halides can undergo is an El elimination. The reaction of ferf-butyl bromide with water to form 2-methyl-propene is an example of an El reaction E stands for elimination and 1 stands for unimolecular. 

Hughes and Ingold observed that the hydrolysis of ferf-butyl bromide, which occurs readily, is characterized by a first-order rate law ... 

Halogenation usually occurs preferentially at the more highly substituted a position of unsymmetrical ketones. Regioselectivity can be a problem in the halogenation of unsymmetrical ketones, but heating a ketone with ferf-butyl bromide and DMSO at 65°C is a mild and selective solution to the problem, and it... 

Since the starting material given is ferf-butyl bromide, a practical synthesis is ... 

Methyl bromide Ethyl bromide Isopropyl bromide ferf-Butyl bromide... 

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