Hydrocarbons and Alkanes

Hydrocarbons are compounds containing only carbon and hydrogen atoms. The simplest family of hydrocarbons is the alkanes, which include methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). All alkanes follow the general formula C₅H₂₂₊₂.

Alkanes belong to a homologous series (similar compounds with similar properties) and are saturated compounds, meaning all carbon atoms form four single bonds. As the carbon chain length increases, so do certain properties:

• Longer chains have higher melting and boiling points
• Shorter chains evaporate more easily and are more flammable
• Longer chains are thicker and stickier (more viscous)

Did you know? Alkanes make excellent fuels because they release significant energy when burned with oxygen, which is why they're widely used in vehicles and heating systems.

Crude oil, a fossil fuel formed from the remains of ancient organisms over millions of years, is our primary source of hydrocarbons. It's extracted by drilling into rocks where it has been stored for millennia. Because crude oil takes so long to form, it's considered a finite resource that will eventually run out.

To separate the different hydrocarbons in crude oil, we use fractional distillation. This process involves heating the oil until it vaporizes, then passing the vapor through a column that's hot at the bottom and cooler at the top. As the gases rise, they condense into liquids at different heights based on their boiling points.

Fractional Distillation and Cracking

In fractional distillation, compounds separate based on their boiling points. Longer chain hydrocarbons like bitumen (used for roads) and heavy fuel oils condense at the bottom of the column where it's hottest. Medium chains like diesel and petrol collect in the middle. Very short chains like propane and butane (components of LPG) may remain gaseous throughout.

Cracking is a thermal decomposition reaction that breaks longer hydrocarbon chains into shorter, more useful ones. There are two main methods:

• Catalytic cracking: The hydrocarbon vapor passes over hot powdered aluminium oxide, causing it to split into smaller molecules
• Steam cracking: The hydrocarbon vapor mixes with steam at very high temperatures to break apart

Remember this! Cracking typically produces a shorter chain alkane plus an alkene - this is how we create the unsaturated hydrocarbons industry needs.

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. This double bond makes alkenes more reactive than alkanes. When bromine water is added to alkenes, it changes from orange to colorless as the bromine reacts with the double bond - this is a key test for unsaturation.

Alkenes can undergo several important reactions:

• With hydrogen (plus a catalyst) to form alkanes
• With water (plus a catalyst and heat) to form alcohols
• With halogens (no catalyst needed) to form halogenated compounds

The high reactivity of alkenes makes them extremely useful in industry, particularly for creating polymers where many alkene molecules join together in long chains.

Esters and Polymers

Esters are organic compounds with the -COO- functional group. They typically have pleasant, fruity smells and evaporate easily, making them perfect for perfumes and food flavourings. To create an ester, a carboxylic acid and an alcohol react in the presence of a catalyst like concentrated sulphuric acid, producing water as a byproduct.

Esters form the basis of important condensation polymers like polyester. These polymers form when two different monomers—typically a dicarboxylic acid $with two -COOH groups$ and a diol $with two -OH groups$—combine. During this reaction, small molecules like water are eliminated as the monomers join together through ester links.

Chemistry tip: Condensation polymers are often biodegradable because microorganisms can easily break the ester links—that's why polyester fabrics eventually break down in nature!

The natural world is full of naturally occurring polymers. These include:

• Polypeptides: Chains of amino acids joined by amide bonds (also called peptide bonds)
• DNA: Two polymer chains of nucleotides coiled in a double helix, with the four bases A, C, G, and T distinguishing each nucleotide
• Carbohydrates: Polymers made of sugar monomers, including starch, glycogen, and cellulose

Amino acids can join together because they have two functional groups: a carboxylic acid group and an amino group. When these react, water is eliminated as the carbon and nitrogen atoms join to form an amide bond. This same chemistry creates the long chains of proteins that make up much of your body's structure.