Alkyl halides 1- bromobutane

For example, the E2 reaction of a 1° alkyl halide (1-bromobutane) forms a monosubstituted alkene, whereas the E2 reaction of a 3° alkyl halide (2-bromo-2-methylpropane) forms a disub-stituted alkene. The disubstituted alkene is more stable, so the 3° alkyl halide reacts faster than the 1° alkyl halide. 

We see that a secondary alkyl halide is needed as the alkylating agent The anion of diethyl malonate is a weaker base than ethoxide ion and reacts with secondary alkyl halides by substitution rather than elimination Thus the synthesis of 3 methylpentanoic acid begins with the alkylation of the anion of diethyl mal onate by 2 bromobutane... 

This reaction sequence is called the acetoacetic ester synthesis. It is a standard procedure for the preparation of ketones from alkyl halides, as the conversion of 1-bromobutane to 2-heptanone illustrates. 

Alkyne alkylation is not limited to acetylene itself. Any terminal alkyne can be converted into its corresponding anion and then alkylated by treatment with an alkyl halide, yielding an internal alkyne. For example, conversion of 1-hexyne into its anion, followed by reaction with 1-bromobutane, yields 5-decyne. 

It has been shown that a complete shift in stereochemistry of the nucleophilic reactions of (29), with alkyl halides such as 2-bromobutane or cis-2-bromomethoxycyclohexane, from racemization to complete inversion, is induced by increase in the inner-sphere stabilization of the transition state from 0 to 3 kcal mol" This has been ascribed to competition between inner-sphere 5)vr2 and outer-sphere electron-transfer processes the former being extremely sensitive towards inner-sphere stabilization. 

When the reactions of alkyl bromides (n-Q-Cg) with phenoxide were carried out in the presence of cosolvent catalyst 51 (n = 1 or 2,17 % RS) under triphase conditions without stirring, rates increased with decreased chain length of the alkyl halide 82). The substrate selectivity between 1-bromobutane and 1-bromooctane approached 60-fold. Lesser selectivity was observed for polymer-supported HMPA analogue 44 (5-fold), whereas the selectivity was only 1,4-fold for polymer-supported phosphonium ion catalyst 1. This large substrate selectivity was suggested to arise from differences in the effective concentration of the substrates at the active sites. In practice, absorption data showed that polymer-supported polyethylene glycol) 51 and HMPA analogues 44 absorbed 1-bromobutane in preference to 1-bromooctane (6-7 % excess), while polymer-supported phosphonium ion catalyst 1 absorbed both bromides to nearly the same extent. 

SAMPLE SOLUTION (a) An alcohol and a hydrogen halide react to form an alkyl halide and water. In this case 2-bromobutane was isolated in 73% yield. 

As stated above, intermolecular coupling reactions between carbon atoms are of limited use. In the classical Wurtz reaction two identical primary alkyl iodide molecules are reduced by sodium. n-Hectane (C100H202), for example, has been made by this method in 60% yield (G. Stallberg, 1956). The unsymmetrical coupling of two alkyl halides can be achieved via dialkylcuprates. The first halide, which may have a branched carbon chain, is lithiated and allowed to react with copper(I) salts. The resulting dialkylcuprate can then be coupled with alkyl or aryl iodides or bromides. Although the reaction probably involves radicals it is quite stereoselective and leads to inversion of chiral halides. For example, lithium diphenyl-cuprate reacts with (R)-2-bromobutane with 90% stereoselectivity to form (S)-2-phenylbutane (G.M. Whitesides, 1969). 

Ylides are prepared by the reaction of an alkyl halide with triphenylphosphine, followed by treatment with strong base. 2-Bromobutane is the alkyl halide needed in this case. 

All these reactions involve addition of a dialkylcopper reagent [(CHsC C CH CuLi] to an alkyl halide. The dialkylcopper is prepared by treating 1-bromobutane with lithium, followed by addition of Cul ... 

In contrast to the behavior of the foregoing nickel(I) complexes as catalysts, the catalytic reactions of alkyl halides with cobalt(I) species such as vitamin Bi2s, cobaloximes(I), and cobalt(I) salen exhibit a significant difference. Cobalt(I) species, acting as potent nucleophiles in Sn2 reactions with alkyl halides, give stable alkylcobalt(III) intermediates. Lexa and coworkers [318] have discussed this mechanistic scheme for the catalytic reduction of l-bromobutane by the electrogenerated cobalt(I) tetraphenylpor-phin complex, where TPP denotes the ligand. Reversible one-electron reduction of the parent cobalt(II) species... 

The nucleophilic acetylide ion uses an electron pair to attack the positively, polarized, electrophilic carbon atom of bromomethane. As the new C-C bond forms, Br" departs, taking with it the electron pair from the former C-Br bond and yielding propyne as product. We call such a reaction an alkyla-l tion because a new alkyl group has become attached to the starting alkyne. Alkyne alkylation is not limited to acetylene itself, Any terminal alkyne s can be converted into its corresponding anion and then alkylated by treat-j ment with an alkyl halide, yielding an internal alkyne. For example, con/ version of 1-hexyne into its anion, followed by reaction with 1-bromobutane,] yields 5-decyne ... 

The essential feature of the 8 2 mechanism is that the reaction takes place in a single step without intermediates when the incoming nucleophile attacks the alkyl halide or tosylate (the substrate) from a direction directly opposite the group that leaves. As the nucleophile comes in on one side of the substrate and bonds to the carbon, the halide or tosylate departs from the other side, thereby inverting the stereochemical configuration. The process is shown in Figure 11.3 for the reaction of (S)-2-bromobutane with HO , leading to (/2)-2-butanol. 

SOLUTION TO 34b (1) The alkyl halide required depends on which phosphonium ylide is used it would be either 1-bromobutane or 2-bromopropane. 

The primary alkyl halide would be more reactive in the 8 2 reaction required to make the ylide, so the best method would be to use acetone and the ylide obtained from 1-bromobutane. 

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