Alkanes Revision Notes for HSC SSCE Chemistry

Alkanes

Introduction to alkanes

Alkanes are a crucial family of hydrocarbons that play a vital role in our daily lives. These compounds provide the energy we use every day - they power our vehicles, heat our water, warm our homes, and help us cook our food. Beyond their use as fuels, alkanes also serve as essential raw materials for manufacturing plastics and many other substances.

Alkanes are saturated compounds, which means they contain only single carbon-carbon bonds and no multiple bonds. This saturation makes them relatively stable and gives them distinct properties.

infoNote

The term "saturated" refers to the fact that each carbon atom is bonded to the maximum possible number of hydrogen atoms. This is because all bonds are single bonds - there are no double or triple bonds that would reduce the number of hydrogens.

The first alkanes

The simplest member of the alkane family is methane ( $\text{CH}_4$ ). This is followed by ethane ( $\text{C}_2\text{H}_6$ ), propane ( $\text{C}_3\text{H}_8$ ), and butane ( $\text{C}_4\text{H}_{10}$ ).

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Different Ways to Represent Molecules

The diagram above shows three different representations of propane and butane:

Ball-and-stick models showing atoms (pink spheres for carbon, gray for hydrogen) connected by bonds
Structural formulae displaying all carbon and hydrogen atoms with their bonds
Space-filling models showing the actual shape and size of the molecules

Each representation provides different insights into the molecule's structure and properties.

Straight-chain alkanes

The first ten members of the alkane family are listed in the table below. These are called straight-chain alkanes because all carbon atoms are connected in one continuous chain.

Name	Formula
Methane	$\text{CH}_4$
Ethane	$\text{C}_2\text{H}_6$
Propane	$\text{C}_3\text{H}_8$
Butane	$\text{C}_4\text{H}_{10}$
Pentane	$\text{C}_5\text{H}_{12}$
Hexane	$\text{C}_6\text{H}_{14}$
Heptane	$\text{C}_7\text{H}_{16}$
Octane	$\text{C}_8\text{H}_{18}$
Nonane	$\text{C}_9\text{H}_{20}$
Decane	$\text{C}_{10}\text{H}_{22}$

chatImportant

Despite being called "straight-chain", these molecules aren't actually straight lines. The carbon bonds arrange themselves in a zigzag pattern due to the tetrahedral geometry around each carbon atom. This is a common misconception - the "straight" refers to the connectivity of atoms, not their spatial arrangement.

The figure above shows the same alkane molecule represented in two ways: condensed formula on the left and the more realistic zigzag arrangement on the right.

Homologous series

Alkanes belong to a homologous series - a group of compounds that share the same general formula. Each successive member of the series is formed by adding one carbon atom and three hydrogen atoms. This follows a predictable pattern, giving us the general molecular formula:

$\text{C}_n\text{H}_{2n+2}$

where $n$ is a whole number representing the number of carbon atoms.

lightbulbExample

Worked Example: Using the General Formula

For propane, we know it has 3 carbon atoms, so $n = 3$ .

Substituting into the general formula: $\text{C}_3\text{H}_{(2 \times 3)+2} = \text{C}_3\text{H}_8$

Therefore, the molecular formula for propane is $\text{C}_3\text{H}_8$ .

Straight-chain versus branched alkanes

Not all alkane molecules have their carbon atoms in one continuous chain. Many alkanes contain branched chains, where carbon atoms branch off from the main chain.

The skeletal structures above show the difference between straight-chain alkanes (left) and branched alkanes (right). This structural difference affects how we name these compounds.

Naming alkanes

The naming of hydrocarbons follows rules set by IUPAC (International Union of Pure and Applied Chemistry). These rules ensure that every organic compound has one universally accepted name, regardless of language or country.

Understanding alkane names

An alkane's name consists of two parts:

Stem - indicates the length of the carbon chain
Ending - identifies the hydrocarbon family (always -ane for alkanes)

Carbon chain stems

The stems for the first ten members are:

Number of carbons	Stem	Number of carbons	Stem
1	meth-	6	hex-
2	eth-	7	hept-
3	prop-	8	oct-
4	but-	9	non-
5	pent-	10	dec-

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Memory Tip

Learn these stems in order - they're essential for naming any organic compound! A helpful mnemonic: "My Elephant Proudly Bows Patiently Helping Every Organic Nomenclature Develop"

Rules for naming branched alkanes

Follow these six steps to name branched alkanes correctly:

Step 1: Identify the hydrocarbon family

All alkanes end in -ane

Step 2: Find the longest continuous carbon chain

This is the main chain or parent structure
It may not be drawn in a straight line
The name comes from the number of carbons in this chain
Example: If the longest chain has 6 carbons, the parent structure is hexane

Step 3: Identify substituents

Substituents are atoms or groups attached to the main chain, forming 'branches'
Carbon group substituents are called alkyl groups
Alkyl groups are named according to their carbon atoms with the ending -yl
Examples: $\text{CH}_3$ — is methyl, $\text{CH}_3\text{CH}_2$ — is ethyl

Step 4: Number the main chain

Number the carbons so that branches have the lowest possible numbers
The position of each substituent is shown by the number of the carbon it's attached to
Numbers are separated from words by hyphens

Step 5: List substituents alphabetically

When multiple substituents exist, list them in alphabetical order
Words and numbers are separated by hyphens

Step 6: Handle multiple identical substituents

Use prefixes: di- (two), tri- (three), tetra- (four), penta- (five)
Separate numbers with commas
Important: Ignore these prefixes when arranging substituents alphabetically

chatImportant

Common Mistake to Avoid

When alphabetizing substituents, remember to ignore prefixes like di-, tri-, tetra-, etc. For example, in "4-ethyl-2,3-dimethylhexane", ethyl comes before dimethyl because we ignore the "di-" prefix.

Worked example: naming an alkane

lightbulbExample

Worked Example: Naming a Branched Alkane

Let's name this compound step by step:

Step 1: Find the longest chain

Count the carbons: 6 carbons in the longest continuous chain
Base name: hexane

Step 2: Identify and number substituents

One ethyl group at position 4: 4-ethyl
Two methyl groups at positions 2 and 3: 2,3-dimethyl

Step 3: Arrange alphabetically

"Ethyl" comes before "methyl" alphabetically
Ignore the "di-" prefix when alphabetising

Step 4: Write the complete name

Final answer: 4-ethyl-2,3-dimethylhexane
Written as one word with hyphens separating numbers from words

Drawing alkanes from their names

To draw an alkane structure when given its systematic name, reverse the naming process by following these systematic steps:

lightbulbExample

Worked Example: Drawing 2,2-dimethylbutane

Step 1: Draw the main carbon chain

Use the base name to determine chain length
"Butane" tells us we need 4 carbons in a chain
Number the carbons starting from either end

     4    3    2    1
     C — C — C — C       (main chain)

Step 2: Identify and position substituents

"2,2-dimethyl" tells us we have two methyl groups
Both are attached to carbon 2

Step 3: Add the substituents

Attach each methyl group to carbon 2

           C
           |
     C — C — C — C       (add substituents)
           |
           C

Step 4: Complete with hydrogen atoms

Add hydrogen atoms to satisfy carbon's valency
Check that each carbon has exactly four bonds

          CH₃
           |
    CH₃—CH₂—C —CH₃      (complete structure)
           |
          CH₃

infoNote

Key Points for Drawing Structures

Always start by drawing the main carbon chain based on the parent name
Use the numbers in the name to position substituents correctly
Remember that each carbon must have exactly four bonds
Double-check that you've added all substituents mentioned in the name

Remember!

Here are the essential concepts you need to master about alkanes:

bookmarkSummary

Key Points to Remember

Alkanes have the general formula $\text{C}_n\text{H}_{2n+2}$ , making them a homologous series
Learn the carbon chain prefixes: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10)
Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds
When naming branched alkanes, always find the longest continuous carbon chain first - this becomes your parent structure
Number the main chain to give substituents the lowest possible numbers, then list substituents alphabetically (ignoring prefixes like di-, tri-)

Alkanes (HSC SSCE Chemistry): Revision Notes

Alkanes

Introduction to alkanes

The first alkanes

Straight-chain alkanes

Homologous series

Straight-chain versus branched alkanes

Naming alkanes

Understanding alkane names

Carbon chain stems

Rules for naming branched alkanes

Worked example: naming an alkane

Drawing alkanes from their names

Remember!

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