IUPAC Naming and Writing Formula of Straight Chain Alkanes Chemistry Tutorial

Key Concepts

The International Union of Pure and Applied Chemistry (IUPAC) is developing the rules for naming compounds.⁽¹⁾

Alkanes are hydrocarbons
(compounds containing ONLY carbon and hydrogen)

A single covalent bond joins one carbon atom in the chain to another carbon atom.

The name of an alkane is made up of two parts:
(i) a prefix or stem (first part of the name, alk)

(ii) a suffix (last part of the name, ane)

The suffix when naming an alkane is always "ane"

The prefix or stem is dependent on the number of carbon atoms in the chain of carbon atoms as shown below:

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

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

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

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Bonding and Structure of Straight Chain Alkanes

Alkanes are hydrocarbons, that is, alkanes are compounds containing ONLY carbon and hydrogen atoms.

Each carbon atom in the alkane molecule has 4 valence electrons.
In a Lewis structure (electron dot diagram), each valence electron is represented by a single dot surrounding the symbol for the element as shown below:

.

. C .

.

Each hydrogen atom in the alkane molecule has 1 valence electron.
The Lewis structure (electron dot diagram) for hydrogen is shown below:

. H

Each carbon atom in the alkane molecule makes 4 covalent bonds by sharing a valence electron with another atom to make an octet of electrons (4 electron pairs) as shown below:

Lewis Structure
(electron dot diagram)

Valence Structure

	. .
. .	C	. .
	. .

	\|
-	C	-
	\|

Each carbon atom can covalently bond to hydrogen atoms and to other carbon atoms as shown below:

C-C

C-H

Lewis Structure

.
C
.

^._.

.
C
.

^._.

Valence Structure

.
C
.

In a straight chain alkane there are only single covalent bonds made up of one pair of electrons joining one carbon atom to another carbon atom, as shown below:

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 as shown below:

Lewis Structure
(electron dot diagram)

Valence Structure

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

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

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Steps for Naming Straight Chain Alkanes

The name of an alkane is made up of two parts:
(i) a prefix or stem (first part of the name) : alk

(ii) a suffix (last part of the name) : ane

The name of a straight chain alkane always ends in the suffix ane

The first part of the name of a straight chain alkane, its prefix or stem, is determined by the number of carbon atoms in the chain:

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

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

To name a straight chain alkane:

Step 1: Write the general name for your organic compound (carbon compound) (alkane)

Step 2: Number the carbon atoms in the carbon chain from left to right (or from right to left).

Step 3: Use the table of prefixes above to convert the number of carbon atoms to a prefix

Step 4: Substitute the alk prefix in the general name for the prefix determined by the number of carbon atoms.

Example of Naming a Striaght Chain Alkane

Name the straight chain alkane shown below:

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

Step 1: Write the general name for your organic (carbon) compound

alkane
(the name of straight chain alkanes always ends in "ane")

Step 2: Number the carbon atoms in the carbon chain from left to right (or from right to left).

		H \|		H \|
H	-	C¹	-	C²	-	H
		\| H		\| H

Step 3: Use the table of prefixes above to convert the number of carbon atoms to a prefix

Number of carbon atoms:	2
Prefix:	eth

Step 4: Substitute the alk prefix in the general name for the prefix determined by the number of carbon atoms.

General name :	alk	ane
Name of this alkane :	eth	ane

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Steps for Drawing the Structure of Straight Chain Alkanes

Step 1: Divide the name of the straight chain alkane into a prefix and a suffix

Step 2: The suffix "ane" tells us that there are ONLY single covalent bonds between the carbon atoms in the chain.

Step 3: Use the table above to convert the prefix to the number of carbon atoms in the carbon chain

Step 4: Draw the required number of carbon atoms in a line with a dash (-) between each pair of carbon atoms to represent the single covalent bond

Step 5: Place more dashes (- or | ) to represent other covalent bonds around each carbon atom so that each carbon atom is surrounded by 4 dashes (covalent bonds)

Step 6: Place a hydrogen atom (H) at the end of any covalent bond (dash) that is not used to bond to a carbon atom (C)

Example

Draw a structure⁽²⁾ for the straight chain alkane butane.

Step 1: Divide the name of the straight chain alkane into a prefix and a suffix

name: butane

prefix is but

suffix is ane

Step 2: The suffix ane tells us that there are ONLY single covalent bonds between the carbon atoms in the chain.

Step 3: Use the table above to convert the prefix to the number of carbon atoms in the carbon chain

Number of carbon atoms:	4
Prefix:	but

Step 4: Draw the required number of carbon atoms in a line with a dash (-) between each pair of carbon atoms to represent the single covalent bond

C¹

C²

C³

C⁴

Step 5: Place more dashes (- or | ) to represent other covalent bonds around each carbon atom so that each carbon atom is surrounded by 4 dashes (covalent bonds)

	\|		\|		\|		\|
-	C	-	C	-	C	-	C	-
	\|		\|		\|		\|

Step 6: Place a hydrogen atom (H) at the end of any covalent bond (- or |) that is not used to bond to a carbon atom (C)

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

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Steps for Writing the Molecular Formula of Straight Chain Alkanes

A molecular formula tells us the number of atoms of each element present in a molecule of the compound.

For an alkane, only two elements are present, carbon (C) and hydrogen (H).

When writing the molecular formula of an alkane, the number of carbon atoms is written before the number of hydrogen atoms, that is, C is written before H

Step 1: Draw the structure of the alkane molecule.

Step 2: Write a skeleton molecular formula using the symbols for carbon (C) and hydrogen (H)

Step 3: Count the number of carbon atoms in the straight chain.

Step 4: Write the number of of carbon atoms into the skeleton molecular formula as a subscript number to the right of the symbol for carbon (C).

Step 5: Count the number of hydrogen atoms in the straight chain.

Step 6: Write the number of of hydrogen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for hydrogen (H).

Step 7: Check that your completed molecular formula makes sense (C_nH_2n+2)

Example of Writing the Molecular Formula for a Straight Chain Alkane

Write the molecular formula for propane.

Step 1: Draw the structure of the alkane molecule.

propane

prefix or stem: prop means there are 3 carbon atoms (C) in the chain

ane means there are only single covalent bonds between carbon atoms in the chain (C-C)

structure of propane:

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

Step 2: Write a skeleton molecular formula using the symbols for carbon (C) and hydrogen (H)

C H

Step 3: Count the number of carbon atoms in the straight chain.

		H		H		H
		\|		\|		\|
H	-	C¹	-	C²	-	C³	-	H
		\|		\|		\|
		H		H		H

Step 4: Write the number of of carbon atoms into the skeleton molecular formula as a subscript number to the right of the symbol for carbon (C).

C₃H

Step 5: Count the number of hydrogen atoms in the straight chain.

		H²		H³		H⁴
		\|		\|		\|
H¹	-	C	-	C	-	C	-	H⁵
		\|		\|		\|
		H⁸		H⁷		H⁶

Step 6: Write the number of of hydrogen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for hydrogen (H).

C₃H₈

Step 7: Check that your completed molecular formula makes sense (C_nH_2n+2)

propane: 3 carbon atoms so n=3
general alkane formula: C_nH_2n+2
for n=3: C₃H_2x3+2 which is C₃H₈

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Animated YouTube Tutorial for Naming Straight Chain Alkanes

Worked Examples of Names and Formulae of Straight Chain Alkanes

The table below gives the name, molecular formula and valence structure (structural formula) for straight chain alkanes from 1 to 10 carbon atoms:

Name

no. C atoms
(n)

no. H atoms
(2n + 2)

Molecular Formula
C_nH_2n+2

Valence Structure

methane

(2 x 1) + 2 = 4

CH₄

	H \|
H-	C	-H
	\| H

ethane

(2 x 2) + 2 = 6

C₂H₆

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

propane

(2 x 3) + 2 = 8

C₃H₈

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

butane

(2 x 4) + 2 = 10

C₄H₁₀

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

pentane

(2 x 5) + 2 = 12

C₅H₁₂

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

hexane

(2 x 6) + 2 = 14

C₆H₁₄

H
|

H-

-H

|
H

heptane

(2 x 7) + 2 = 16

C₇H₁₆

H
|

H-

-H

|
H

octane

(2 x 8) + 2 = 18

C₈H₁₈

H
|

H-

-H

|
H

nonane

(2 x 9) + 2 = 20

C₉H₂₀

H
|

H-

-H

|
H

decane

(2 x 10) + 2 = 22

C₁₀H₂₂

H
|

H-

-H

|
H

Footnotes:

(1) 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.

(2) "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