Butyl bromide nucleophilic substitution

Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of tert-butyl bromide in methanol and outline a reason able mechanism for its formation... 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. 

In the first systematic study on nucleophilic substitutions of chiral halides by Group IV metal anions, Jensen and Davis showed that (S )-2-bromobutane is converted to the (R)-2-triphenylmetal product with predominant inversion at the carbon center (Table 5)37. Replacement of the phenyl substituents by alkyl groups was possible through sequential brominolysis and reaction of the derived stannyl bromides with a Grignard reagent (equation 16). Subsequently, Pereyre and coworkers employed the foregoing Grignard sequence to prepare several trialkyl(s-butyl)stannanes (equation 17)38. They also developed an alternative synthesis of more hindered trialkyl derivatives (equation 18). 

Consider a proposed nucleophilic substitution reaction on the secondary alcohol shown using aqueous HBr. As a secondary alcohol, either Sn2 or SnI mechanisms are possible (see Section 6.2.3), but SnI is favoured because of the acidic environment and the large fert-butyl group hindering approach of the nucleophile. The expected SnI bromide product is formed, together with a smaller amount of the El-derived alkene in a competing reaction. 

Kenawy 64) immobilized ammonium and phosphonium peripheral functionalized dendritic branches on a montmorillonite supported chloromethylstyrene/methyl methacrylate copolymer (74-75). These polymer/montmorillonite-supported dendrimers were used as phase transfer catalysts (PTC) for the nucleophilic substitution reaction between -butyl bromide and thiocyanate, cyanide, and nitrite anions in a toluene or a benzene/water system. These PT catalysts could be recycled by filtration of the functionalized montmorillonite from the reaction mixture. Generally,... 

The kinetics of nucleophilic substitution reactions have been studied in greater detail than any other type of reaction because they don t always proceed through the same mechanism. Consider the reaction between the OH ion and t-butyl bromide, for example. 

Because the slowest step of this reaction only involves t-butyl bromide, the overall rate of reaction only depends on the concentration of this species. This is therefore a unimolecular nucleophilic substitution, or SN1, reaction. 

One equivalent of NaOEt in EtOH deprotonates diethyl malonate completely to give the sodium enolate A (Figure 13.36). This enolate is monoalkylated upon addition of an alkylating reagent such as BuBr, and a substituted malonic ester C is formed. During the alkylation reaction, the substituted malonic ester C reacts to a certain extent with some of the enolate A, resulting in the butylated enolate B and unsubstituted neutral malonic ester. It is for this reason that the reaction mixture contains two nucleophiles—the original enolate A and the butylated enolate B. The alkylation of A with butyl bromide is much faster than that of B, since A is less sterically hindered than B. The main product is therefore the product of monoalkylation. Distillation can be used to separate the main product from small amounts of the product of dialkylation. 

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. 

This type of substitution is called an SN1 reaction, for Substitution, Nucleophilic, unimolecular. The term unimolecular means there is only one molecule involved in the transition state of the rate-limiting step. The mechanism of the SnI reaction of tert-butyl bromide with methanol is shown here. Ionization of the alkyl halide (first step) is the rate-limiting step. 

The elimination reaction of f-butyl bromide happens because the nucleophile is basic You will recall from Chapter 12 that there is some correlation between basicity and nucleophilicity strong bases are usually good nucleophiles. But being a good nucleophile doesn t get hydroxide anywhere in the substitution reaction, because it doesn t appear in the first-order rate equation. But being a good base does get it somewhere in the elimination reaction, because hydroxide is involved in the rate-determining step of the elimination, and so it appears in the rate equation. This is the mechanism. 

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... 

Another nucleophilic aliphatic substitution is the reaction between sodium hydroxide and tert-butyl bromide (TBB) ... 

With methyl and ethyl bromide the rate of reaction is principally proportional to the concentration of hydroxide ion and of the halide. With isopropyl bromide no simple mathematical relationship exists, and with Jcrf-butyl bromide rate of reaction is dependent only upon the concentration of the halide. At this point in the series, then, the rate of a substitution reaction at a saturat ed carbon atom is completely independent of the concentration of the nucleophilic reagent (hydroxide ion). 

They found that the rate of hydrolysis depends only on the concentration of tert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. Just as second-order kinetics was interpreted as indicating a bimolecular rate-determining step, first-order kinetics was interpreted as evidence for a unimolecular rate-determining step—a step that involves only the alkyl halide. 

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