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Isomerism in Amines (C4H11N): Chain, Position and Metamerism

Chemistry · Amines · NEET

The molecular formula C4H11N gives 8 isomeric amines in total: 4 primary (1 degree), 3 secondary (2 degree) and 1 tertiary (3 degree). They differ by chain isomerism (branched vs straight carbon chain), position isomerism (where the amino group sits) and metamerism (different alkyl groups on the same nitrogen). Memory hook: "4-3-1 makes 8" for primary-secondary-tertiary amines of C4H11N.
C4H11N — 8 isomeric aminesPrimary (4)n-butylamineisobutylaminesec-butylaminetert-butylaminechain + positionSecondary (3)CH3-NH-C3H7 (n)CH3-NH-C3H7 (iso)C2H5-NH-C2H5metamerismTertiary (1)(CH3)2N-C2H5N,N-dimethyl-ethanamineonly one
The 8 isomeric amines of C4H11N split as 4 primary, 3 secondary and 1 tertiary. Primary amines differ by chain and position isomerism; secondary amines are metamers; the tertiary amine is unique.

Your doubts, answered

How many total amine isomers does C4H11N have?

Eight. There are 4 primary amines (n-butylamine, isobutylamine, sec-butylamine, tert-butylamine), 3 secondary amines (N-methylpropan-1-amine, N-methylpropan-2-amine, N-ethylethanamine/diethylamine) and 1 tertiary amine (N,N-dimethylethanamine). This is the exact NCERT exercise, so remember the split 4 + 3 + 1 = 8.

What is metamerism in amines and how is it different from position isomerism?

Metamerism happens in secondary and tertiary amines when different alkyl groups are attached to the same nitrogen atom, keeping the total carbon count the same. Example for C4H11N: N-methylpropylamine (CH3-NH-C3H7) and diethylamine (C2H5-NH-C2H5) are metamers. Position isomerism instead moves the amino group along the SAME carbon chain, like n-butylamine (NH2 on C1) versus sec-butylamine (NH2 on C2).

How do I tell chain isomers apart from position isomers here?

Chain isomers differ in the carbon skeleton: n-butylamine has a straight 4-carbon chain, while isobutylamine has a branched chain, yet both keep -NH2 as a primary amine. Position isomers keep the same skeleton but move -NH2 to a different carbon. Compare the backbone first (chain), then the location of NH2 (position).

Why does tertiary amine give only one isomer for C4H11N?

A tertiary amine needs three alkyl groups on nitrogen with only 4 carbons total. The only split that works is two methyls and one ethyl, giving N,N-dimethylethanamine (CH3)2N-C2H5. No other combination of three alkyl groups adds up to 4 carbons, so there is exactly one tertiary amine.

Are cyclic structures or nitriles counted in C4H11N isomers?

No. C4H11N is fully saturated with no ring and no double bond (degree of unsaturation is 0). So only open-chain saturated amines are counted. Nitriles and imines have unsaturation and a different hydrogen count, so they are excluded.

⚠️ The NEET trap
✗ Counting only the 4 primary amines (or forgetting the tertiary amine) and answering 4 or 7 isomers.
✓ Count all three classes: 4 primary + 3 secondary + 1 tertiary = 8 isomeric amines for C4H11N.
🧠 NTA loves 'total number of isomeric amines'. Always sweep 1 degree, 2 degree AND 3 degree before answering.

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Frequently asked

What are the 4 primary amine isomers of C4H11N?

n-Butylamine (butan-1-amine), isobutylamine (2-methylpropan-1-amine), sec-butylamine (butan-2-amine) and tert-butylamine (2-methylpropan-2-amine).

Which pairs of C4H11N amines show metamerism?

The secondary amines show metamerism: N-methylpropan-1-amine, N-methylpropan-2-amine and diethylamine all have the formula C4H11N but different alkyl groups on nitrogen.

Does C4H11N show functional isomerism?

Within amines it shows chain, position and metamerism. Amines can be functional isomers of other nitrogen classes, but for the NEET amine-counting question you count only the amine structures, which give 8.

How many secondary amines are possible for C4H11N?

Three: N-methylpropylamine, N-methylisopropylamine and diethylamine.

Is n-butylamine a chain or position isomer of sec-butylamine?

Position isomer. Both have a straight 4-carbon chain; only the position of -NH2 changes (C1 versus C2).

Related concepts:
Hydrogen Bonding in Amines and Boiling PointAscent and Descent of the Amine SeriesWhy Primary Amines Boil Higher Than TertiaryWhy Ammonolysis Gives a Mixture of AminesSolubility of Amines in Water
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