Alkane

1.0Introduction

Alkanes are a group of chemical compounds made up only of carbon and hydrogen atoms, and all the atoms in alkanes are joined by single covalent bonds. Because they contain only single bonds, they are called saturated hydrocarbons.

Alkanes follow a general molecular formula:+C_nH_2n+2

This means that for every ‘n’ carbon atom, there will be 2_n+2 hydrogen atoms.

2.0Structure and Bonding

In an alkane molecule:

• Each carbon atom forms four single bonds
• Each hydrogen atom forms one bond

This results in a stable, saturated structure. Chemists often use line-angle formulas to represent alkanes because they're quick to draw and easy to interpret.

For example, the simplest alkane, methane, contains just one carbon atom and four hydrogen atoms.
Its formula is CH₄, and its structure looks like a central carbon bonded to four hydrogen atoms.

Long-Chain Alkanes

In larger alkanes, carbon atoms link to one another in a chain using single bonds. Each carbon is also bonded to enough hydrogen atoms so that it makes four total bonds.

An example of this is octane, which has eight carbon atoms.
Its molecular formula is C₈H₁₈.

List of the first ten alkanes, along with their molecular formulas:

3.0Physical Properties of Alkanes

1. Solubility

• Alkanes are non-polar molecules because the electronegativity difference between carbon and hydrogen is very small.
• Non-polar molecules like alkanes do not dissolve in water (which is polar), making them hydrophobic.
• However, they do dissolve in organic solvents because the energy needed to break and form van der Waals forces is similar.

2. Boiling Point

• Boiling point increases as the size of the alkane molecule increases.
• Straight-chain alkanes generally have a higher boiling point than their branched isomers because they have a larger surface area for van der Waals interactions.

3. Melting Point

• The melting point of alkanes also increases with molecular weight.
• Even-numbered alkanes tend to have higher melting points than odd-numbered ones. This is because they pack better in the solid state, creating stronger intermolecular forces.

4.0Structural Formulas of Alkanes

Here are the condensed structural formulas for the first five straight-chain alkanes:

Alkanes can be represented in different ways:

• Molecular Formula – shows how many atoms of each element are present (e.g., C₈H₁₈).
• Structural Formula – shows how atoms are connected.
• Condensed Formula – a simplified way of writing the structure.
• Line-Angle Formula – each vertex or end of a line represents a carbon atom.

Sometimes, different compounds may share the same molecular formula but have different structures. These are called structural isomers.

5.0Types of Alkanes

Alkanes can exist in three main forms:

1. Straight-Chain Alkanes – all carbon atoms are arranged in a single continuous chain.
2. Branched-Chain Alkanes – the carbon chain has side branches.
3. Cycloalkanes – carbon atoms are arranged in a ring. Their general formula is CnH₂n, different from straight/branched alkanes.

Even if the number of carbon atoms is the same, the structure can be different. For example, a molecule with 8 carbon atoms can exist as:

• A straight chain
• A branched chain
• Or even a ring (cycloalkane).

6.0Alkyl Groups and Substitution Reactions in Alkanes

When a substituent—such as a halogen—attaches itself to an alkane molecule, it does so by replacing one of the hydrogen atoms bonded to a carbon. This results in the formation of a new bond between the carbon atom and the substituent.

The reaction between methane (CH₄) and chlorine (Cl₂) as an example. When methane reacts with chlorine (in the presence of light or heat), one hydrogen atom from methane is substituted by a chlorine atom. This gives rise to a new compound called chloromethane (CH₃Cl). In this compound, a CH₃ group (methyl group) is bonded to a chlorine atom.

Alkyl Groups

When an alkane loses a hydrogen atom, the resulting fragment is called an alkyl group. This group still contains carbon and hydrogen atoms, but it's now ready to bond with other atoms or groups.

For convenience, chemists often represent an alkyl group with the symbol R—just like halogens are often denoted by the letter X.

For example, if we remove one hydrogen atom from methane (CH₄), we get the methyl group (CH₃–). This methyl group can now form a bond with another atom, such as chlorine, to make chloromethane (CH₃Cl).

This substitution reaction can be generalized as:

RH + X₂ → RX + HX

Where:

• R is an alkyl group (from an alkane),
• X is a halogen like Cl or Br,
• RX is the resulting haloalkane (like chloromethane),
• HX is the hydrogen halide by-product (like HCl).