---
title: "But-1-Yne | Organic Chemistry"
description: "But-1-yne is a terminal alkyne with formula C4H6 and a triple bond at carbon 1, used to practice naming and alkyne reactivity in Organic Chemistry."
canonical: "https://fiveable.me/organic-chem/key-terms/but-1-yne"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 9"
---

# But-1-Yne | Organic Chemistry

## Definition

But-1-yne is a four-carbon terminal alkyne with formula C4H6, meaning its triple bond starts at carbon 1. In Organic Chemistry, it is a model example for naming and alkyne reactions.

## What It Is

But-1-yne is a four-carbon alkyne with the triple bond on the first carbon, so it is a terminal alkyne. Its formula is C4H6, and the name tells you both the chain length, but-, and the triple bond, -yne.

The key structure idea is that the triple bond sits at the end of the chain rather than in the middle. That makes one of the two alkyne carbons bonded to only one other carbon and leaves a terminal hydrogen on the end. In organic chemistry, that terminal hydrogen matters because it makes the molecule behave differently from an internal alkyne.

You’ll often see but-1-yne used when a question wants you to apply IUPAC naming rules. The longest chain has four carbons, and numbering starts from the end closest to the triple bond, which gives the lowest possible number for the multiple bond. So the name is not just a label, it encodes the structure directly.

Because it is an alkyne, but-1-yne has a carbon-carbon triple bond, which contains one sigma bond and two pi bonds. That makes the bond shorter and stronger than a single bond, but also gives it the reactive electron-rich pi system that many addition reactions target. If you are drawing it, the structure is linear around the triple-bonded carbons, so the atoms in that region sit much straighter than in an alkane.

But-1-yne also sits inside the homologous series of terminal alkynes, alongside compounds that follow the same naming pattern but differ by added CH2 units. That is why it shows up so often in naming practice: once you can identify but-1-yne, you can usually handle similar molecules by changing the chain length or the position of the triple bond.

## Why It Matters

But-1-yne shows up in Organic Chemistry because it connects naming, structure, and reactivity in one small molecule. If you can read this name correctly, you are already using core IUPAC skills: finding the parent chain, numbering from the correct end, and recognizing that a triple bond changes the suffix to -yne.

It also gives you a clean example of a terminal alkyne, which matters when comparing reaction behavior. Terminal alkynes have a hydrogen on the end carbon, so they can be treated differently from internal alkynes in synthesis problems and mechanism questions. That difference shows up when you predict products, identify functional groups, or choose a starting material for a sequence.

In class, but-1-yne can also be used as a stepping stone to bigger synthesis ideas. A molecule like this helps you see how a simple carbon skeleton can be extended, functionalized, or used as a building block for more complex compounds. Instead of memorizing names as isolated facts, you start seeing how the name tells you what the molecule can do.

It is also a nice check for structure drawing. If you can sketch but-1-yne correctly from the name, you are practicing one of the most common organic chemistry moves, going from a name to a structure and back again.

## Connections

### [Alkyne](/organic-chem/key-terms/alkyne)

But-1-yne is one specific alkyne, so the broader alkyne rules apply to it. When you see this term, you should think of a carbon-carbon triple bond and the naming suffix -yne. That puts it in the same family as other unsaturated hydrocarbons, but with reactivity that comes from the triple bond rather than a single or double bond.

### Terminal Alkyne

This is the most useful relationship for but-1-yne. The triple bond is at the end of the chain, so one end of the alkyne has a hydrogen attached. That terminal position changes how you number the chain and helps you recognize why but-1-yne behaves differently from an internal alkyne on reaction questions.

### [Homologous Series](/organic-chem/key-terms/homologous-series)

But-1-yne belongs to the homologous series of terminal alkynes, where each successive member differs by a CH2 unit but keeps the same functional group pattern. That makes it a good example for spotting trends in naming and formula patterns. Once you know one member, you can often predict the next names and structures more easily.

### [IUPAC Nomenclature](/organic-chem/key-terms/iupac-nomenclature)

The name but-1-yne is a direct IUPAC name, so it is a good practice item for chain selection and numbering rules. The prefix tells you the carbon count, the locant tells you where the triple bond begins, and the suffix tells you the bonding pattern. In problem sets, that naming logic is often the first step before you discuss reactivity.

## On the AP Exam

A naming question may give you a structure and ask for the correct IUPAC name, or it may give you the name but-1-yne and ask you to draw it. The move is to identify the four-carbon chain, place the triple bond at carbon 1, and check that the chain is numbered from the correct end so the multiple bond gets the lowest number.

On reaction problems, you may use but-1-yne to recognize a terminal alkyne starting material. That matters when you predict addition products or compare it to an internal alkyne. If a prompt asks which molecule fits a terminal alkyne pattern, but-1-yne is a straightforward example you can identify fast.

If your class uses mechanism questions, you may be asked to mark the triple bond, count sigma and pi bonds, or explain why the terminal carbon still has a hydrogen attached. Those are small details, but they are the ones that separate a correct structure from a nearly-correct one.

## but-1-yne vs prop-1-ynyl

But-1-yne is a complete four-carbon molecule, while prop-1-ynyl is a substituent group, not a standalone parent chain. If you see prop-1-ynyl, it is being named as a side chain attached to something else. But-1-yne is the full alkyne skeleton by itself.

## Key Takeaways

- But-1-yne is a four-carbon terminal alkyne with the formula C4H6.
- The triple bond starts at carbon 1, so the name tells you the bond position and the chain length.
- Because it is terminal, but-1-yne has a hydrogen on the end alkyne carbon and behaves differently from an internal alkyne.
- The molecule is a useful IUPAC naming example because you have to number the chain from the end closest to the triple bond.
- In Organic Chemistry, but-1-yne often shows up as a simple starting material or reference structure for alkyne reactions.

## FAQs

### What is but-1-yne in Organic Chemistry?

But-1-yne is a four-carbon alkyne with the triple bond at the first carbon. It is a terminal alkyne, so one end of the triple bond has a hydrogen attached. In naming practice, it is a simple example of how IUPAC locants work.

### How do you draw but-1-yne from its name?

Start with a four-carbon chain, then place the triple bond at carbon 1. That means the triple bond is at the end of the chain, not in the middle. After that, check that the remaining valences are filled correctly so the structure matches C4H6.

### Is but-1-yne a terminal alkyne?

Yes. The triple bond is located at the end of the carbon chain, which makes it terminal. That detail matters because terminal alkynes have different naming and reaction behavior than internal alkynes.

### Why is but-1-yne useful in Organic Chemistry?

It is a compact example of several core skills at once, including naming, structure drawing, and recognizing terminal alkynes. It also gives you a clean molecule for thinking about alkyne reactivity and comparing one triple bond placement to another.

## Related Study Guides

- [9.1 Naming Alkynes](/organic-chem/unit-9/naming-alkynes/study-guide/ILmhHCUqBdQ4iKxp)

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