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ChemistryIsomerism
Which of the following pairs of compounds represent metamers?
Options
1
Pentan-1-ol and pentan-2-ol
2
Diethyl ether and methyl propyl ether
3
Propanal and propanone
4
1-butanol and diethyl ether
Correct Answer
Option 2 : Diethyl ether and methyl propyl ether
Step-by-Step Solution
1

Definition of Metamerism:

Metamers are compounds that have the same molecular formula and same functional group, but differ in the alkyl groups attached on either side of the functional group (i.e., different distribution of carbon atoms on either side of the same functional group).

2

Check Option 2 — Ethers:

Diethyl ether: CH₃CH₂—O—CH₂CH₃ (C₂H₅—O—C₂H₅)

Methyl propyl ether: CH₃—O—CH₂CH₂CH₃ (CH₃—O—C₃H₇)

Both have molecular formula C₄H₁₀O

Both have ether functional group (—O—)

They differ in alkyl groups on either side of —O—

→ These are METAMERS ✓

3

Why other options fail:

Option 1: Pentan-1-ol & pentan-2-ol — same functional group (—OH) but differ in position → position isomers, not metamers

Option 3: Propanal & propanone — different functional groups (aldehyde vs ketone) → functional group isomers

Option 4: 1-butanol & diethyl ether — different functional groups (—OH vs —O—) → functional group isomers

Theory: Types of Isomerism
1. Structural Isomerism — Overview

Structural (constitutional) isomers have the same molecular formula but different structural arrangements of atoms. They differ in the way atoms are connected to each other. Structural isomerism is further classified into several subtypes based on the nature of the structural difference.

2. Chain Isomerism

Compounds with same molecular formula but different carbon chain arrangements (branching). Example: n-butane (CH₃CH₂CH₂CH₃) and isobutane (CH₃CH(CH₃)₂) — both C₄H₁₀ but different chain lengths. Properties like boiling point differ significantly.

3. Position Isomerism

Same molecular formula, same functional group, same carbon chain, but the functional group is at a different position. Example: 1-propanol (CH₃CH₂CH₂OH) and 2-propanol (CH₃CH(OH)CH₃) — both C₃H₈O with —OH group but at different carbons.

4. Functional Group Isomerism

Same molecular formula but different functional groups. Example: C₂H₆O — ethanol (CH₃CH₂OH, alcohol) and dimethyl ether (CH₃OCH₃, ether) are functional group isomers. Another example: C₃H₆O — propanal (aldehyde) and propanone (ketone).

5. Metamerism

A special type of structural isomerism where compounds have the same molecular formula and the same functional group, but different alkyl groups on either side of the functional group. It is exhibited by compounds containing a divalent functional group such as —O— (ether), —S— (thioether), —CO— (ketone), —NH— (secondary amine).

Metamers: same molecular formula + same functional group

+ different distribution of carbon atoms on either side of functional group

Functional groups showing metamerism: —O—, —S—, —CO—, —NH—

📌 Diethyl ether (C₂H₅—O—C₂H₅) and methyl propyl ether (CH₃—O—C₃H₇) — C₄H₁₀O

📌 Diethyl ketone (C₂H₅—CO—C₂H₅) and methyl propyl ketone (CH₃—CO—C₃H₇) — C₅H₁₀O

📌 Diethylamine (C₂H₅—NH—C₂H₅) and methyl propylamine (CH₃—NH—C₃H₇) — C₄H₁₁N

📌 Alcohols (—OH) and carboxylic acids (—COOH) do NOT show metamerism (monovalent functional groups)

6. Tautomerism

Tautomers are structural isomers that rapidly interconvert in equilibrium. The most common is keto-enol tautomerism: the keto form (C=O with adjacent C–H) and enol form (C=C with adjacent C–OH) interconvert rapidly. Example: acetaldehyde (keto) ⇌ vinyl alcohol (enol). Acetylacetone exists predominantly as the enol form due to stabilisation by intramolecular hydrogen bonding. Glucose exists in open chain (aldehyde) and cyclic (hemiacetal) forms.

7. Stereoisomerism — Overview

Stereoisomers have the same molecular formula and same connectivity of atoms, but differ in the spatial arrangement of atoms. Two main types: geometrical (cis-trans) isomerism and optical isomerism. Geometrical isomers arise due to restricted rotation (double bonds or rings). Optical isomers (enantiomers) are non-superimposable mirror images due to a chiral centre (usually a carbon with 4 different groups).

8. Optical Isomerism and Chirality

A molecule is chiral if it is non-superimposable on its mirror image. The most common cause is a chiral carbon (asymmetric carbon) — a carbon bonded to four different groups. Enantiomers rotate plane-polarised light equally but in opposite directions: d (dextrorotatory, +) and l (laevorotatory, −). A racemic mixture (equal d and l) shows no net rotation. Diastereomers are stereoisomers that are not enantiomers — they have different physical properties.

Frequently Asked Questions
1. What is the key feature that defines metamers? ⌄
Metamers have: (1) same molecular formula, (2) same functional group, (3) same total carbon count but different distribution of carbons on each side of the divalent functional group. Example: C₂H₅-O-C₂H₅ vs CH₃-O-C₃H₇ — both have the same ether functional group and formula C₄H₁₀O but different alkyl groups on each side of oxygen.
2. Can alcohols show metamerism? ⌄
No. Metamerism requires a DIVALENT functional group (—O—, —S—, —CO—, —NH—) so that different alkyl groups can be attached on BOTH sides. Alcohols have the monovalent —OH group attached on only ONE side of one carbon — so they cannot show metamerism. They show position isomerism instead.
3. What is the difference between metamers and functional group isomers? ⌄
Functional group isomers have the same molecular formula but DIFFERENT functional groups (e.g., alcohol vs ether — C₂H₆O). Metamers have the same molecular formula and the SAME functional group but different alkyl groups on each side. Diethyl ether and methyl propyl ether are metamers (both ethers). Ethanol and dimethyl ether are functional group isomers (alcohol vs ether).
4. Give an example of metamers in ketones. ⌄
Diethyl ketone (pentan-3-one): C₂H₅—CO—C₂H₅ (C₅H₁₀O). Methyl propyl ketone (pentan-2-one): CH₃—CO—C₃H₇ (C₅H₁₀O). Both are ketones with molecular formula C₅H₁₀O but different alkyl groups on each side of the carbonyl group. These are metamers.
5. What is keto-enol tautomerism? ⌄
Rapid interconversion between keto form (C=O, no C-OH) and enol form (C=C-OH). For acetaldehyde: CH₃CHO (keto, 99.9%) ⇌ CH₂=CHOH (enol, 0.1%). The equilibrium strongly favours keto form in most cases. For acetylacetone (1,3-diketone), enol form is stabilised by intramolecular H-bonding and conjugation — about 80% enol at equilibrium.
6. How many stereoisomers does a compound with 2 chiral carbons have? ⌄
Maximum = 2ⁿ where n = number of chiral carbons. For 2 chiral carbons: max 4 stereoisomers (2 pairs of enantiomers). But if the compound has a plane of symmetry (meso compound), actual count is less. Tartaric acid (2 chiral carbons) has only 3 stereoisomers: (+), (−), and meso form.
7. What is geometrical isomerism and when does it occur? ⌄
Geometrical (cis-trans) isomerism occurs when rotation is restricted — in alkenes (C=C) or cyclic compounds. Conditions: each doubly bonded carbon must have two DIFFERENT groups. Cis: same groups on same side of double bond. Trans: same groups on opposite sides. Example: but-2-ene — cis (both CH₃ same side, bp=3.7°C) and trans (both CH₃ opposite, bp=0.9°C) have different physical properties.
8. What is a racemic mixture? ⌄
A racemic mixture (racemate) is an equimolar mixture of two enantiomers. It shows no optical activity (rotations cancel out). Denoted as (±) or dl. Many synthetic reactions produce racemic mixtures since attack can occur from either face of a planar intermediate. Optical resolution (separating enantiomers) is important in pharmaceuticals — often only one enantiomer is therapeutically active (e.g., only S-ibuprofen is active; R-form is inactive).
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