Naming of Alkynes

Q: What is the general formula of an alkyne?

For an open-chain hydrocarbon with exactly one carbon-carbon triple bond the general formula is CnH2n-2 with n at least 2. Each extra triple bond or ring removes two more hydrogens, so a diyne follows CnH2n-6.

Q: How do you name an alkyne in the IUPAC system?

Select the longest chain that contains the triple bond, replace the -ane of the parent alkane with -yne, then number the chain from the end nearer the triple bond and cite that number before the suffix. Substituents are added as alphabetically ordered prefixes with their own locants, as in 3-methylbut-1-yne.

Q: Why do alkynes not show cis-trans isomerism?

The two triply bonded carbons are sp hybridised with a bond angle of 180 degrees, so the whole C-C triple bond-C unit is linear. Each carbon carries just one substituent lying on the bond axis, leaving no two distinguishable sides of the bond.

Q: What is the difference between a terminal and an internal alkyne?

A terminal alkyne has the triple bond at the end of the chain and therefore an acidic hydrogen on sp carbon, as in propyne and but-1-yne. An internal alkyne has the triple bond between two carbon chains, as in but-2-yne. Only terminal alkynes give a precipitate with ammoniacal silver nitrate or cuprous chloride.

Q: How many isomers does C5H8 have as an alkyne?

Three: pent-1-yne, pent-2-yne and 3-methylbut-1-yne. The first two are position isomers and the third is a chain isomer. If non-alkyne structures are allowed the same formula also covers pentadienes, cyclopentene and several small-ring compounds.

Q: How are compounds containing both a double and a triple bond named?

They are named as alkenynes. The chain is numbered so the double and triple bond locants taken together are as low as possible, and if both directions tie the double bond gets the lower number. The ene suffix is written first, so CH3CH=CH-C(triple bond)CH is pent-3-en-1-yne.

Q: Why is the carbon-carbon triple bond the shortest carbon-carbon bond?

Triply bonded carbons use sp hybrid orbitals with 50 percent s character, and s electrons sit closer to the nucleus than p electrons. The higher s character pulls the bonding pair inward and contracts the internuclear distance to 1.20 angstrom, against 1.34 for a double bond and 1.54 for a single bond.

Q: Is propyne written as prop-1-yne or just propyne?

Both are acceptable. With only three carbons the triple bond can occupy just one position, so the locant carries no information and is usually dropped. From four carbons onwards the locant is compulsory, which is why but-1-yne and but-2-yne must always be numbered.

Q: Which alkyne is used in welding torches and why?

Ethyne, better known as acetylene. Burning it in pure oxygen releases a very large amount of heat per mole and gives a flame near 3500 degrees Celsius, hot enough to melt steel. In air it burns with a sooty luminous flame because of its high carbon percentage.

ChemistryHydrocarbonsFor JEE aspirants

Naming of alkynes follows the IUPAC pattern for any unsaturated hydrocarbon: find the longest carbon chain that contains the triple bond, replace the -ane ending of the parent alkane with -yne, and number the chain from the end that reaches the triple bond first. Alkynes have the general formula $C_{n} H_{2 n - 2}$ for one triple bond, so ethyne is $C_{2} H_{2}$ and propyne is $C_{3} H_{4}$ . When a chain carries a double bond as well, it is named as an alkenyne with the lowest possible pair of locants. This page covers the naming of alkynes rule by rule, with worked examples, common names and the isomerism you are expected to recognise for JEE and NEET.

SYLLABUS Alkynes are prescribed for JEE Main, JEE Advanced and NEET under Hydrocarbons (NCERT Class 11, Unit 13). Nomenclature, structure, isomerism, preparation and properties are all examinable.

Key Formulas - Quick Reference

General formula (one triple bond): $C_{n} H_{2 n - 2}$ , with $n \geq 2$
Degree of unsaturation: $DoU = \frac{2 n + 2 - H}{2}$ ; one $C \equiv C$ contributes $2$
Naming pattern: alkane $\to$ drop -ane $\to$ add -yne $+$ position number
Bond data: $C \equiv C$ length $= 1.20 \overset{˚}{A}$ , bond energy $\approx 835 kJ mo l^{- 1}$ ( $192 kcal mo l^{- 1}$ )
Hybridisation of triply bonded carbon: $s p$ , bond angle $= 18 0^{\circ}$ , $s$ character $= 50%$
Bond length order: $C \equiv C < C = C < C - C$
Enyne rule: lowest locant set to ${ene, yne}$ together; on a tie the double bond gets the lower number

1. What Is an Alkyne?

Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. They are also called acetylenes, because every member can be viewed as a derivative of the first member, acetylene ( $HC \equiv CH$ ).

Definition. An alkyne is an aliphatic hydrocarbon whose molecule contains at least one

C \equiv C

bond. For a single triple bond and an open chain, the molecular formula is

C_{n} H_{2 n - 2}

A triple bond removes four hydrogen atoms relative to the alkane, so it contributes two degrees of unsaturation.
An alkyne whose triple bond sits at the chain end ( $- C \equiv CH$ ) is called a terminal alkyne; one with the triple bond inside the chain is an internal or non-terminal alkyne. This distinction drives almost every reaction difference you will meet later.
Alkynes are much less common in nature than alkenes, though some plants make them as chemical defences. Acetylene is by far the most important industrial alkyne, used mainly as the fuel of the oxy-acetylene welding torch.

The homologous series

$n$	Molecular formula	IUPAC name	Structure	Type
2	$C_{2} H_{2}$	Ethyne	$HC \equiv CH$	terminal
3	$C_{3} H_{4}$	Propyne	$C H_{3} - C \equiv CH$	terminal
4	$C_{4} H_{6}$	But-1-yne	$C H_{3} C H_{2} - C \equiv CH$	terminal
4	$C_{4} H_{6}$	But-2-yne	$C H_{3} - C \equiv C - C H_{3}$	internal
5	$C_{5} H_{8}$	Pent-1-yne	$C H_{3} C H_{2} C H_{2} - C \equiv CH$	terminal
5	$C_{5} H_{8}$	Pent-2-yne	$C H_{3} C H_{2} - C \equiv C - C H_{3}$	internal

2. Structure and Bonding of the Triple Bond

Each carbon of the triple bond is $s p$ hybridised. The two $s p$ orbitals point in opposite directions, which forces the $H - C \equiv C - H$ framework to be linear.

Figure 1: The

C \equiv C

triple bond. One

σ

bond from

s p

s p

overlap plus two

π

bonds at

9 0^{\circ}

to each other give a linear molecule with a cylindrical

π

cloud.

One $σ$ bond from $s p$ - $s p$ overlap along the internuclear axis.
Two $π$ bonds from side-on overlap of the two unhybridised $p$ orbitals on each carbon. These two $π$ clouds lie at right angles, so together they form a cylindrical sheath of electrons around the $σ$ bond.
Bond length $1.20 \overset{˚}{A}$ and bond energy $192 kcal mo l^{- 1}$ ( $\approx 835 kJ mo l^{- 1}$ ): the shortest and strongest carbon-carbon bond.
Because $s$ character rises from $25%$ ( $s p^{3}$ ) to $33.3%$ ( $s p^{2}$ ) to $50%$ ( $s p$ ), the bonding electrons sit closer to the nucleus and the internuclear distance contracts. Hence $C \equiv C < C = C < C - C$ .

Why this matters for naming: the linear geometry is the reason alkynes have no

c i s

t r an s

forms, so an alkyne name never carries an

E

Z

c i s

t r an s

prefix at the triple bond.

3. IUPAC Rules for Naming Alkynes

In the IUPAC system, the compound is named as an alkyne in which the final -ane of the parent alkane is replaced by the suffix -yne. The position of the triple bond is indicated by a number.

Pick the parent chain. Choose the longest continuous carbon chain that contains the triple bond, even if a longer chain exists elsewhere in the molecule.
Name the parent. Drop -ane from the alkane name and add -yne: ethane $\to$ ethyne, propane $\to$ propyne, butane $\to$ butyne.
Number the chain. Start from the end that gives the triple bond the lower locant. The triple bond outranks every alkyl branch in this decision.
Place the locant. Cite the number of the first triply bonded carbon, written just before the suffix: but-1-yne, pent-2-yne.
Add substituents. Name and number branches as usual, cite them alphabetically as prefixes, and separate numbers from words with hyphens: $3$ -methylbut- $1$ -yne.
More than one triple bond? Use $d i$ -, $t r i$ - with the full parent name kept intact: hexa- $1, 5$ -diyne (the "a" of hexa is retained before a consonant).

Figure 2: The triple bond, not the branch, decides the direction of numbering.

HC \equiv C - CH (C H_{3}) - C H_{3}

is but-1-yne substituted at

C_{3}

, never but-3-yne.

Remember the ranking. Chain length decides first, then the triple bond position, then the substituents. A student who numbers "from the branch end" will get but-3-yne, which is simply not a valid name for that compound.

Solved Example 1

Give the IUPAC name of

C H_{3} - C \equiv CH

and of

(C H_{3})_{2} CH - C \equiv CH

Solution:

The first has three carbons with the triple bond at the end: parent propane $\to$ propyne (the locant $1$ is unnecessary because no other position is possible in a three-carbon chain, though prop- $1$ -yne is also correct).

The second has a four-carbon parent chain, $C H_{3} - CH (C H_{3}) - C \equiv CH$ . Numbering from the alkyne end: $C_{1} \equiv C_{2}$ , methyl at $C_{3}$ . Name: $3$ -methylbut- $1$ -yne.

Solved Example 2

Name

C H_{3} C H_{2} - C \equiv C - C H_{2} C H_{3}

and

C H_{3} - C \equiv C - C H_{2} C H_{2} C H_{3}

Solution:

The first is a symmetrical six-carbon chain with the triple bond between $C_{3}$ and $C_{4}$ : hex- $3$ -yne. Numbering from either end gives the same answer.

The second is a six-carbon chain. From the left the triple bond starts at $C_{2}$ ; from the right it starts at $C_{4}$ . Choose the lower: hex- $2$ -yne.

Solved Example 3

A hydrocarbon has the molecular formula

C_{6} H_{10}

. How many degrees of unsaturation does it contain, and what structural features are possible?

Solution:

$DoU = \frac{2 ( 6 ) + 2 - 10}{2} = \frac{4}{2} = 2$ .

Two degrees of unsaturation can mean one triple bond, or two double bonds, or one double bond plus one ring, or two rings. If the compound is stated to be an alkyne, then exactly one $C \equiv C$ accounts for both degrees and the chain is otherwise saturated and acyclic.

4. Common (Derived) Names

Alkynes are still widely named as substituted acetylenes: name the groups attached to the two triply bonded carbons and follow with the word acetylene.

Structure	IUPAC name	Common (derived) name
$HC \equiv CH$	Ethyne	Acetylene
$C H_{3} - C \equiv CH$	Propyne	Methylacetylene
$C H_{3} - C \equiv C - C H_{3}$	But-2-yne	Dimethylacetylene
$C H_{3} C H_{2} - C \equiv CH$	But-1-yne	Ethylacetylene
$C H_{3} - C \equiv C - C H_{2} C H_{3}$	Pent-2-yne	Methylethylacetylene

Exam papers mix both systems freely. "Ethylcyclohexylacetylene" and "

1

-cyclohexylbut-

1

-yne" are the same compound; translate to IUPAC before you start solving.

5. Naming Alkenynes: Double and Triple Bonds Together

When both a double bond and a triple bond are present, the hydrocarbon is named as an alkenyne. Both suffixes appear, -ene first and -yne last, each with its own locant.

Choose the longest chain containing both multiple bonds.
Number so that the set of locants for the double and triple bonds together is lowest, comparing the two sets term by term.
If both directions give the same set, the double bond receives the lower number.
Write the name as $\dots$ - $a$ -en- $b$ -yne.

Figure 3: Enyne numbering. Compare the full locant set from each end and keep the lower set; the name still reads

-en-

before

-yne

Solved Example 4

Name

C H_{3} - CH = CH - C \equiv CH

and

HC \equiv C - C H_{2} - CH = C H_{2}

Solution:

First compound. Five carbons. From the alkyne end the locant set is ${1, 3}$ ; from the other end it is ${2, 4}$ . Since ${1, 3}$ is lower, number from the alkyne end: pent- $3$ -en- $1$ -yne.

Second compound. Five carbons again. From the alkene end the set is ${1, 4}$ ; from the alkyne end it is ${1, 4}$ as well, so the two sets tie. The tie-break gives the double bond the lower number: pent- $1$ -en- $4$ -yne.

Solved Example 5

What is the IUPAC name of

C H_{2} = CH - C \equiv CH

, the dimer of acetylene?

Solution:

Four carbons, double bond starting at $C_{1}$ and triple bond starting at $C_{3}$ when numbered from the alkene end. The locant set ${1, 3}$ beats ${1, 3}$ from the other end only on the tie-break, which again favours the double bond. Name: but- $1$ -en- $3$ -yne, commonly called vinyl acetylene.

6. Isomerism in Alkynes

Alkynes from $C_{4}$ onwards show isomerism, but not every kind.

Figure 4: Isomerism in

C_{5} H_{8}

alkynes: two position isomers (pent-1-yne, pent-2-yne) and one chain isomer (

3

-methylbut-

1

-yne).

Position isomerism (from $C_{4}$ ): but- $1$ -yne and but- $2$ -yne differ only in where the triple bond sits.
Chain isomerism (from $C_{5}$ ): pent- $1$ -yne versus $3$ -methylbut- $1$ -yne.
Ring-chain and functional isomerism: an alkyne $C_{n} H_{2 n - 2}$ is isomeric with dienes, cyclic alkenes and bicyclic alkanes of the same formula. For example $C_{4} H_{6}$ covers but- $1$ -yne, but- $2$ -yne, buta- $1, 3$ -diene, buta- $1, 2$ -diene, cyclobutene and methylcyclopropene.
No geometrical isomerism: the triple bonded unit is linear, so there is no "same side" to define.

Figure 5: Geometry decides isomerism. A

12 0^{\circ}

alkene has two faces; a

18 0^{\circ}

alkyne has none, so

C H_{3} - C \equiv C - C H_{3}

exists in one form only.

Solved Example 6

Why is

c i s

t r an s

isomerism impossible in alkynes?

Solution:

Because of the $18 0^{\circ}$ bond angle at the triply bonded carbons. Geometrical isomerism requires restricted rotation and two different groups on each of the two doubly bonded atoms lying in a plane. In an alkyne each triply bonded carbon carries only one substituent, and that substituent lies on the bond axis itself, so the two possible "arrangements" are identical molecules.

Solved Example 7

Draw and name all the alkyne isomers of

C_{6} H_{10}

that are terminal alkynes.

Solution:

Terminal means the chain ends in $- C \equiv CH$ , so carbons $1$ and $2$ are fixed and the remaining four carbons form the tail.

$C H_{3} C H_{2} C H_{2} C H_{2} - C \equiv CH$ : hex- $1$ -yne
$(C H_{3})_{2} CH - C H_{2} - C \equiv CH$ : $4$ -methylpent- $1$ -yne
$C H_{3} C H_{2} - CH (C H_{3}) - C \equiv CH$ : $3$ -methylpent- $1$ -yne
$(C H_{3})_{3} C - C \equiv CH$ : $3, 3$ -dimethylbut- $1$ -yne

Four terminal alkynes in all. (Hex- $2$ -yne and hex- $3$ -yne exist too, but they are internal.)

7. Quick Comparison: Alkane, Alkene, Alkyne

Property	Alkane	Alkene	Alkyne
General formula	$C_{n} H_{2 n + 2}$	$C_{n} H_{2 n}$	$C_{n} H_{2 n - 2}$
Suffix	-ane	-ene	-yne
Hybridisation	$s p^{3}$	$s p^{2}$	$s p$
$s$ character	$25%$	$33.3%$	$50%$
Bond length	$1.54 \overset{˚}{A}$	$1.34 \overset{˚}{A}$	$1.20 \overset{˚}{A}$
Bond energy	$347 kJ mo l^{- 1}$	$615 kJ mo l^{- 1}$	$835 kJ mo l^{- 1}$
Shape at the bond	Tetrahedral	Planar, $12 0^{\circ}$	Linear, $18 0^{\circ}$
Geometrical isomerism	No	Yes	No

Common Mistakes to Avoid

Watch out

Numbering from the substituent end. The triple bond always outranks an alkyl branch. It is $3$ -methylbut- $1$ -yne, never $2$ -methylbut- $3$ -yne.
Choosing the longest chain in the molecule instead of the longest chain through the triple bond. The parent chain must contain the $C \equiv C$ even if a longer chain exists elsewhere.
Writing $c i s$ or $t r an s$ for an alkyne. A linear unit has no geometry to specify.
Using $C_{n} H_{2 n - 2}$ for a cyclic compound. That formula assumes one triple bond and an open chain; a cycloalkene fits the same formula with completely different bonding.
Forgetting that the alkyne locant cites the first of the two carbons. In pent- $2$ -yne the triple bond spans $C_{2}$ and $C_{3}$ , and only $2$ appears in the name.
Reversing the suffix order in enynes. It is always "-en-" then "-yne", so pent- $4$ -en- $1$ -yne is a valid form while pent- $1$ -yn- $4$ -ene is not.
Assuming "acetylene" means only $C_{2} H_{2}$ . In derived nomenclature every alkyne is an "acetylene", so dimethylacetylene is but- $2$ -yne.

Frequently Asked Questions

What is the general formula of an alkyne?

For an open-chain hydrocarbon with exactly one carbon-carbon triple bond, the general formula is $C_{n} H_{2 n - 2}$ with $n \geq 2$ . Each extra triple bond or ring removes two more hydrogens, so a diyne follows $C_{n} H_{2 n - 6}$ .

How do you name an alkyne in the IUPAC system?

Select the longest chain that contains the triple bond, replace the -ane of the parent alkane with -yne, then number the chain from the end nearer the triple bond and cite that number before the suffix. Substituents are added as alphabetically ordered prefixes with their own locants, as in $3$ -methylbut- $1$ -yne.

Why do alkynes not show cis-trans isomerism?

The two triply bonded carbons are $s p$ hybridised with a bond angle of $18 0^{\circ}$ , so the whole $C - C \equiv C - C$ unit is linear. Each carbon carries just one substituent and that substituent lies on the bond axis, leaving no two distinguishable sides of the bond.

What is the difference between a terminal and an internal alkyne?

A terminal alkyne has the triple bond at the end of the chain, so it carries an acidic $\equiv C - H$ hydrogen; propyne and but- $1$ -yne are examples. An internal alkyne has the triple bond between two carbon chains, as in but- $2$ -yne. Only terminal alkynes give a precipitate with ammoniacal silver nitrate or cuprous chloride.

How many isomers does $C_{5} H_{8}$ have as an alkyne?

Three: pent- $1$ -yne, pent- $2$ -yne and $3$ -methylbut- $1$ -yne. The first two are position isomers and the third is a chain isomer. If non-alkyne structures are allowed, the same formula also covers pentadienes, cyclopentene and several cyclopropane and cyclobutane derivatives.

How are compounds containing both a double and a triple bond named?

They are named as alkenynes. The chain is numbered so the double and triple bond locants taken together are as low as possible, and if both directions tie the double bond gets the lower number. Write the ene suffix first: $C H_{3} CH = CH - C \equiv CH$ is pent- $3$ -en- $1$ -yne.

Why is the $C \equiv C$ bond the shortest carbon-carbon bond?

Triply bonded carbons use $s p$ hybrid orbitals with $50%$ $s$ character, and $s$ electrons sit closer to the nucleus than $p$ electrons. The higher $s$ character pulls the bonding pair inward and contracts the internuclear distance to $1.20 \overset{˚}{A}$ , against $1.34 \overset{˚}{A}$ for $C = C$ and $1.54 \overset{˚}{A}$ for $C - C$ .

Is propyne written as prop- $1$ -yne or just propyne?

Both are acceptable. With only three carbons the triple bond can occupy just one position, so the locant carries no information and is usually dropped. From four carbons onwards the locant is compulsory, which is why but- $1$ -yne and but- $2$ -yne must always be numbered.

Which alkyne is used in welding torches and why?

Ethyne, better known as acetylene. Burning it in pure oxygen releases a very large amount of heat per mole and produces a flame near $3500^{\circ} C$ , hot enough to melt steel. In air it burns with a sooty luminous flame because of its high carbon percentage.

Previous year questions on Naming of Alkynes

2 questions from past papers, each with a step-by-step solution.

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1. What Is an Alkyne?

The homologous series

2. Structure and Bonding of the Triple Bond

3. IUPAC Rules for Naming Alkynes

4. Common (Derived) Names

5. Naming Alkenynes: Double and Triple Bonds Together

6. Isomerism in Alkynes

7. Quick Comparison: Alkane, Alkene, Alkyne

Common Mistakes to Avoid

Frequently Asked Questions

What is the general formula of an alkyne?

How do you name an alkyne in the IUPAC system?

Why do alkynes not show cis-trans isomerism?

What is the difference between a terminal and an internal alkyne?

How many isomers does C5​H8​ have as an alkyne?

How are compounds containing both a double and a triple bond named?

Why is the C≡C bond the shortest carbon-carbon bond?

Is propyne written as prop-1-yne or just propyne?

Which alkyne is used in welding torches and why?

Previous year questions on Naming of Alkynes

Ready to master Hydrocarbons?

How many isomers does $C_{5} H_{8}$ have as an alkyne?

Why is the $C \equiv C$ bond the shortest carbon-carbon bond?

Is propyne written as prop- $1$ -yne or just propyne?