Functional Isomerism

Functional isomers share a formula but have different functional groups, so they behave differently. The key skill is knowing which groups share a general formula.

Builds on: Part 1 · Isomers: Chain, Position and Ring-Chain.

CₙH₂ₙO

Seven isomers of C₃H₆O

One C=O, one C=C or one ring: propanal, propanone, allyl alcohol, methyl vinyl ether, 1,2-epoxypropane, oxetane and cyclopropanol. Don't stop at the aldehyde and ketone.

Seven C3H6O isomers. — The functional group of each is lit.

Five C3H6O2 isomers. — Hydroxy carbonyls fit CₙH₂ₙO₂ too.

CₙH₂ₙO₂

Acids, esters and hydroxy carbonyls

C₃H₆O₂: propionic acid, methyl acetate, ethyl formate, and the often-forgotten hydroxy carbonyls 2-hydroxypropanal and 1-hydroxypropan-2-one.

Nitrogen

1°, 2° and 3° amines

C₃H₉N: propan-1-amine and propan-2-amine (1°), N-methylethanamine (2°), N,N-dimethylmethanamine (3°). Amines of different degree are functional isomers. Also: CH₃CN / CH₃NC, C₂H₅NO₂ / C₂H₅ONO, acetaldoxime / acetamide, ethanethiol / dimethyl sulphide.

Four amines of formula C3H9N. — Primary, secondary and tertiary amines of C₃H₉N.

Five C7H8O isomers. — One alcohol, three phenols, one ether.

Aromatic

C₇H₈O

Benzyl alcohol (an alcohol), o-, m- and p-cresol (phenols) and anisole (an ether). The three cresols are position isomers of each other.

JEE-style question

Your turn

Which pair are NOT functional isomers?

(a)CH₃CN and CH₃NC

(b)propanal and propanone

(c)ethanol and dimethyl ether

(d)propan-1-amine and propan-2-amine

Show the answer and the traps

Both are primary amines; only the NH₂ moves. They're position isomers: option (d).

Cyanide and isocyanide, aldehyde and ketone, alcohol and ether: each of those pairs changes the functional group.

Watch out

Common mistakes

Listing only the aldehyde and ketone for CₙH₂ₙO.Unsaturated alcohols, unsaturated ethers and cyclic compounds fit too.

Forgetting hydroxy carbonyls for CₙH₂ₙO₂.2-Hydroxypropanal and 1-hydroxypropan-2-one are C₃H₆O₂.

Practice

Try these

1. Give two isomers of C₃H₆O that are not carbonyl compounds.

Any two of allyl alcohol, methyl vinyl ether, oxetane, 1,2-epoxypropane, cyclopropanol.

2. Ethyl acetate is C₄H₈O₂. Name a functional isomer from a different class.

Butanoic acid (or 2-methylpropanoic acid): a carboxylic acid.

3. Are nitroethane and ethyl nitrite functional isomers?

Yes: a nitroalkane (bonded through N) and an alkyl nitrite (bonded through O).