2-Methylpropan-2-ol is commonly known as:

The correct answer is tert-butyl alcohol.

Let's break down the naming step-by-step to understand why 2-methylpropan-2-ol is commonly known as tert-butyl alcohol:

1. IUPAC Name Breakdown: 2-Methylpropan-2-ol
- The parent chain is "propan", which indicates a three-carbon chain:
$C_1-C_2-C_3$
- The suffix "-2-ol" indicates that a hydroxyl group ($-OH$) is attached to the second carbon ($C_2$).
- The prefix "2-methyl" indicates that a methyl group ($-C{H}_3$) is also attached to the second carbon ($C_2$).
- Combining these, the structural formula is:
$(C{H}_3{)}_{3}C-OH$

2. Common Name Derivation: tert-Butyl Alcohol
- The molecule contains a total of four carbon atoms (three in the main propan chain and one in the methyl branch). A four-carbon alkyl group is called a butyl group.
- The carbon atom that is bonded to the hydroxyl group ($-OH$) is directly attached to three other carbon atoms (the two outer carbons of the propane chain and the carbon of the methyl group).
- A carbon atom bonded to three other carbons is classified as a tertiary (or tert-) carbon.
- Therefore, the alkyl radical is a tert-butyl group, and the alcohol is commonly named tert-butyl alcohol (or tertiary-butyl alcohol).