IUPAC Name, Structure and Formula of Straight-Chain Alkynes Chemistry Tutorial

Key Concepts

Alkynes are hydrocarbons in which there is a triple bond between two carbon atoms⁽¹⁾:
C≡C

The systematic IUPAC⁽²⁾ name of all simple straight-chain alkynes ends in "yne".

The systematic IUPAC name of a straight-chain alk-n-yne is made up of three parts⁽³⁾:

(i) A prefix which tells us how many carbon atoms are in the chain (alk).

Number of carbon atoms:	1	2	3	4	5	6	7	8	9	10
Prefix:	meth	eth	prop	but	pent	hex	hept	oct	non	dec

(ii) An infix which is a number that tells us the location of the triple bond (-n-)

(ii) The suffix yne telling us that a triple bond is present within the chain.

Note that the preferred IUPAC name⁽⁴⁾ may not be the same as the systematic IUPAC name:

molecular formula

structure

systematic IUPAC name

trivial name

Preferred IUPAC Name

C₂H₂

H-C

≡

C-H

ethyne

acetylene

IUPAC Naming of Straight-Chain Alkynes:

Identify the longest carbon chain containing the triple bond.

Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.

Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.

Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).
Note that the infix may not be required if the longest carbon chain contains only two or three carbon atoms.

Determine the suffix for the name of the alkyne. All straight chain alkynes containing one triple bond will end in "yne".

Write the name for the alkyne in the form of prefix-infix-suffix

Drawing the structure⁽⁵⁾ of straight-chain alkynes:

Break the systematic IUPAC name of the alkyne into its three parts:

alk -n- yne

prefix infix suffix

Determine the number of carbon atoms in the longest carbon chain using the prefix.

Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.

Number the carbon atoms in the chain from left to right.

Determine the location of the triple bond using the infix.
We know there is a triple bond because the molecule's name has the suffix yne.

Draw a second and then a third dash, a second and third covalent bond, between the carbon with the same number as the infix, and the carbon atom with a number equal to the infix + 1 (that is, the adjacent carbon atom on the right hand side if you have numbered your carbon chain from left to right).

Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.
Note that the carbon atoms involved in the triple bond have already used 3 out of the 4 possible bonds in making the triple bond!

Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

Molecular formula of straight-chain alkynes:⁽⁶⁾

Draw the structure of the straight-chain alkyne.

Write a skeletal molecular formula using the symbols for carbon and hydrogen:
C H

Count the number of carbon atoms in total in the structure.
total number of carbon atoms = n

Write the number of carbon atoms as a subscript number to the right of the symbol for carbon
C_nH

Count the number of hydrogen atoms in total in the structure.
total number of hydrogen atoms = y

Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
C_nH_y

The general molecular formula for a straight-chain alkyne is C_nH_2n-2
where n = number of carbon atoms in the carbon chain

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Structure and Bonding of Straight-Chain Alkynes

Straight-chain alkynes are hydrocarbons, that is, molecules containing ONLY carbon atoms and hydrogen atoms.

Carbon is a Group 14 element.
Each carbon atom in the alkyne molecule has 4 valence electrons.

.

. C .

.

Each hydrogen atom in the alkyne molecule has 1 valence electron.

. H

Each carbon atom in the alkyne molecule makes 4 covalent bonds by sharing a valence electron with another atom to make an octet of electrons (4 electron pairs).

Lewis Structure
(electron dot diagram)

Valence Structure

	. .
. .	C	. .
	. .

	\|
-	C	-
	\|

Each carbon atom can form a single covalent bond to hydrogen atoms and to other carbon atoms:

C-C

C-H

Lewis Structure

.
C
.

^._.

.
C
.

^._.

Valence Structure

.
C
.

It is also possible for a carbon atom to form two covalent bonds with another carbon atom (a double bond) by each carbon atom sharing two of its valence electrons, as in the case of straight-chain alkenes.

In the case of alkynes, a carbon atom forms three covalent bonds with another carbon atom (a triple bond) by each carbon atom sharing three of its valence electrons:

single covalent bond
C-C

double covalent bond
C=C

triple covalent bond
C≡C

Lewis Structure

.
C
.

^._.

.
C
.

..
..
..

Valence Structure

.
C
.

≡

In a straight chain alkyne there will be a triple covalent bond between two of the carbon atoms making up the chain of carbon atoms, and single bonds between any other carbon atoms in the chain.

Lewis Structure
(electron dot diagram)

Valence Structure


.	C	.. .. ..	C	_.^.	. C .	.


.	C	≡	C	-	. C .	.

In order to complete the octet of electrons around each carbon atom, any unpaired electron will pair up with the valence electron from an atom of hydrogen.

Lewis Structure
(electron dot diagram)

Valence Structure

					H
H_.	^.C	.. .. ..	C	_.^.	.. C ..	^._.H
					H

					H \|
H-	C	≡	C	-	C	-H
					\| H

A hydrocarbon that contains a triple bond (C≡C) within the carbon chain belongs to the group of organic (carbon) compounds known as alkynes.

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Worked Examples of Naming Straight-Chain Alkynes using IUPAC System

Two Carbon Chain

Give the systematic IUPAC name for the alkyne shown below:

H-C

≡

C-H

Identify the longest carbon chain containing the triple bond.

H-C ≡ C-H

Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.
Longest carbon chain contains two carbon atoms.

prefix is eth

Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.

H-C¹ ≡ C²-H

Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).

H-C¹ ≡ C²-H

Note that the infix may not be required if the longest carbon chain contains only two or three carbon atoms.

The triple bond can ONLY be between carbon atoms 1 and 2 so the infix is not required in the final name.

Determine the suffix for the name of the alkyne.
All straight chain alkynes containing one triple bond will end in "yne".

Suffix is yne

Write the name for the alkyne in the form of prefix-infix-suffix
Systematic IUPAC name is eth-1-yne

Preferred systematic IUPAC name is ethyne

Preferred IUPAC name is acetylene (which is also the trivial or traditional name for this compound).

Three Carbon Chain

Give the systematic IUPAC name for the alkyne shown below:

H-C

≡

H
|
C
|
H

-H

Identify the longest carbon chain containing the triple bond.

H-C ≡ C - H
|
C
|
H -H

Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.
Longest carbon chain contains three carbon atoms.

prefix is prop

Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.

numbering from left to right

numbering from right to left

H-C¹

≡

C²

H
|
C³
|
H

-H

H-C³

≡

C²

H
|
C¹
|
H

-H

location of triple bond: C¹

location of triple bond: C²

Choose to number the carbon chain from left to right because this gives the lowest number for the location of the triple bond.

Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).

H-C¹ ≡ C - H
|
C
|
H -H

infix is -1-

Determine the suffix for the name of the alkyne.
All straight chain alkynes containing one triple bond will end in "yne".

Suffix is yne

Write the name for the alkyne in the form of prefix-infix-suffix
Systematic IUPAC name is prop-1-yne

Other systematic names are propyne and 1-propyne

Four Carbon Chain

Give the systematic IUPAC name for the alkyne shown below:

H-

H
|
C
|
H

≡

H
|
C
|
H

-H

Identify the longest carbon chain containing the triple bond.

H- H
|
C
|
H - C ≡ C - H
|
C
|
H -H

Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.
Longest carbon chain contains four carbon atoms.

prefix is but

Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.

numbering from left to right

numbering from right to left

H-

H
|
C¹
|
H

C²

≡

C³

H
|
C⁴
|
H

-H

H-

H
|
C⁴
|
H

C³

≡

C²

H
|
C¹
|
H

-H

location of triple bond: C²

In this case the triple bond occurs between C² and C³ in both cases.

Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).

H- H
|
C
|
H - C² ≡ C - H
|
C
|
H -H

infix is -2-

Determine the suffix for the name of the alkyne.
All straight chain alkynes containing one double bond will end in "yne".

Suffix is yne

Write the systematic IUPAC name for the alkyne in the form of prefix-infix-suffix
Systematic IUPAC name is but-2-yne

Another systematic name is 2-butyne

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Worked Example: Drawing the Structure of Straight-Chain Alkynes

Draw the structure for the molecule with the systematic IUPAC name of hex-2-yne.

Break the systematic IUPAC name of the alkene into its three parts:

hex -2- yne

prefix infix suffix

Determine the number of carbon atoms in the longest carbon chain using the prefix.
Prefix is hex so there are 6 carbon atoms in the longest carbon chain.

Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.

C - C - C - C - C - C

Number the carbon atoms in the chain from left to right.

C¹ - C² - C³ - C⁴ - C⁵ - C⁶

Determine the location of the triple bond using the infix.
We know there is a triple bond because the molecule's name has the suffix yne.

Infix is 2, so this is the location of the triple bond.

C - C² - C - C - C - C

Draw a second and a third dash, a second and third covalent bond, between the carbon with the same number as the infix, and the carbon atom with a number equal to the infix + 1 (that is, the adjacent carbon atom on the right hand side if you have numbered your carbon chain from left to right).
The triple bond will occur between C² and C³ (C²⁺¹⁼³).

C - C² ≡ C³ - C - C - C

Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.

Note that the carbon atoms involved in the triple bond have already used 3 out of the 4 possible bonds in making the triple bond!

≡

Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

H
|

H-

≡

-H

|
H

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Worked Example: Writing the Molecular Formula of Straight-Chain Alkynes

Write the molecular formula for the molecule with the systematic IUPAC name of but-1-yne.

Draw the structure of the straight-chain alkyne.

					H \|		H \|
H-	C	≡	C	-	C	-	C	-H
					\| H		\| H

Write a skeletal molecular formula using the symbols for carbon and hydrogen:
C H

Count the number of carbon atoms in total in the structure.

					H \|		H \|
H-	C¹	≡	C²	-	C³	-	C⁴	-H
					\| H		\| H

total number of carbon atoms = n = 4

Write the number of carbon atoms as a subscript number to the right of the symbol for carbon
C₄H

Count the number of hydrogen atoms in total in the structure.

					H² \|		H³ \|
H¹-	C	≡	C	-	C	-	C	-H⁴
					\| H⁵		\| H⁶

total number of hydrogen atoms = y = 6

Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
C₄H₆

The general molecular formula for a straight-chain alkyne is C_nH_2n-2
where n = number of carbon atoms in the carbon chain

When n = 4 = number of carbon atoms in alkyne, then, the number of hydrogen atoms in the alkyne will equal 2 × 4 - 2 = 8 - 2 = 6

Summary Table for Straight-Chain Alkynes: IUPAC Name, Structure, and Molecular Formula

no. C atoms
(n)

Systematic IUPAC Name

Other Names

Molecular Formula

Structure

ethyne

acetylene
(preferred IUPAC name)

C₂H₂

H-

≡

-H

prop-1-yne

1-propyne
propyne

C₃H₄

					H \|
H-	C	≡	C	-	C	-H
					\| H

but-1-yne

1-butyne

C₄H₆

					H \|		H \|
H-	C	≡	C	-	C	-	C	-H
					\| H		\| H

but-2-yne

2-butyne

C₄H₆

	H \|						H \|
H-	C	-	C	≡	C	-	C	-H
	\| H						\| H

pent-1-yne

1-pentyne

C₅H₈

					H \|		H \|		H \|
H-	C	≡	C	-	C	-	C	-	C	-H
					\| H		\| H		\| H

pent-2-yne

2-pentyne

C₅H₈

	H \|						H \|		H \|
H-	C	-	C	≡	C	-	C	-	C	-H
	\| H						\| H		\| H

hex-1-yne

1-hexyne

C₆H₁₀

H
|

H-

≡

-H

|
H

hex-2-yne

2-hexyne

C₆H₁₀

H
|

H-

≡

-H

|
H

hex-3-yne

3-hexyne

C₆H₁₀

H
|

H-

≡

-H

|
H

hept-1-yne

1-heptyne

C₇H₁₂

H
|

H-

≡

-H

|
H

hept-2-yne

2-heptyne

C₇H₁₂

H
|

H-

≡

-H

|
H

hept-3-yne

3-heptyne

C₇H₁₂

H
|

H-

≡

-H

|
H

oct-1-yne

1-octyne

C₈H₁₄

H
|

H-

≡

-H

|
H

oct-2-yne

2-octyne

C₈H₁₄

H
|

H-

≡

-H

|
H

oct-3-yne

3-octyne

C₈H₁₄

H
|

H-

≡

-H

|
H

oct-4-yne

4-octyne

C₈H₁₄

H
|

H-

≡

-H

|
H

(1) We are ignoring the possibility of more than one triple bond existing in the carbon chain during this discussion.

(2) IUPAC is the abbreviation for the International Union of Pure and Applied Chemistry

(3) The preferred IUPAC systematic name places the infix for the locant immediately before that part of the name to which it relates, except when the preferred IUPAC name is the traditional contracted name in which case the infix is placed at the front of the name.
For the simple straight chain alkynes we are discussing, either nomenclature is acceptable since the name in each case is unambiguous.

(4) The systematic IUPAC name is derived from a set of general "rules" designed to ensure that each organic molecule can be given an unambiguous name.
The rules for naming organic compounds are still being developed. The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).

(5) "Structure" here will refer to a valence structure, which can be used to represent the 2-dimensional structural formula.
Once you have drawn the valence structure or 2-dimensional structural formula you can use this to draw
a condensed (semi) structural formula
or a skeletal structure

(6) We are going to ignore the possibility of cyclic compounds here as well as the possibility of there being more than triple bond.

alk	-n-	yne
prefix	infix	suffix

IUPAC Name, Structure and Formula of Straight-Chain Alkynes Chemistry Tutorial

Key Concepts

IUPAC Naming of Straight-Chain Alkynes:

Drawing the structure(5) of straight-chain alkynes:

Molecular formula of straight-chain alkynes:(6)

Structure and Bonding of Straight-Chain Alkynes

Worked Examples of Naming Straight-Chain Alkynes using IUPAC System

Worked Example: Drawing the Structure of Straight-Chain Alkynes

Worked Example: Writing the Molecular Formula of Straight-Chain Alkynes

Summary Table for Straight-Chain Alkynes: IUPAC Name, Structure, and Molecular Formula

Drawing the structure⁽⁵⁾ of straight-chain alkynes:

Molecular formula of straight-chain alkynes:⁽⁶⁾