Fert-Butoxide ion

Potassium fert-butoxide ions, (CH3)3COK, can be generated similarily. 

It was observed that there were only slight differences in composition between the products of the MW and conventionally heated reactions. The greatest change in composition was observed in the reaction of 1-bromooctane with fert-butoxide ion where the percent of the elimination product (the alkene) increased from 5%, under conventional reflux to 14% under MW heating. On the other hand there... 

To serve as a nucleophile, an ion or molecule must get in close to a carbon atom to attack it. Bulky groups on the nucleophile hinder this close approach, and they slow the reaction rate. For example, the ferf-butoxide ion is a stronger base (for abstracting protons) than ethoxide ion, but terf-butoxide ion has three methyl groups that hinder any close approach to a more crowded carbon atom. Therefore, ethoxide ion is a stronger nucleophile than fert-butoxide ion. When bulky groups interfere with a reaction by virtue of their size, we call the effect steric hindrance. 

The fert-butoxide ion, (CH3)3CO, in t rt-butyl alcohol, (CH3)3COH, is a stronger base than the ethoxide ion in ethyl alcohol, and it can be prepared in a similar way ... 

Therefore, ferf-butoxide ion, with its three methyl groups, is a poorer nucleophile than ethoxide ion, even though fert-butoxide ion is a stronger base (p Tg of ferf-butanol = 18) than ethoxide ion (pK of ethanol = 16). 

For example, in the reactions that follow, acetate ion is a weaker base than ethoxide ion because acetic acid is a stronger acid (p Tg = 4.76) than ethanol (pisTa = 15.9). No elimination product is formed from the reaction of 2-chloropropane with the weakly basic acetate ion, whereas the elimination product is the major product formed with the strongly basic ethoxide ion. The percentage of elimination product produced would be increased further if the bulky fert-butoxide ion were used instead of ethoxide ion. 

The rate of an Sj 2 reaction is strongly affected by bulky groups near the reaction center, which hinder the approach of the nucleophile (Section 9.9). Therefore, the size of the nucleophile is also important. Steric crowding in the transition state increases with the size of the nucleophile. We find, for example, that the larger fert-butoxide ion is a poorer nucleophile in S 2 reactions than the smaller ethoxide ion. 

The order of basicities of alkoxides is opposite to the order of nucleophihcity. Thus, fert-butoxide ion is a stronger base than ethoxide ion. Steric hindrance has little effect on the ease of abstraction of a proton in acid-base reactions. Steric repulsions are less severe when the base approaches a small hydrogen atom than when it approaches the more crowded environment at a tetravalent carbon atom. 

If a primary haloalkane is treated with ftrr-butoxide ion instead of ethoxide, the amount of ehmination product increases significantly. The fert-butoxide ion is not only more basic than the ethoxide ion also much more sterically hindered. The combination of these two factors favors elimination by an E2 process over substitution by an Sj 2 process. 

Problem 10.10 The amount of elimination product for the reaction of 1-bromooctadecane with an alkoxide in the corresponding alcohol solvent is about 1 % for methoxide ion and 85% for fert-butoxide ion. Explain these data. 

Because the Williamson synthesis is an Sx2 reaction, it is subject to all the usual constraints, as discussed in Section 11.2. Primary halides and tosylatcs work best because competitive 2 elimination can occur with more hindered substrates. Unsymmetrical ethers should therefore be synthesized by leciclion between the more hindered alkoxide partner and less hindered halide partner rather than vice versa. For example, fert-butvi methyl ether, a substance used in the 1990s as an octane booster in gasoline, is best prepared by reaction of tert-butoxide ion with iodomethane rather than by reaction of metho.xide ion with 2-chloro-2-mcthylpropane. 

Dialkylation The monoalkylacetoacetic ester shown above still has one appreciably acidic hydrogen, and, if we desire, we can carry out a second alkylation. Because a monoalkylacetoacetic ester is somewhat less acidic than acetoacetic ester itself due to the electron-donating effect of the added alkyl group, it is usually helpful to use a stronger base than ethoxide ion for the second alkylation. Use of potassium fert-butoxide is common because it is a stronger base than sodium ethoxide. Potassium fcrf-butoxide, because of its steric bulk, is also not likely to cause transesterification. 

The pA values indicate that r rf-butanol is less acidic than ethanol, by two orders of magnitude. This difference in acidity is best explained by a steric effect. The ethoxide ion is not sterically hindered and is therefore easily solvated (stabilized) by the solvent, while fert-butoxide is steri-cally hindered and is less easily solvated (Figure 13.4). The conjugate base of ferr-butanol is less stabilized than the conjugate base of ethanol, rendering r rt-butanol less acidic. 

Sodium and potassium alkoxides are often used as bases in organic syntheses (Section 6.15B). We use alkoxides, such as ethoxide and fert-butoxide, when we carry out reactions that require stronger bases than hydroxide ion but do not require exceptionally powerful bases, such as the amide ion or the anion of an alkane. We also use alkoxide ions when (for reasons of solubility) we need to carry out a reaction in an alcohol solvent rather than in water. 

When treated with fert-butoxide (a strong base), an alcohol is deprotonated to give an alkoxide ion. 

Alkoxide ions are nearly the same or somewhat stronger bases than the hydroxide ion. In addition to sodium methoxide, the following metal salts of alcohols are commonly used in organic reactions requiring a strong base in a nonaqueous solvent, as, for example, sodium ethoxide in ethanol and potassium ffrf-butoxide in 2-methyl-2-propanol (fert-butyl alcohol). 

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