C3H9N Isomers (Amine Structural Guide)
C₃H₉N has four neutral, saturated, acyclic amine constitutional isomers: propan-1-amine, propan-2-amine, N-methylethanamine, and trimethylamine. They all have a molar mass of about 59.11 g·mol⁻¹ and a degree of unsaturation of zero. Their atoms connect in different ways, and the number of carbon groups attached to nitrogen determines whether each amine is primary, secondary, or tertiary.
A useful surprise: all four compounds share the same molecular formula, yet one has no N–H bond while the other three do. A formula tells you how many atoms are present, but not how they connect. That difference is the key to finding every isomer without counting the same structure twice.
When I work through this kind of question, I use the formula to set limits first, then build and check each structure. This method is more reliable than trying to recall a list from memory. It also makes clear why N-methylethanamine belongs on the list: its carbon atoms connect to nitrogen in a different pattern from those in either propylamine.
C₃H₉N Isomer Overview and Classification
A constitutional isomer has the same molecular formula as another compound but a different connection between its atoms. For C₃H₉N, four neutral, saturated, acyclic amines meet the formula. Each has three carbon atoms, nine hydrogen atoms, and one nitrogen atom, but the carbon groups are arranged differently around the nitrogen.
An amine is a compound in which nitrogen is bonded to one or more carbon groups. To classify an amine, count the carbon groups directly attached to nitrogen. Do not count the total number of carbons in the whole molecule.
| Isomer | Condensed structure | Carbon groups attached to N | Amine class |
|---|---|---|---|
| Propan-1-amine | CH₃CH₂CH₂NH₂ | 1 | Primary |
| Propan-2-amine | (CH₃)₂CHNH₂ | 1 | Primary |
| N-methylethanamine | CH₃CH₂NHCH₃ | 2 | Secondary |
| Trimethylamine | (CH₃)₃N | 3 | Tertiary |
A primary amine has one carbon group bonded to nitrogen; nitrogen also has two hydrogen atoms in these examples. A secondary amine has two carbon groups and one hydrogen bonded to nitrogen. A tertiary amine has three carbon groups and no N–H bond.
Notice that the two primary amines have the same nitrogen classification but different carbon arrangements. In propan-1-amine, nitrogen is attached at the end of a three-carbon chain. In propan-2-amine, it is attached to the middle carbon. Classification describes nitrogen’s attachments; it does not tell the whole structure.
Structures, Naming, and Degree of Unsaturation
A structural formula shows which atoms are bonded to which. A name gives a standard way to describe that structure. The degree of unsaturation is a formula-based check for rings and multiple bonds; here it equals zero, so the listed neutral structures have neither.
Calculate the degree of unsaturation
For a formula containing carbon, hydrogen, and nitrogen, use:
DBE = (2C + 2 + N − H) / 2
DBE means degree of unsaturation. Substitute the atom counts for C₃H₉N:
DBE = (2 × 3 + 2 + 1 − 9) / 2 = 0
A value of zero means there is no unsaturation to account for. For the neutral, closed-shell structures considered here, that rules out rings and carbon–carbon or carbon–nitrogen multiple bonds. It supports looking for saturated, acyclic amines.
The formula also passes a simple valence check. Carbon normally forms four bonds, nitrogen in these neutral amines forms three, and hydrogen forms one. If a drawing gives a carbon five bonds or a neutral nitrogen four ordinary bonds, check the structure and its charge.
Read the four structures
Propan-1-amine, CH₃CH₂CH₂NH₂, has a straight three-carbon chain. The NH₂ group is attached to carbon 1, the chain end. Its common name is 1-propylamine.
Propan-2-amine, (CH₃)₂CHNH₂, has NH₂ attached to carbon 2. Its common name is isopropylamine. The central carbon is bonded to two identical CH₃ groups, so it is not a stereogenic center.
N-methylethanamine, CH₃CH₂NHCH₃, has an ethyl group and a methyl group attached to nitrogen. The prefix N-methyl tells you that the methyl group is bonded to nitrogen, not added to the carbon chain.
Trimethylamine, (CH₃)₃N, has three methyl groups bonded to nitrogen. The neutral nitrogen has no hydrogen attached.
These names help prevent a common reading error. In N-methylethanamine, the name does not describe a new three-carbon chain with an amine at one end. It describes two carbon groups attached to the same nitrogen.
Drawing, Counting, and Verifying Amine Isomers
A reliable count comes from considering how the three carbon atoms can be placed in groups around nitrogen. First check the formula and DBE. Then list the possible carbon arrangements, draw each connection, and remove duplicates. This makes the secondary amine easy to spot rather than miss.
Step 1: Set the formula limits
C₃H₉N contains only carbon, hydrogen, and nitrogen. With DBE equal to zero, build saturated structures without rings or multiple bonds. Keep nitrogen neutral and give it three bonds. These rules define the set being counted.
Step 2: Place nitrogen at different positions
Start with a three-carbon chain. Attaching NH₂ at either end gives the same structure when the chain is numbered from the end nearest the amine: propan-1-amine. Attaching NH₂ to the middle carbon gives propan-2-amine.
The two arrangements are different because the nitrogen attaches to different carbons. But reversing the drawing of propan-1-amine does not make a new isomer. It is the same connectivity viewed from the other direction.
Step 3: Divide the carbons among nitrogen’s attachments
Now consider nitrogen bonded to more than one carbon group. The three carbons can be divided around nitrogen in these ways:
- 1 + 2: one methyl group and one ethyl group, giving N-methylethanamine.
- 1 + 1 + 1: three methyl groups, giving trimethylamine.
The first pattern is the one that can be overlooked. It is not a new carbon skeleton in the usual chain sense. It is a distinct arrangement because nitrogen sits between two carbon groups.
Step 4: Check formula and duplicates
For each candidate, count every atom in the drawing and confirm nitrogen’s bonds. Then compare connections, not how the sketch looks on the page.
| Check | What to confirm | Example |
|---|---|---|
| Atom count | 3 C, 9 H, 1 N | CH₃CH₂NHCH₃ fits |
| Nitrogen bonds | Three bonds for neutral amine N | Two C–N bonds and one N–H bond |
| Carbon attachment | Count carbon groups bonded to N | N-methylethanamine has two |
| Duplicate test | Same connections means same isomer | Reversing a chain drawing adds none |
| Formula check | DBE equals zero | No ring or multiple bond is included |
A quick final check is to sort the results by amine class: two primary, one secondary, and one tertiary. If your list contains only primary amines, look again for the 1 + 2 and 1 + 1 + 1 arrangements.
Structural Differences and Chemical Significance
The four compounds have the same atom counts and molar mass, but their connectivity changes the shape and bonding around nitrogen. Those changes matter when you compare structures, name compounds, or predict basic features of their behavior. They do not change the basic count of four constitutional isomers.
All four are constitutional isomers, not stereoisomers. Stereoisomers have the same atom-to-atom connections but differ in spatial arrangement. Here, each listed structure differs in connectivity. None has a stereogenic center: in propan-2-amine, for example, the central carbon has two identical methyl groups.
The N–H bond count offers another useful comparison. Propan-1-amine and propan-2-amine each have two N–H bonds. N-methylethanamine has one, while trimethylamine has none. All four have a lone pair on nitrogen in their neutral amine form. In introductory chemistry, these features help describe how amines can interact or take part in reactions, though a specific prediction needs the reaction conditions.
Do not confuse an alternate drawing with an alternate isomer. A molecule may be rotated or drawn with a different zigzag shape while keeping every atom connected to the same neighbors. That drawing still represents the same compound. A new constitutional isomer requires a change in connectivity, such as moving the amine attachment from the end carbon to the middle carbon.
For identification tasks, keep the goal narrow: confirm the formula, check the DBE, and compare atom connections. Physical-property comparisons may be useful in another question, but they are not needed to establish the four structures.
Worked Identification Checks
Worked checks show how to apply the counting method to a proposed structure. Each example asks whether the candidate fits C₃H₉N and whether it duplicates another listed structure. The same checks work on a formula, a condensed structure, or a line drawing.
Candidate: CH₃CH₂CH₂NH₂. It has three carbons, nine hydrogens, and one nitrogen. Nitrogen bonds to one carbon group, so it is a primary amine. The structure is propan-1-amine.
Candidate: CH₃CH₂NHCH₃. It also has three carbons, nine hydrogens, and one nitrogen. Nitrogen bonds to two carbon groups, so it is a secondary amine. It is not propan-1-amine drawn differently: the nitrogen lies between an ethyl group and a methyl group.
Candidate: (CH₃)₃N. Three methyl groups give three carbons, and neutral nitrogen has three C–N bonds with no N–H bond. This is trimethylamine, the tertiary isomer.
A proposed structure with a ring or a double bond does not fit this set because the formula’s DBE is zero. A protonated ammonium ion also does not count as another neutral isomer: it has a charge and a different hydrogen count.
Frequently Asked Questions
These answers address common points in formula-based isomer problems. The key distinction is between atom count and atom connectivity. Use the formula and DBE to set limits, then use nitrogen’s carbon attachments to classify each amine and check whether two drawings are truly different.
How many amine isomers have the formula C₃H₉N?
There are four neutral, saturated, acyclic amine constitutional isomers: propan-1-amine, propan-2-amine, N-methylethanamine, and trimethylamine.
What is the molar mass of C₃H₉N?
Its molar mass is about 59.11 g·mol⁻¹. This follows from three carbon atoms, nine hydrogen atoms, and one nitrogen atom using standard atomic masses.
What is the degree of unsaturation for C₃H₉N?
It is zero: (2 × 3 + 2 + 1 − 9) / 2 = 0. The neutral, closed-shell structures in this guide therefore have no rings or multiple bonds.
Why are propan-1-amine and propan-2-amine different isomers?
The NH₂ group bonds to a different carbon. It attaches to an end carbon in propan-1-amine and to the middle carbon in propan-2-amine.
Why is N-methylethanamine a separate isomer?
Its nitrogen bonds to an ethyl group and a methyl group. That connectivity differs from both propylamines, where nitrogen bonds to just one carbon group.
Which isomers are primary amines?
Propan-1-amine and propan-2-amine are primary. Each has one carbon group attached to nitrogen.
Which isomer is secondary?
N-methylethanamine is secondary because nitrogen bonds to two carbon groups and one hydrogen.
Which isomer is tertiary?
Trimethylamine is tertiary. Nitrogen bonds to three carbon groups and has no N–H bond.
Are any of these isomers stereoisomers?
No. The four are constitutional isomers because their atom connections differ. None of the listed structures has a stereogenic center.
Does a different drawing count as a new isomer?
No. Turning, flipping, or redrawing a structure does not make a new isomer if the atom connections stay the same.
(This article was written by one of our staff writers, Alan Westbrook. Visit our Meet the Team page.)
