Hydrocarbons (VCE SSCE Chemistry): Revision Notes

Hydrocarbons

Introduction to hydrocarbons

Hydrocarbons represent the simplest category of organic molecules, containing only carbon and hydrogen atoms. Despite having just two elements, the different ways these atoms can be arranged create an enormous diversity of compounds. Crude oil is a naturally occurring mixture of many different hydrocarbons used throughout industry.

The main families of hydrocarbons you will study are alkanes and alkenes, each with distinctive properties and uses. Understanding how to name these compounds and represent their structures is fundamental to organic chemistry.

Homologous series

To simplify the study of millions of organic compounds, chemists group related molecules into families called homologous series. Within a homologous series, compounds share four key characteristics:

• Similar molecular structures
• Similar chemical properties
• The same general formula
• A pattern in their physical properties

An important feature of homologous series is that each successive member differs from the previous one by a single $-CH_2-$ unit. This regular pattern makes it easier to predict properties and understand relationships between compounds. Both alkanes and alkenes are examples of hydrocarbon homologous series.

Alkanes

What are alkanes?

Alkanes are saturated hydrocarbons, meaning all the carbon-carbon bonds in the molecule are single bonds. This saturation distinguishes them from other hydrocarbon families. Alkanes follow the general formula:

Naming straight-chain alkanes

The systematic naming of alkanes uses a prefix that indicates the number of carbon atoms in the main chain (called the parent molecule), followed by the suffix "-ane" to show that all carbon-carbon bonds are single bonds.

The prefixes used for carbon chains are shown in the table above. For example:

• Methane: one carbon atom
• Ethane: two carbon atoms
• Propane: three carbon atoms

Representing alkane structures

Alkanes can be represented in several different ways, each providing different levels of detail:

• Molecular formula: Shows only the types and numbers of atoms (e.g. $CH_4$)
• Structural formula: Shows all atoms and how they are bonded together
• Semi-structural formula: A condensed representation (e.g. $CH_3CH_3$)

Notice that each successive member of the alkane series differs by one $-CH_2-$ unit, confirming their membership in the same homologous series.

Alkyl groups

When alkanes lose one hydrogen atom, they form alkyl groups or alkyl side chains. These groups act as branches on the main carbon chain. Alkyl groups are named after their parent alkane but with a "-yl" ending instead of "-ane". They have one less hydrogen atom than the corresponding alkane, giving them the general formula:

$C_nH_{2n+1}$

Understanding alkyl groups is essential for naming branched alkanes.

Naming branched alkanes

When alkanes contain more than three carbon atoms, they can form isomers with branches. These branched structures are named systematically using the following steps:

Worked example: naming branched alkanes

Cycloalkanes

Carbon atoms can also form ring structures. Cycloalkanes are saturated hydrocarbons arranged in a ring. The most common example is cyclohexane:

Cyclohexane contains six carbon atoms arranged in a hexagonal ring. Because an additional carbon-carbon bond is needed to close the ring, cyclohexane has two fewer hydrogen atoms than straight-chain hexane. This gives cyclohexane the molecular formula $C_6H_{12}$, compared to $C_6H_{14}$ for hexane.

Alkenes

What are alkenes?

The alkenes are a homologous series of unsaturated hydrocarbons that contain one carbon-carbon double bond. Because the double bond must be shared between two carbon atoms, all alkene molecules contain at least two carbon atoms.

Ethene and its applications

Ethene (also called ethylene) is the first member of the alkene series and has important industrial applications:

Ethene serves as the starting material for manufacturing many polymers, including the plastics used in food storage containers shown in the image above.

Representing alkene structures

Like alkanes, alkenes can be represented in multiple ways:

The table above shows ethene represented as:

• Molecular formula: $C_2H_4$
• Structural formula: Shows all atoms and the double bond
• Semi-structural formula: $CH_2=CH_2$ or $CH_2CH_2$
• Skeletal formula: A simple line representing the double bond

Here are examples of the first three alkenes:

Each successive member differs by one $-CH_2-$ unit, characteristic of a homologous series.

Naming alkenes

The rules for naming alkenes build on the rules used for alkanes, with these additional conventions:

1. Use the suffix "-ene" for the parent name (instead of "-ane")
2. Number the carbon atoms from the end of the chain closest to the carbon-carbon double bond
3. Indicate the position of the double bond by including the number of the first carbon in the double bond
4. Place this number immediately before "-ene" in the parent name
5. Name and number any alkyl side chains at the beginning of the molecule's name, following the same rules as for alkanes

For example, but-1-ene indicates a four-carbon chain with the double bond starting at carbon 1.

Structural isomers of alkenes

When alkenes contain four or more carbon atoms, structural isomers can exist where the double bond is located at different positions in the carbon chain. These isomers are distinguished by the numbering system in their names.

Degree of unsaturation

Understanding the concept

When determining the structure of an unknown molecule, the degree of unsaturation (also called the index of hydrogen deficiency) provides valuable information. This concept is based on the observation that every time a molecule forms a double bond or a ring structure, it contains two fewer hydrogen atoms than the fully saturated equivalent.

The degree of unsaturation tells you how many combinations of double bonds and/or rings are present in a molecule.

Calculating degree of unsaturation

To calculate the degree of unsaturation, compare the molecular formula to what a fully saturated alkane would have:

$\text{Degree of unsaturation} = \frac{\text{maximum number of H possible} - \text{actual number of H}}{2}$

The maximum number of hydrogen atoms possible is calculated using the alkane general formula: $(2n + 2)$, where $n$ is the number of carbon atoms.

Worked example: calculating degree of unsaturation

Benzene

Structure and bonding

Benzene is a special unsaturated cyclic hydrocarbon consisting of six carbon atoms arranged in a ring. What makes benzene unique is its bonding structure:

Three of the four outer-shell electrons from each carbon atom form normal covalent bonds. The fourth electron from each carbon is delocalised (shared) around all six carbon atoms, creating a ring of electrons above and below the plane of the molecule.

Representing benzene

Benzene is sometimes drawn with alternating single and double bonds (as shown in part b of the figure above), but this is not the most accurate representation. In reality, all the carbon-carbon bonds in benzene are identical and have properties intermediate between single and double bonds.

The most accurate representation uses a circle inside the hexagon of carbon atoms (part c), symbolising the delocalised ring of electrons.

Historical context: Kathleen Lonsdale

Although Michael Faraday first isolated benzene in 1825 and August Kekulé proposed a ring structure in 1865, the true three-dimensional structure remained a mystery. In 1929, Kathleen Lonsdale used X-ray crystallography to determine that benzene has a planar, hexagonal structure with all carbon-carbon bonds of equal length.

The phenyl group

When a benzene ring is bonded to an alkyl group or functional group, the ring structure is called a phenyl functional group with the formula $C_6H_5-$. This notation indicates that one hydrogen has been removed from benzene to allow attachment to another structure.

Summary