2-Bromo-2-methylbutane elimination reactions

One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2-bromo-2-methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures ... 

Dehydrohalogenation may give a mixture of products if the halogen is unsymmetrically located on the carbon skeleton. Eor example, 2-bromo-2-methylbutane (6), the substrate you will use in this experiment, yields both 2-methyl-2-butene (7) and 2-methyl-l-butene (8) on reaction with strong base (Eq. 10.5). Because such elimination reactions are normally irreversible under these experimental conditions, the alkenes 7 and 8 do not undergo equilibration subsequent to their production. Consequently, the ratio of 7 and 8 obtained is defined by the relative rates of their formation. These rates, in turn, are determined by the relative free energies of the two transition states, 9 and 10, respectively, rather than by the relative free energies of the alkenes 7 and 8 themselves. 

Within experimental error, the first entry (2-bromo-2-methylbutane) in the table provides results the same as those of Scheme 7.23. Interestingly, 2-bromo-2,4,4-trimethylpentane (the last entry in the table) undergoes El elimination to yield a larger amount of the less-substituted alkene, which equilibration studies show to be the more stable (see Saunders, W. H. Jr. Cockerill, A. F. Mechanisms of Elimination Reactions, 1973, Wiley, New York). 

When treated with a strong base, 2-bromo-2,3-di-methylbutane will undergo an elimination reaction to produce two products. The choice of base (ethoxide vs. tert-butox-ide) will determine which of the two products predominates. Draw both products and determine how you could distinguish between them using IR spectroscopy. 

Tertiary haloalkanes undergo substitution reactions only by an S l mechanism because there is too much steric hindrance for an Sj 2 reaction to occur. However, a tertiary haloalkane can undergo an elimination reaction by either an E2 or an El process. The mechanism depends on the basicity of the nucleophile and the polarity of the solvent. If the nucleophile is a weak base, S l and El processes compete, and the amounts of the two types of products depend only on the stability of the carbocation that forms as an intermediate. For example, 2-bromo-2-methylbutane reacts in ethanol to give about 64% of an ether product from an S l process. The ratio of S l to El products is about 2 1. 

Ethoxide-promoted E2 elimination from 2-iodo-3-methylbutane produces 18% of the 1-alkene and 82% of the 2-alkene (equation 10.78), while elimination from 2-bromo-2-methylbutane produces 29% of the 1-alkene and 71% of the 2-alkene (equation 10.79). Similarly, E2 elimination of (CH3)2S from dimethyl-sec-butylsulfonium ion produces 74% of the 1-alkene and 26% of the 2-alkenes (equation 10.80), while the bimolecular elimination of (CH3)2S from dimethyl-f-amylsulfonium produces 86% of the 1-alkene and 14% of the 2-alkene (equation 10.81). In each case, an additional methyl substituent on the a-carbon atom leads to a greater proportion of the less substituted alkene. Explain the product distributions from all four reactions in terms of the factors that determine the distribution of products from E2 reactions. 

The Zaitsev rule results because the double bond is partially formed in the transition state for the E2 reaction. Thus, increasing the stability of the double bond by adding R groups lowers the energy of the transition state, which increases the reaction rate. For example, E2 elimination of HBr from 2-bromo-2-methylbutane yields alkenes C and D. D, having the more substituted double bond, is the major product, because the transition state leading to its formation is lower in energy. 

In Chapter 7 we discussed how haloalkanes (or alkyl sulfonates) in the presence of strong base can nndergo elimination of the elements of HX with simultaneons formation of a carbon-carbon donble bond. With many substrates, removal of a hydrogen can take place from more than one carbon atom in a molecule, giving rise to constitutional (donble-bond) isomers. In snch cases, can we control which hydrogen is removed—that is, the regio-selectivity of the reaction (Section 9-9) The answer is yes, to a limited extent. A simple example is the elimination of hydrogen bromide from 2-bromo-2-methylbutane. Reaction with sodinm ethoxide in hot ethanol fnmishes mainly 2-methyl-2-butene, but also some 2-methyl-1 -butene. 

The ratio of elimination to substitution products for the reaction of 2-bromo-2-methylbutane depends on the concentration of the base. For 0.05 and 1.0 M sodium ethoxide, the percentages of elimination product are 56% and 98%, respectively. Explain these data. 

Now let s draw the forward scheme. The 3° alcohol is converted to 2-methylpropene using strong acid. Anti-Markovnikov addition of HBr (with peroxides) produces l-bromo-2-methylpropane. Subsequent reaction with sodium acetylide (produced from the 1° alcohol by dehydration, bromination and double elimation/deprotonation as shown) produces 4-methyl-1-pentyne. Deprotonation with sodium amide followed by reaction with 1-bromopentane (made from the 2° alcohol by tosylation, elimination and anfi -Markovnikov addition) yields 2-methyl-4-decyne. Reduction using sodium in liquid ammonia produces the E alkene. Ozonolysis followed by treatment with dimethylsulfide produces an equimolar ratio of the two products, 3-methylbutanal and hexanal. 

Now, let s draw the forward scheme. Radical bromination of 2-methylbutane produces the tertiary alkyl hahde, selectively. Then, elimination with NaOEt, followed by awti-Markovnikov addition (HBr / peroxides), and then elimination with iert-butoxide, followed by another awri-Markovnikov addition (HBr / peroxides) produces l-bromo-3-methylbutane. This alkyl hahde will then undergo an Sn2 reaction when treated with an acetylide ion to give 5-methyl-1-hexyne. Ozonolysis of this terminal alkyne cleaves the CC triple bond, producing the carboxylic acid. Deprotonation (with NaOH) produces a carboxylate nucleophile that subsequently reacts with bromomethane in an Sn2 reaction to give the desired ester. 

---