IUPAC Name and Formula of Straight-Chain Alkenes Chemistry Tutorial

Key Concepts

The International Union of Pure and Applied Chemistry (IUPAC) is developing the rules for naming compounds.

Alkenes are hydrocarbons, molecules made up of only carbon and hydrogen atoms, in which there is a double bond between two carbon atoms⁽¹⁾:
C=C

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

The systematic IUPAC name of a straight-chain alk-n-ene 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 double bond (-n-)

(iii) The suffix ene telling us that a double 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

C₂H₄

H
|
C
|
H

ethene

ethylene

Naming Straight-Chain Alkenes:

Identify the longest carbon chain containing the double bond.

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

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

Determine the infix for the name of the alkene based on the location of the double bond (use the lowest number out of the two carbon atoms joined by the double 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 alkene.
All straight chain alkenes containing one double bond will end in "ene".

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

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

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

alk -n- ene

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 double bond using the infix.
We know there is a double bond because the molecule's name has the suffix ene.

Draw a second dash, a second 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 double bond have already used 2 out of the 4 possible bonds in making the double bond!

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

Molecular formula of straight-chain alkenes:⁽⁶⁾

Draw the structure of the straight-chain alkene.

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 alkene is C_nH_2n
where n = number of carbon atoms in the carbon chain

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Structure and Bonding of Straight-Chain Alkenes

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

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

.

. C .

.

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

. H

Each carbon atom in the alkene 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:

single covalent bond
C-C

double covalent bond
C=C

Lewis Structure

.
C
.

^._.

.
C
.

Valence Structure

.
C
.

In a straight chain alkene there will be a double 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 ..		H ..
C	::	C	_.^.	C	^._.H
.. H				.. H

H \|		H \|		H \|
C	=	C	-	C	-H
\| H				\| H

A hydrocarbon that contains a double bond (C=C) within the carbon chain belongs to the group of organic (carbon) compounds known as alkenes.

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Examples of Naming Straight-Chain Alkenes using IUPAC System

Two Carbon Chain

Give the systematic IUPAC name for this alkene:

H
|
C
|
H

Identify the longest carbon chain containing the double bond.

H
|
C
|
H = H
|
C
|
H

Determine the prefix for the name of the alkene 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 double bonded carbon atoms have the lowest possible number.

H
|
C¹
|
H = H
|
C²
|
H

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

H
|
C¹
|
H = H
|
C²
|
H

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

The double 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 alkene.
All straight chain alkenes containing one double bond will end in "ene".

Suffix is ene

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

Preferred systematic IUPAC name is ethene

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

Three Carbon Chain

Give the systematic IUPAC name for this alkene:

H
|
C
|
H

H
|
C

H
|
C
|
H

-H

Identify the longest carbon chain containing the double bond.

H
|
C
|
H = H
|
C

- H
|
C
|
H -H

Determine the prefix for the name of the alkene 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 double bonded carbon atoms have the lowest possible number.

numbering from left to right

numbering from right to left

H
|
C¹
|
H

H
|
C²

H
|
C³
|
H

-H

H
|
C³
|
H

H
|
C²

H
|
C¹
|
H

-H

location of double bond: C¹

location of double bond: C²

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

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

H
|
C¹
|
H = H
|
C

- H
|
C
|
H -H

infix is -1-

Determine the suffix for the name of the alkene.
All straight chain alkenes containing one double bond will end in "ene".

Suffix is ene

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

Other systematic names are propene and 1-propene

Trivial name is propylene.

Four Carbon Chain

Give the systematic IUPAC name for this alkene:

H-

H
|
C
|
H

H
|
C

H
|
C
|
H

-H

Identify the longest carbon chain containing the double bond.

H- H
|
C
|
H - H
|
C

= H
|
C

- H
|
C
|
H -H

Determine the prefix for the name of the alkene 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 double bonded carbon atoms have the lowest possible number.

numbering from left to right

numbering from right to left

H-

H
|
C¹
|
H

H
|
C²

H
|
C³

H
|
C⁴
|
H

-H

H-

H
|
C⁴
|
H

H
|
C³

H
|
C²

H
|
C¹
|
H

-H

location of double bond: C²

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

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

H- H
|
C
|
H - H
|
C²

= H
|
C

- H
|
C
|
H -H

infix is -2-

Determine the suffix for the name of the alkene.
All straight chain alkenes containing one double bond will end in "ene".

Suffix is ene

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

Another systematic name is 2-butene

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Example: Drawing the Structure of Straight-Chain Alkenes

Draw the structure for the molecule with the systematic IUPAC name of hex-2-ene (2-hexene).

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

hex -2- ene

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 double bond using the infix.
We know there is a double bond because the molecule's name has the suffix ene.

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

C - C² - C - C - C - C

Draw a second dash, a second 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 double 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 double bond have already used 2 out of the 4 possible bonds in making the double 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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Example: Writing the Molecular Formula of Straight-Chain Alkenes

Write the molecular formula for the molecule with the systematic IUPAC name of but-1-ene (1-butene).

Draw the structure of the straight-chain alkene.

	H \|		H \|		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 \|		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 \|		H⁴ \|		H⁵ \|
H¹-	C	=	C	-	C	-	C	-H⁶
					\| H⁸		\| H⁷

total number of hydrogen atoms = y = 8

vWrite the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
C₄H₈

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

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

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

no. C atoms
(n)

Systematic IUPAC Name

Other Names

Molecular Formula

Structure

ethene

ethylene
(preferred IUPAC name)

C₂H₄

H \|		H \|
C	=	C
\| H		\| H

prop-1-ene

1-propene
propene
propylene

C₃H₆

H \|				H \|
C	=	C	-	C	-H
\| H		\| H		\| H

but-1-ene

1-butene

C₄H₈

H \|				H \|		H \|
C	=	C	-	C	-	C	-H
\| H		\| H		\| H		\| H

but-2-ene

2-butene

C₄H₈

	H \|		H \|		H \|		H \|
H-	C	-	C	=	C	-	C	-H
	\| H						\| H

pent-1-ene

1-pentene

C₅H₁₀

H \|				H \|		H \|		H \|
C	=	C	-	C	-	C	-	C	-H
\| H		\| H		\| H		\| H		\| H

pent-2-ene

2-pentene

C₅H₁₀

	H \|		H \|		H \|		H \|		H \|
H-	C	-	C	=	C	-	C	-	C	-H
	\| H						\| H		\| H

hex-1-ene

1-hexene

C₆H₁₂

H
|

-H

|
H

hex-2-ene

2-hexene

C₆H₁₂

H
|

H-

-H

|
H

hex-3-ene

3-hexene

C₆H₁₂

H
|

H-

-H

|
H

hept-1-ene

1-heptene

C₇H₁₄

H
|

-H

|
H

hept-2-ene

2-heptene

C₇H₁₄

H
|

H-

-H

|
H

hept-3-ene

3-heptene

C₇H₁₄

H
|

H-

-H

|
H

oct-1-ene

1-octene

C₈H₁₆

H
|

-H

|
H

oct-2-ene

2-octene

C₈H₁₆

H
|

H-

-H

|
H

oct-3-ene

3-octene

C₈H₁₆

H
|

H-

-H

|
H

oct-4-ene

4-octene

C₈H₁₆

H
|

H-

-H

|
H

Footnotes:

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

(2) IUPAC is the abbreviation for the International Union of Pure and Applied Chemistry.
IUPAC nomenclature began in 1892 when an international assembly of Chemists met in Geneva, Switzerland, to try to come up with a rational system for naming organic molecules.
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).
This document supercedes 'Nomenclature of Organic Chemistry, 1979 Edition' and 'A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993'.
The current document recognises that there is more than one way to systematically name organic compounds, and develops rules for assigning "preferred IUPAC names", but any other name, as long as it is unambiguous and follows the principles of the IUPAC recommendations is acceptable as a "general IUPAC name".
As a result, there are examples of "preferred IUPAC" names that are based on the tradition of use, rather than a system of naming. Examples of these traditional names that students may be expected to know are acetone, acetic acid and its derivatives such as ethyl acetate and acetic anhydride, formic acid and its derivatives such as ethyl formate.

(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 alkenes 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 double bond.

alk	-n-	ene
prefix	infix	suffix

IUPAC Name and Formula of Straight-Chain Alkenes Chemistry Tutorial

Key Concepts

Naming Straight-Chain Alkenes:

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

Molecular formula of straight-chain alkenes:(6)

Structure and Bonding of Straight-Chain Alkenes

Examples of Naming Straight-Chain Alkenes using IUPAC System

Example: Drawing the Structure of Straight-Chain Alkenes

Example: Writing the Molecular Formula of Straight-Chain Alkenes

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

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

Molecular formula of straight-chain alkenes:⁽⁶⁾