Methylcyclohexane bromination

The parent hydrocarbon of this compound is cyclohexane. There are two bromine atoms attached at position numbers 1 and 3. Therefore, part of the prefix is 1,3-dibromo-. There is also a methyl group at position number 4. Because the groups are put in alphabetical order, the full prefix is l,3-dibromo-4-methyl-. (The ring is numbered so that the two bromine atoms have the lowest possible position numbers. See the Problem Tip on page 18.) The full name of the compound is l,3-dibromo-4-methylcyclohexane. 

Would chlorination or bromination of methylcyclohexane produce a greater yield of 1-halo-1-methylcyclohexane ... 

Halo-1-methylcyclohexane is a tertiary alkyl halide, so the question becomes, Will bromination or chlorination produce a greater yield of a tertiary alkyl halide Because bromination is more selective, it will produce a greater yield of the desired compound. Chlorination will form some of the tertiary alkyl halide, but it will also form significant amounts of primary and secondary alkyl halides. 

Route 1 involves the substitution of a hydrogen atom on the methyl group in methylcyclohexane (Structure 2.9) by bromine. However, there are 14 hydrogen atoms in the molecule (Figure 2.2), and other brominated products are possible, such as the four shown below. In practice, the direct bromination reaction shown in Route 1 would probably give a mixture of all of the possible brominated products. It would then be difficult to separate the products, and chemicals would be wasted. 

In a reaction resembling halohydrin formation (Section 6.17), vicinal haloethers are prepared from alkenes by reaction with an alcohol in the presence of halogens— usually bromine or iodine. This haleotherification proceeds through a cyclic halonium ion, which reacts with the alcohol. 1-Methylcyclohexene undergoes iodoetherification with ethanol in the presence of iodine to give ran -l-ethoxy-2-iodo-l-methylcyclohexane. 

A(-bromosuccinimide (NBS) is, however, one of the best known bromine donors. For exanq>le, it can brominate cyclohexane to give a 30% yield of the corresponding bromide. 7 NBS/dibenzoyl peroxide is not very selective, giving mixtures with methylcyclohexane, although decalin gives tetrabromides in a reasonably well-defined manner (equation 81). 

A problem arises when l-bromo-2-methylcyclohexane is treated with base. There are two diastereomers one where the bromine and methyl groups are irons... 

While relatively little work has been done on the stereochemistry of 5 2 reactions, the evidence suggests that they do in fact lead to retention of configuration. A good example is provided by the reactions of cis- and trans-methylcyclohexylmercuric bromides with bromine and pyridine to form l-bromo-4-methylcyclohexane, the cis isomer giving almost exclusively the cis product while the trans isomer gives 91% of the trans product. The reactions probably involve an electrophilic (S 2) attack by bromine on the pyridine complexes, as indicated below. 

Reaction of (H Si—CH2)3 with bromine allows selective formation of partially brominated derivatives, because formation of an SiBr group reduces the reactivity of the remaining Si—H bond in the SiHBr group. If appropriate amounts of bromine, dissolved in methylcyclohexane are added dropwise at —50 °C to a 0.2 molar solution of (H2Si—CH2)3 in methylcyclohexane, three bromination products are obtained ... 

Further bromination of (HBrSi—CH2)3 is achieved in methylcyclohexane. However, the decreased reactivity means that the time required for complete reaction at room temperature is considerably longer. Partly brominated 1,3,5-trisilacyclohexanes exist at room temperature as colorless liquids which maintain an oily appearance as the extent of bromination is increased. 

Example 3. What is the product of the radical bromination of methylcyclohexane ... 

Conclusions. The bromine atom must now be located at the same position as the CH3 substituent, as in the achiral 1-bromo-l-methylcyclohexane ... 

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