Bromination of 2-methylpropane

In contrast to chlorination, the bromination of alkanes is highly regioselective. For example, in the photochemical bromination of 2-methylpropane, more than 99% of the product results from substitution of bromine for the tertiary hydrogen atom. 

The reaction of 2-methylpropane and bromine, for example, gives almost exclusive replacement of the tertiary hydrogen atom ... 

Figure 3-11 Potential-energy diagram for the abstraction of a primary or a tertiary hydrogen of 2-methylpropane by a bromine atom. The two iate transition states are dissimilar in energy, indicative of the energy difference between the resulting primary and tertiary radicals, respectively, leading with greater selectivity to the products.

The product ratios for bromination of propane and 2-methylpropane are quite different from those seen above in the chlorination reaction, in that the more-favoured products by far are the secondary and tertiary halides respectively. Abstraction of a hydrogen atom by a bromine atom is now much more difficult than with a chlorine atom. The favoured product may be rationalized in terms of the relative strength of the C-H bond being broken, and the... 

Place 92.5 g (115 ml, 1.25 mol) of isobutyl alcohol (2-methylpropan-l-ol) and 8.55 g (0.275 mol) of purified red phosphorus (Section 4.2.59, p. 458) in a 500-ml three-necked flask fitted with a sealed mechanical stirrer, a reflux condenser and a dropping funnel containing 100 g (32 ml, 0.62 mol) of bromine (for precautions in the use of bromine, see Section 4.2.9, p. 422). Start the stirrer, heat the flask (e.g., in an oil bath) so that the contents reflux gently, and introduce the bromine at such a rate that it appears to react completely so that there is little bromine vapour above the surface of the reaction mix-... 

A-10. Using the data in Table B-l of this Study Guide, calculate the heat of reaction (AH0) for the light-initiated reaction of bromine (Br2) with 2-methylpropane to give 2-bromo-2-methylpropane and hydrogen bromide. 

In contrast, the hydrolysis of tm-butyl bromide (2-bromo-2-methylpropane) occurs in a stepwise manner (reaction 1.1b). In the first slow step, the C-Br bond breaks, with the bromine atom taking both electrons from the bond and leaving as a negatively charged bromide ion. The remainder of the molecule is the positively charged tert-butyl cation (2-methylprop-2-yl cation). This is a highly reactive intermediate, which reacts rapidly with the hydroxide ion to form the corresponding alcohol. 

Explain the following observations (a) although Friedel-Crafts alkylation reactions often result in polysubstitution products, Friedel-Crafts acylation reactions only give the monosub-stituted product (b) bromination of ethyl 4-methy benzoate gives only one product (c) treatment of 4-chlorotoluene with NaNH, in liquid ammonia gives two products (d) reaction of benzene with I-chloro-2-methylpropane and AlCl gives rm-butylbenzene. 

Methylcyclopropane shows strikingly different reactivity toward chlorine and bromine under radical chain conditions in CHjClj solution. The main product with chlorine is chloromethylcyclopropane (56%), along with smaller amounts of 1,3-dichlorobutane and l,3-dichloro-2-methylpropane. Bromine gives only... 

If 2-methylpropane is brominated at 125 °C in the presence of light, what percent of the product will be 2-bromo-2-methylpropane Compare your answer with the percent given in Problem 4 for chlorination. 

Bromination reactions are much more selective than chlorination reactions. For example, when 2-methylpropane is treated with bromine in the presence of light at 127 °C the product is almost exclusively 2-bromo-2-methylpropane (Scheme 4.35). 

The greater selectivity of bromination can be illustrated by comparing the reaction coordinate diagrams for the formation of primary and tertiary free radicals by hydrogen atom abstraction from 2-methylpropane with bromine versus chlorine radicals (Figure 4.20). 

A reaction with 2-bromo-2-methylpropane generates a cation intermediate by direct loss of the bromine. Draw the transition state for this reaction as well as the cation intermediate. 

As we saw in Sections 6.3A and 6.3D, haloalkanes can be prepared by the addition of HX and X2 to alkenes. They are also prepared by replacement of the —OH group of alcohols by halogen (Section 10.5). Many of the simpler low-molecular-weight haloalkanes are prepared by the halogenation of alkanes, illustrated here by treating 2-methylpropane with bromine at an elevated temperature. 

When 2-methylpropane is treated with bromine in the presence of UV light, one product predominates. 

These potential energy diagrams for the formation of primary and tertiary alkyl radicals by halogen atom abstraction from 2-methylpropane illustrate a larger difference in the activation energies for the reaction with a bromine atom ib) than with a chlorine atom (a). This difference is consistent with the higher selectivity of bromination. 

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. 

Methylpropane has two types of H atoms. The H atoms shown in blue are primary H atoms (see Figure 26-4) and the H atom shown in red is a tertiary H atom. From a statistical standpoint, we should expect l-bromo-2-methylpro-prane to be the major product. (There are nine primary H atoms but only one tertiary H atom.) However, the major product is the one formed when the tertiary H atom is replaced. The selectivity of bromine for different H atoms is summarized on the next page. (Chlorine is more reactive and not as selective as bromine.)... 

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