Hydrocarbons (VCE SSCE Chemistry): Revision Notes

Hydrocarbons

Why carbon forms so many compounds

Carbon is unique among elements in its ability to form an enormous variety of compounds. This remarkable property exists for several key reasons:

• Carbon has four valence electrons, allowing it to form covalent bonds with up to four different atoms
• Carbon atoms can bond strongly with other carbon atoms, creating stable carbon chains
• These carbon-carbon bonds can be single, double, or triple bonds, adding to the diversity of possible structures

Because of these properties, carbon atoms can link together to form molecules of varying lengths and shapes. Each different structure has unique properties and practical applications.

Naming hydrocarbons: the IUPAC system

The International Union of Pure and Applied Chemists (IUPAC) has established a systematic naming convention for hydrocarbons. This system ensures that each compound's name provides information about its structure.

The stem name (or parent name) indicates the number of carbon atoms in the longest continuous chain. For example, "prop-" tells you there are three carbon atoms in the main chain.

Alkanes: saturated hydrocarbons

What are alkanes?

Alkanes are hydrocarbons in which all carbon-carbon bonds are single covalent bonds. Because they contain only single bonds, alkanes are described as saturated molecules.

Methane ($CH_4$) is the simplest alkane. From methane onwards, each member of the alkane series differs from the previous one by a $-CH_2-$ unit. This type of series is called a homologous series.

Characteristics of a homologous series

Members of the same homologous series share several features:

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

For alkanes, the general formula is $C_nH_{2n+2}$, where $n$ represents the number of carbon atoms.

Alkanes are named by adding the suffix -ane after the stem name. A five-carbon alkane is called pentane ($C_5H_{12}$).

Different ways to represent alkanes

Chemists use various methods to show the structure of alkane molecules, each providing different levels of detail:

Molecular formula: Shows only the total number of each type of atom (e.g., $C_2H_6$)

Electron dot diagram (Lewis structure): Shows all valence electrons and how they are shared in bonds

Semi-structural formula: Shows the arrangement of carbon atoms and which atoms are bonded to each carbon, but doesn't show all the bonds. Also called condensed structural formula. For example, butane can be written as $CH_3CH_2CH_2CH_3$ or $CH_3(CH_2)_2CH_3$

Structural formula: Shows all atoms and all bonds in the molecule

Structural isomers of alkanes

For alkanes with one, two, or three carbon atoms (methane, ethane, propane), only one possible structure exists for each molecular formula. However, when you have four or more carbon atoms, multiple structural arrangements become possible.

Structural isomers are molecules that have the same molecular formula but different arrangements of atoms. The more carbon atoms present, the more possible isomers exist.

Butane ($C_4H_{10}$) has two structural isomers:

• Straight-chain isomer: The four carbon atoms form a continuous, unbranched chain
• Branched isomer: One carbon chain branches off from the main chain

Alkyl groups and side chains

Alkyl groups (or alkyl side chains) are fragments derived from alkanes by removing one hydrogen atom. They are named after their parent alkane with a -yl ending instead of -ane.

Since alkyl groups have one less hydrogen than their parent alkane, their general formula is $-C_nH_{2n+1}$.

Common examples:

• $-CH_3$: methyl group (from methane)
• $-CH_2CH_3$: ethyl group (from ethane)
• $-CH_2CH_2CH_3$: propyl group (from propane)

Physical properties of structural isomers

Structural isomers are distinct compounds with different physical and chemical properties. The table below shows the isomers of hexane ($C_6H_{14}$) and their melting and boiling points:

Rules for naming alkanes systematically

The IUPAC system uses specific rules for naming alkanes. Follow these steps carefully:

Important tip: The longest carbon chain is not always drawn in a straight line. Carefully trace through the molecule to find the longest continuous chain.

Worked example: naming an alkane

Physical properties of alkanes

The physical properties of organic compounds follow predictable patterns within each homologous series. Physical properties (such as melting point, boiling point, and solubility) are determined by intermolecular forces rather than by breaking chemical bonds.

Alkanes are non-polar molecules. This has important consequences:

• They are insoluble in water
• The only attractive forces between alkane molecules are weak dispersion forces

The table above shows the first three alkanes with their structural formulas, physical properties, and common uses. Notice how the boiling point increases from methane ($-164°C$) to ethane ($-87°C$) to propane ($-42°C$).

Chemical properties of alkanes

Chemical properties describe how a substance reacts with other substances. Alkanes are relatively unreactive compared to other hydrocarbons, but they do undergo combustion reactions readily.

Combustion is the reaction between a fuel and oxygen. When alkanes burn in a plentiful supply of oxygen, complete combustion occurs, producing carbon dioxide and water:

$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$

This reaction releases significant amounts of energy, which is why alkanes are excellent fuels. Natural gas (mainly methane) for cooking and petrol (a mixture of alkanes) for vehicles both rely on combustion reactions.

For example, a Bunsen burner with closed air holes produces a yellow flame due to incomplete combustion:

$2CH_4(g) + 3O_2(g) \rightarrow 2CO(g) + 4H_2O(l)$

$CH_4(g) + O_2(g) \rightarrow C(s) + 2H_2O(l)$

The yellow colour comes from glowing carbon particles. When the air holes are open, enough oxygen reaches the methane for complete combustion, producing the hotter blue flame.

Alkenes: unsaturated hydrocarbons

What are alkenes?

Alkenes are hydrocarbons that contain one carbon-carbon double bond ($C=C$). A double bond forms when two pairs of electrons are shared between two carbon atoms.

Because alkenes contain at least one double bond, they are classified as unsaturated hydrocarbons. Alkenes form a homologous series with the general formula $C_nH_{2n}$ (for alkenes with one double bond).

The simplest alkene is ethene ($C_2H_4$). The next member is propene ($C_3H_6$), which has an additional $-CH_2-$ unit compared to ethene.

Representing alkenes

Like alkanes, alkenes can be represented using different formula types:

In semi-structural formulas, the double bond can be shown (e.g., $CH_2=CH_2$) or sometimes omitted (e.g., $CH_2CH_2$), though showing it is clearer. In structural formulas, the double bond must always be explicitly shown.

Structural isomers of alkenes

Alkenes with more than three carbon atoms can have structural isomers. Isomerism in alkenes can arise from:

• Different positions of the double bond along the carbon chain
• Branching of the carbon chain

The figure above shows three isomers of $C_4H_8$:

• But-1-ene: double bond between carbons 1 and 2
• But-2-ene: double bond between carbons 2 and 3
• 2-methylprop-1-ene: a branched structure with the double bond between carbons 1 and 2

Rules for naming alkenes systematically

Naming alkenes follows similar principles to alkanes, with additional rules for the double bond:

Worked example: naming an alkene

Let's work through naming an alkene systematically:

More examples of correctly named alkenes demonstrate these principles in action.

Physical properties of alkenes

Alkenes have very similar physical properties to alkanes with the same chain length. This is because they differ by only two hydrogen atoms.

Like alkanes, alkenes are non-polar molecules. This means:

• Alkenes are insoluble in water
• Only weak dispersion forces exist between alkene molecules
• Alkenes have relatively low boiling points

As the carbon chain lengthens, melting points and boiling points increase due to stronger dispersion forces between larger molecules.

The table below shows physical properties and uses of the first three alkenes:

Name and molecular formula	Structural formula	Physical properties	Uses
Ethene, $C_2H_4$	$H_2C=CH_2$	Non-polar, gas, BP $-78.4°C$	Manufacture of polyethene and other chemicals
Propene, $C_3H_6$	$H_2C=CHCH_3$	Non-polar, gas, BP $-47.7°C$	Manufacture of propene oxide and polymers
But-1-ene, $C_4H_8$	$H_2C=CHCH_2CH_3$	Non-polar, gas, BP $-6.3°C$	Manufacture of butanol and polymers

Chemical properties of alkenes

Alkenes undergo similar combustion reactions to alkanes. For example, complete combustion of ethene produces carbon dioxide and water:

$C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(l)$

However, the most important chemical property of alkenes is their ability to undergo addition reactions. The carbon-carbon double bond is more reactive than a single bond, making these reactions possible.

The table above shows important addition reactions of ethene:

1. Addition with bromine ($Br_2$):

$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$

Product: 1,2-dibromoethane

2. Addition with hydrogen chloride ($HCl$):

$CH_2=CH_2 + HCl \rightarrow CH_3-CH_2Cl$

Product: Chloroethane (once used as a refrigerant)

3. Polymerisation:

Many ethene molecules can join together in an addition reaction to form long-chain polymers:

$n(CH_2=CH_2) \rightarrow -(CH_2-CH_2)_n-$

Product: Polyethene (a common plastic)

This ability to form polymers makes alkenes extremely valuable in the manufacture of plastics and other materials.