Final answer:
The major monobromination product of pentane with Br2 is 2-bromopentane due to the higher reactivity of secondary hydrogens. For hexane with a methyl group, the main product is 4-bromo-2-methylhexane following IUPAC nomenclature rules.
Step-by-step explanation:
The major monobromination product formed by heating an alkane with Br2 depends on the structure of the alkane. For example, when pentane is reacted with bromine, the major product is likely to be 2-bromopentane. This is because secondary hydrogens are more reactive than primary ones, leading to the bromine atom most commonly attaching at a secondary position. In the case of hexane carrying a methyl group, the reaction would yield 4-bromo-2-methylhexane when substituents are listed in alphabetical order for IUPAC naming.
Bromination of alkanes, like chlorination, occurs via a radical mechanism under the influence of ultraviolet light or heat. The reaction of chlorine with excess methane to produce methyl chloride is another example of such a halogenation reaction, although this particular reaction would yield a chlorinated alkane rather than a brominated one.