Question
Are C₂H₅OH and CH₃OCH₃ functional group isomers? Justify your answer and explain the difference in their properties.
Solution — Step by Step
C₂H₅OH (ethanol): C₂H₆O → 2 carbons, 6 hydrogens, 1 oxygen.
CH₃OCH₃ (dimethyl ether): C₂H₆O → 2 carbons, 6 hydrogens, 1 oxygen.
Both have the molecular formula . ✓ This is the first requirement for any type of isomerism.
C₂H₅OH — contains the hydroxyl group (–OH) → it is an alcohol (primary alcohol, ethanol).
CH₃OCH₃ — contains the ether linkage (–O–) → it is an ether (dimethyl ether, methoxymethane).
The functional groups are different. This confirms they are functional group isomers — same molecular formula, different functional groups.
Functional group isomers (also called functional isomers): Compounds with the same molecular formula but different functional groups. They belong to different homologous series and have different physical and chemical properties.
C₂H₅OH and CH₃OCH₃ are textbook examples of this type of isomerism.
| Property | Ethanol (C₂H₅OH) | Dimethyl ether (CH₃OCH₃) |
|---|---|---|
| Boiling point | 78.4°C | −24°C |
| State at room temp | Liquid | Gas |
| Solubility in water | Miscible (hydrogen bonding with water) | Slightly soluble |
| With Na metal | Reacts — releases H₂ gas | No reaction |
| With HI | No reaction at room temp | Reacts (cleavage of ether) |
| IUPAC name | Ethanol | Methoxymethane |
The dramatically different boiling points (78.4°C vs −24°C) arise because ethanol forms strong hydrogen bonds (–OH group), while ether cannot act as a hydrogen bond donor.
Why This Works
The molecular formula tells us the raw material available — 2 carbons, 6 hydrogens, 1 oxygen. The functional group tells us how those atoms are arranged relative to each other and how the compound behaves. Placing the oxygen as –OH (bonded to one carbon) versus –O– (bridging two carbons) gives completely different architectures and therefore completely different reactivity and physical properties.
This is the power of structural isomerism — same atoms, different connections, totally different substances.
CBSE Class 12 and JEE both test functional group isomerism. Common pairs to know:
- Alcohols ↔ Ethers ()
- Aldehydes ↔ Ketones ()
- Carboxylic acids ↔ Esters ()
- Primary amines ↔ Secondary/tertiary amines ↔ Imines ( etc.)
Alternative Method
You can verify isomerism by checking degrees of unsaturation (DoU):
- C₂H₅OH: DoU = 0 (saturated, no rings/double bonds)
- CH₃OCH₃: DoU = 0 (same)
Equal DoU is consistent with being isomers. Since the molecular formula and DoU match but the functional groups differ, they are functional group isomers.
Common Mistake
Students sometimes say ethanol and dimethyl ether are “metamers” (a subtype of structural isomers where the same functional group is attached to different alkyl groups). Metamerism requires the same functional group — here, one is an alcohol and the other is an ether. They are functional group isomers, not metamers. Metamers would be something like CH₃–O–CH₂CH₃ and C₂H₅–O–C₂H₅ (both ethers, different alkyl groups).