Question
Draw the complete structural formula (expanded and condensed) of 2-methylpropan-1-ol. Also write its molecular formula and identify the class of compound it belongs to.
Solution — Step by Step
The name 2-methylpropan-1-ol contains three pieces of information:
- Parent chain: “propan” → 3 carbon chain (propane backbone)
- Substituent: “2-methyl” → a methyl group (–CH₃) attached at carbon 2
- Functional group: “1-ol” → hydroxyl group (–OH) attached at carbon 1
Note: “1-ol” means –OH is on carbon 1 of the parent chain. IUPAC rules say the chain is numbered to give the lowest possible locant to the principal functional group (OH in alcohols).
Start with a 3-carbon chain:
Number from the end closest to the –OH group. –OH is at C1, so:
- C1 gets the –OH group
- C2 gets the –CH₃ substituent
Expanded structural formula:
CH₃
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HO - CH₂ - CH - CH₃Or drawn differently:
CH₃
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H-O-CH₂-CH-CH₃Count all atoms to verify:
- C1: CH₂ (bonded to OH and C2)
- C2: CH (bonded to CH₃, C1, and C3, with 1 H)
- C3: CH₃ (terminal carbon)
- Methyl substituent at C2: CH₃
Carbon atoms: C1 + C2 + C3 + CH₃ substituent = 4 carbons total
Hydrogen atoms:
- C1 (CH₂): 2H
- C2 (CH): 1H
- C3 (CH₃): 3H
- Substituent CH₃: 3H
- OH: 1H
- Total H = 2 + 1 + 3 + 3 + 1 = 10H
Oxygen atoms: 1 (from –OH)
Molecular formula: C₄H₁₀O
Condensed structural formula: (CH₃)₂CHCH₂OH or (CH₃)₂CHCH₂OH
Class: This is an alcohol (contains –OH group on a carbon chain, not on benzene ring which would make it a phenol).
Degree: 2-methylpropan-1-ol is a primary alcohol — the carbon bearing the –OH group (C1) is attached to only ONE other carbon. Primary alcohols are easier to oxidise and less hindered than secondary or tertiary.
Compare with:
- 2-methylpropan-2-ol (same molecular formula C₄H₁₀O) — tertiary alcohol (C with OH has three carbon attachments)
- These two are isomers of each other
Note: 2-methylpropan-1-ol is commonly known as isobutanol (or isobutyl alcohol) — a commercially important solvent.
Why This Works
IUPAC nomenclature is a systematic code where every part of the name maps to a specific structural feature. Decoding in order — parent chain → substituents → functional groups → locants — always gives the unique correct structure. The reverse process (structure → name) follows the same rules in reverse.
The key insight here: “propan” is only 3 carbons, but the total carbon count is 4 because of the methyl substituent. Many students confuse the parent chain (3C) with the total carbon count (4C).
Alternative Method
Draw the isomers with molecular formula C₄H₁₀O and identify each:
- n-butan-1-ol (butanol): CH₃CH₂CH₂CH₂OH — 4C straight chain, OH at C1
- butan-2-ol: CH₃CH₂CH(OH)CH₃ — 4C chain, OH at C2 (secondary)
- 2-methylpropan-1-ol: (CH₃)₂CHCH₂OH — 3C parent + methyl branch (our compound)
- 2-methylpropan-2-ol (tert-butanol): (CH₃)₃COH — tertiary
All four are constitutional isomers (same molecular formula, different connectivity).
For JEE and CBSE Class 12, remember to always count all carbons in a structure, not just those in the parent chain. A “2-methylpropan” compound has 3 + 1 = 4 carbons total. Misidentifying the molecular formula is a common source of error in isomer-counting questions.
Common Mistake
Students often draw 2-methylpropan-1-ol as a branched structure at the wrong carbon or number the chain from the wrong end. Remember: in IUPAC nomenclature for alcohols, number the chain from the end that gives the lowest locant to the OH group. The OH must be at C1 in this compound. If you numbered from the other end, C1 would have the methyl group (making it butanol, not methylpropanol) — this violates the lowest locant rule.