Question
Name the compound CH₃CH(OH)CH₂CH₃ by IUPAC rules.
Solution — Step by Step
Count the longest continuous carbon chain that includes the carbon bearing the functional group (−OH).
The structure CH₃—CH(OH)—CH₂—CH₃ has 4 carbons in a continuous chain:
C1—C2—C3—C4 (with OH on C2)
The parent chain has 4 carbons → butane → butan
The compound has a hydroxyl group (−OH). This makes it an alcohol.
The suffix for alcohols is −ol.
So the base name is butan-?-ol.
We must number from the end that gives −OH the lowest possible position number.
From left (CH₃ end): C1=CH₃, C2=CH(OH), C3=CH₂, C4=CH₃ → OH is at position 2
From right (CH₃ end): C1=CH₃, C2=CH₂, C3=CH(OH), C4=CH₃ → OH is at position 3
We choose numbering from the left → OH at C2 (lower number).
IUPAC name: butan-2-ol
Parent chain: butan (4 carbons) Principal group: −ol (alcohol) at carbon 2
Also acceptable: 2-butanol (older style), but butan-2-ol is the current IUPAC recommendation.
This compound is a secondary alcohol (the carbon bearing −OH is attached to two other carbon atoms).
Why This Works
IUPAC nomenclature follows a strict hierarchy:
- Find the longest carbon chain containing the principal functional group
- Name the chain (methane, ethane, propane, butane…)
- Replace terminal “−e” with the functional group suffix (−ol for alcohols, −al for aldehydes, −one for ketones, −oic acid for carboxylic acids)
- Number the chain to give the functional group the lowest locant
- Name and number all substituents
The system is designed so that any chemist worldwide can draw the molecule from the name, and any molecule can have exactly one correct IUPAC name. This unambiguity is the whole point.
Alternative Method
Common name: This compound is also known as sec-butanol or 2-butanol (common names). For exams, either the IUPAC name (butan-2-ol) or the common name (2-butanol) is acceptable unless the question specifically asks for IUPAC.
Structural features summary: 4-carbon chain, OH on carbon 2, secondary alcohol (carbon 2 is bonded to 2 other carbons, C1 and C3, plus H and OH).
Common Mistake
A common error is numbering from the wrong end and writing “butan-3-ol.” Remember the rule: always number from the end that gives the functional group (here, −OH) the lowest possible number. Carbon 2 (from the left) < carbon 3 (from the right), so we number from the left to give position 2, not 3.
For JEE naming questions, the trickiest cases involve multiple functional groups. Priority order: carboxylic acids > aldehydes > ketones > alcohols. If a compound has both −OH and −COOH, the COOH is the principal group (suffix: −oic acid), and the −OH becomes a prefix (hydroxy−). Example: HOCH₂CH₂COOH = 3-hydroxypropanoic acid.