There are 8 constitutional isomers and a pair of stereoisomers of molecular formula C₄H₁₁N

Structural isomerism  - isomers based on different connectivity's of the constituent atoms, so cannot be spatially identical (but can be defined as having the same shape).

This includes (a) carbon chain variation (usually need a minimum of 4 atoms), (b) change in position of a substituent or functional group and (c) functional group isomerism where the atoms have a different connectivity configuration, usually with significant differences in chemical and physical properties.

All three apply here e.g.

(a) carbon chain linear or branched, different sequences of C and N atoms.

(b) Position of amine group in the linear primary amines.

(c) Primary, secondary or tertiary amines.

Stereoisomerism - isomers based on the same connectivity of the atoms (same constitutional formula), but in some way, they are 2D or 3D spatially different non-superimposable images (e.g. E/Z 'geometrical' isomers or mirror image R/S 'optical' isomers)

This is where molecules have the same basic constitutional structural formula, but isomers differ in the 2D/3D arrangement of the atoms.

For stereoisomers, the (CIP) abbreviation means the IUPAC Cahn-Ingold-Prelog priority order rule for assigning E/Z (geometrical) and R/S (optical) stereoisomers.

E/Z stereoisomerism was called 'geometrical isomerism' e.g. cis and trans isomers of alkenes or disubstituted cyclic alkanes where there are 2D/3D spatial variations that are not mirror images and not super imposable.

Not applicable here.

R/S stereoisomerism was called 'optical isomerism', the pairs of isomers are called enantiomers which are 3D non-superimposable mirror image forms of the molecule (enantiomers). The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.

One isomer exhibits R/S optical isomerism No. (2)

NOTE

There are 8 constitutional-structural isomers of C₄H₁₁, and a total of 9 distinct unique isomers if you include the one example of R/S isomerism.

All these isomers are saturated aliphatic amines, primary, secondary or tertiary (see APPENDIX).

The nomenclature of amines is very varied so I have just quoted some of the most common names used.

Four primary amines, three secondary amines and one tertiary amine for formula C₄H₁₁N.

(1) butan-1-amine, 1-butanamine,  n-butylamine, 1-aminobutane, 

C₄H₁₁N, ,

A primary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 3 : 2 : 2 : 2 : 2 (for equivalent protons)

Index of ¹H NMR spectra organic compounds and Index of ¹³C NMR spectra organic compounds

 

(2) butan-2-amine, 2-butanamine, 2-aminobutane, sec-butylamine, ,  

It has an asymmetric carbon atom, C2 is a chiral centre, an asymmetric carbon atom attached to 4 different groups.

This primary aliphatic amine has R/S isomers, carbon atom 2 is chiral (enantiomers diagram below).

(R)-2-aminobutane, (R)-butan-2-amine, (S)-2-aminobutane, (S)-butan-2-amine

From the CIP assignment priority rule for R/S isomers: ₇N  >  ₆C₆C  >  ₆C₁H  >  ₁H

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 3 : 1 : 2 : 2 : 3 (for equivalent protons)

 

(3) 2-methylpropan-1-amine, 2-methyl-1-propamine, 1-amino-2-methylpropane, 2-methylpropylamine, iso-butylamine

,

A primary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 6 : 1 : 2 : 2 (for equivalent protons)

 

(4) 2-methylpropan-2-amine, 2-amino-2-methylpropanamine, tert-butylamine

, , a primary aliphatic amine

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 2 ¹³C (email if disagree?)

A very low number of NMR chemical shifts due to the high symmetry of the molecule.

¹H NMR ratio of peak areas: 9 (3x3) : 2 (for equivalent protons)

 

(5) N-methylpropylamine, methylpropylamine, N-methylpropanamine, N-methylpropan-1-amine

  , a secondary aliphatic amine

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 3 : 2 : 2 : 1 : 3 (for equivalent protons)

 

(6) N-ethylethylamine, diethylamine, N-ethylethanamine

, a secondary aliphatic amine

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 2 ¹³C (email if disagree?)

A lower number of NMR chemical shifts due to the high symmetry of the molecule.

¹H NMR ratio of peak areas: 6 (3+3) : 4 (2+2) : 1 (for equivalent protons)

 

(7) N-methylpropan-2-amine, N-methyl-2-propanamine, N-methylisopropylamine

  , , A secondary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 6 (3+3) : 1 : 1 : 3 (for equivalent protons)

 

(8) N,N-dimethylethylamine, ethyldimethylamine, tert-butylamine

  ,   A tertiary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 6 (3+3) : 2 : 3 (for equivalent protons)

 

See also Amines of molecular formula C₂H₇N and C₃H₉N

(3) Key revision points summary and extra information on the isomers of C₄H₁₁N

There are eight constitutional isomers of amines with the molecular formula C₄H₁₁N, including primary, secondary, and tertiary amines.

These isomers exhibit chain, position and functional group isomerism - in the sense of primary (five), secondary (three) and tertiary (one) classification.

Constitutional isomers of C₄H₁₁N

Types of Isomerism of the isomers of C₄H₁₁N

• Chain Isomerism: Different carbon skeletons (e.g., straight versus branched chains).

• Position Isomerism: Amino group attached at different positions (e.g., Butan-1-amine versus Butan-2-amine).

• Functional Group Isomerism: Primary, secondary, and tertiary amines differ in connectivity.

Differences in Physical Properties between the isomers of C₄H₁₁N

• Boiling Point: Primary amines have higher boiling points due to stronger hydrogen bonding. Tertiary amines have the lowest due to lack of N–H bonds.

• Solubility: All are fairly soluble in water, but primary amines are most soluble due to hydrogen bonding.

• Volatility: Tertiary amines are more volatile.

Differences in Chemical Reactions & Reactivity of the isomers of C₄H₁₁N

• Primary Amines: Undergo acylation, alkylation, and diazotization (if aromatic).

• Secondary Amines: React with acid chlorides to form N-substituted amides.

• Tertiary Amines: Do not undergo acylation or diazotization; act as nucleophiles in SN1/SN2 reactions.

Relative Basicity: Secondary > Primary > Tertiary in aqueous solution, from a combination of inductive and steric hindrance effects.

Uses and Applications of isomers of C₄H₁₁N

• Primary Amines: Precursors to pharmaceuticals, dyes, and agrochemicals.

• Secondary Amines: Used in rubber vulcanization, corrosion inhibitors.

• Tertiary Amines: Catalysts in epoxy resins, surfactants, and phase-transfer agents.

Common Student Misconceptions

• Confusing constitutional isomers with stereoisomers.

• Assuming all amines have similar boiling points.

• Overlooking the role of hydrogen bonding in physical properties.

• Misidentifying functional group isomers as chain isomers.

Exam Revision Tips

• Draw all isomers: Practice skeletal and condensed structures.

• Classify each amine: Primary, secondary, tertiary.

• Compare boiling points: Link structure to intermolecular forces.

• Use reaction maps: Show how each amine reacts with acyl chlorides, nitrous acid, etc.

• Mnemonic for basicity: "Secondary is strongest, but water may flip the order."

• Focus on naming conventions, reaction mechanisms, and structure-property relationships.

• Practice past paper questions involving isomer identification and reactivity comparisons.

• Use flashcards for functional group transformations and physical trends.

(4) Expanded data table on the constitutional aliphatic amine isomers of C₄H₁₁N and trend comments

Notes on entries and measurement conventions

• Boiling points are literature-style approximate values for the pure liquids at 1 atm. Values vary with source and purity; the table shows typical rounded figures, but they are all relatively similar in value.

• They are all very soluble and miscible over a wide range of proportions.

• pK_b values are approximate equilibrium basicity measures in water (higher pK_b = weaker base). Many small aliphatic amines cluster in the pK_b ≈ 3.2–3.6 range; reported values differ slightly by method and ionic strength.

• The distinct 1H NMR signal counts assume rapid free rotation and no symmetry-breaking aggregation, using a simple first‑order counting of chemically distinct environments in CDCl₃ at room temperature. Exchangeable NH protons may broaden or exchange with trace water and may not always appear as sharp signals; counts shown treat NH as one signal for primary/secondary amines when observable.

• The distinct 13C NMR signal counts reflect distinct carbon environments.

Short commentary on trends and explanations

1. Boiling point trends

• Primary amines with more linear chains (n‑butylamine, butan‑1‑amine) show the highest boiling points because they have greater surface area (stronger van der Waals) and can form two hydrogen bonds per NH site (donor + acceptor interactions in aggregates). Branched primaries (tert‑butylamine) have much lower b.p. because branching reduces surface area and packing efficiency.

• Secondary amines generally have b.p. lower than comparable linear primary amines but higher than highly branched primaries of similar mass; tertiary amine (N,N‑dimethylethylamine) often shows the lowest b.p. because it cannot donate an N–H hydrogen bond and is more compact.

2. Water solubility

• Small amines are generally water‑soluble because the protonated forms are ionic; solubility decreases with increasing hydrophobic surface area and increased branching can reduce solubility slightly. Tertiary amines remain soluble because they are protonated in water, but neutral tertiary amines are less able to H‑bond as donors.

3. Basicity (pK_b)

• Aliphatic amines cluster in a narrow pK_b range around 3–4. Electronic and steric effects shift basicity slightly: electron‑donating alkyl groups increase electron density on N and slightly increase basicity (lower pKb), but steric hindrance around nitrogen (as in tert‑butylamine) can reduce solvation of the protonated form and lower observed basicity in water (raise pKb). Solvation and hydrogen bonding of the conjugate acid are key determinants, so trends are subtle.

4. NMR signal counts

• More symmetric or more highly branched molecules give fewer distinct 1H and 13C signals (e.g., tert‑butylamine — three equivalent methyl groups produce a single methyl signal, fewer carbon environments).

• Secondary and tertiary amines often show fewer distinct N–H signals (tertiary amine: no N–H), and exchange of NH protons can broaden or hide signals.

• Positional isomers (e.g., butan‑1‑ versus butan‑2‑amine) have similar numbers of signals but differ in chemical shifts and splitting patterns; stereogenic centers (butan‑2‑amine is chiral) may give more complex patterns if enantiomeric resolution or diastereotopic protons occur.

(5) PRACTICE Exam Multiple Choice Questions on the 8 amine constitutional isomers of molecular formula C₄H₁₁N

You may have to sketch out some molecular structures to work out the answer.

Jot down your responses and check out the answers

 ANSWERS to the questions based on the isomers of C₄H₁₁N

If you think there are any errors, please email me asap at chem55555@hotmail.com

Q1. Which of the following isomers of C₄H₁₁N can exhibit R/S optical isomerism?

1. Butan-1-amine

2. 2-aminobutane

3. N-methylpropan-2-amine

4. 2-methylpropan-1-amine

Q2. Which isomer has the lowest boiling point due to weakest hydrogen bonding?

1. Diethylamine

2. Butan-1-amine

3. Tert-butylamine

4. N-methylpropan-2-amine

Q3. In the ¹H NMR spectrum, which isomer would show a triplet at δ ~1 ppm and quartet at δ ~2.5 ppm?

1. Diethylamine

2. Butan-2-amine

3. Butan-1-amine

4. N-methylpropan-1-amine

Q4. Which isomer would show two distinct CH₃ signals in ¹³C NMR due to non-equivalence?

1. Butan-1-amine

2. Butan-2-amine

3. N-methylpropan-2-amine

4. Diethylamine

Q6. Which isomer would show a total of four distinct carbon signals in ¹³C NMR?

1. Butan-1-amine

2. Trimethylamine

3. N-methylpropan-2-amine

4. Diethylamine

Q7.  Which isomer would show a singlet in ¹H NMR at δ ~2.2 ppm for N–CH₃?

1. Butan-1-amine

2. N-methylpropan-1-amine

3. Diethylamine

4. Butan-2-amine

Q8. Which isomer is a primary amine?

1. Butan-1-amine

2. Diethylamine

3. Triethylamine

4. N-methylpropan-2-amine

Q9. Which isomer is chiral and can rotate plane-polarised light?

1. Butan-1-amine

2. Butan-2-amine

3. Diethylamine

4. Triethylamine

Q10. Which isomer is primary amine showing a total of four distinct ¹H NMR signals?

1. Diethylamine

2. Butan-1-amine

3. Butan-2-amine

4. 2-methylpropan-1-amine

Q11. Which isomer is a secondary amine and would show two distinct CH₃ signal in the ¹H NMR?

1. Butan-2-amine

2. N-methylpropan-2-amine

3. Diethylamine

4. Butan-1-amine

Index of sets of isomers for a given molecular formula

Isomerism: introduction, structural isomerism - chain, positional, functional group, tautomerism

Stereoisomerism: introduction, definition, priority rules, E/Z isomerism (cis/trans isomerism)

Stereoisomerism - R/S isomerism (optical isomerism) - definition - examples explained

Index of all IR, mass, 1H NMR and 13C NMR spectroscopy pages