What is IUPAC name of (CH_3)_3CBr ?

The given compound is $(\text{CH}_3)_3\text{CBr}$.

Its expanded structure is $\text{CH}_3-\text{C}(\text{CH}_3)(\text{Br})-\text{CH}_3$.

The longest continuous carbon chain consists of 3 carbon atoms, which corresponds to the parent alkane propane.

Numbering the chain from either end, the substituents are attached to the second carbon atom.

The substituents present at the second carbon are a bromo group and a methyl group.

Arranging the substituents in alphabetical order, the prefix becomes 2-bromo-2-methyl.

Combining this with the parent chain name, the IUPAC name of the compound is 2-bromo-2-methylpropane.

Note that tert-butyl bromide is the common name for this compound.

Answer: 2-bromo-2-methylpropane