Organic Chemistry Basics

Introduction to Organic Chemistry

Organic chemistry is the study of carbon compounds. Carbon's unique ability to form four strong covalent bonds and link with other carbon atoms to form long chains and rings, a property known as catenation, gives rise to millions of different organic compounds. The simplest of these are hydrocarbons, which are compounds containing only carbon and hydrogen atoms. These form the basis of fuels and plastics.

A homologous series is a family of organic compounds with the same functional group and similar chemical properties. All members of a homologous series can be represented by a general formula, and each successive member differs from the next by a –CH₂– group.

Alkanes: Saturated Hydrocarbons

Alkanes are the simplest homologous series, with the general formula CₙH₂ₙ₊₂. They are known as saturated hydrocarbons because their carbon atoms are joined by single covalent bonds only, meaning they contain the maximum possible number of hydrogen atoms for their carbon skeleton.

The first four members are:

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)

Properties and Reactions of Alkanes:

1. Reactivity: Alkanes are relatively unreactive due to the high strength of the C-C and C-H single bonds, which require a large amount of energy to break.
2. Combustion: Their primary reaction is combustion (burning in oxygen), which releases a large amount of energy. This makes them excellent fuels.

• Complete Combustion: Occurs with a plentiful supply of oxygen, producing only carbon dioxide and water. This is the principle behind the clean blue flame seen on a gas stove using Sui Gas (mainly methane) in Pakistan.

`CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)`

• Incomplete Combustion: Occurs in a limited supply of oxygen. It produces water along with harmful carbon monoxide (CO), a toxic gas, and carbon (soot), which is a respiratory irritant. This is seen in poorly maintained vehicle exhausts.

`2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l)`

Alkenes: Unsaturated Hydrocarbons

Alkenes are a homologous series with the general formula CₙH₂ₙ. They are unsaturated because they contain at least one carbon-carbon double bond (C=C). This C=C double bond is the molecule's functional group—the site of its chemical reactions.

The first two members are:

• Ethene (C₂H₄)
• Propene (C₃H₆)

Properties and Reactions of Alkenes:

1. Reactivity: The C=C double bond is a region of high electron density, making alkenes much more reactive than alkanes. They undergo addition reactions, where the double bond breaks and atoms are added across it.
2. Test for Unsaturation: The standard test to distinguish an alkene from an alkane is to add bromine water (an orange solution of bromine, Br₂). Alkenes decolourise bromine water, while alkanes do not.

`C₂H₄ (ethene) + Br₂ (orange) → C₂H₄Br₂ (1,2-dibromoethane - colourless)`

Crude Oil and Fossil Fuels

Crude oil is a finite resource and a type of fossil fuel, formed from the remains of ancient marine organisms over millions of years. It is a complex mixture of thousands of different hydrocarbons, mostly alkanes. To be useful, it must be separated into simpler, more useful mixtures called fractions.

This separation is done by fractional distillation in a fractionating column. The process relies on the fact that longer-chain hydrocarbons have stronger intermolecular forces and therefore higher boiling points.

Process:

1. Crude oil is heated to ~350°C, causing most of it to vaporise.
2. The hot liquid-vapour mix is pumped into the bottom of a tall fractionating column, which is hot at the bottom and cooler at the top.
3. Vapours rise, cool, and condense at different levels (on trays) according to their boiling points.
4. Fractions with low boiling points (small molecules) rise highest, while those with high boiling points (large molecules) condense lower down or remain as a liquid residue.

Key Fractions and their Uses in Pakistan:

• Refinery Gas: LPG for cooking.
• Gasoline (Petrol): Fuel for cars and motorcycles.
• Kerosene: Fuel for jet engines (PIA aircraft) and paraffin lamps in rural areas.
• Diesel: Fuel for buses, trucks, and trains, vital for Pakistan's logistics.
• Fuel Oil: Fuel for ships and power plants.
• Bitumen: A thick, black residue used for surfacing roads and waterproofing roofs.

Cracking

Fractional distillation often produces more long-chain hydrocarbons than needed and not enough of the more valuable short-chain ones like petrol. Cracking is a process that breaks down large, less useful hydrocarbon molecules into smaller, more useful ones.

Conditions: High temperature (600-700°C) and a catalyst (e.g., aluminium oxide, silicon dioxide).

Products: Cracking always produces a smaller alkane and at least one alkene. The alkene (e.g., ethene) is a vital feedstock for the chemical industry.

`Example: C₁₀H₂₂ (decane) → C₈H₁₈ (octane, for petrol) + C₂H₄ (ethene, for polymers)`

Polymers

A polymer is a very large molecule made from many small, repeating units called monomers. Alkenes are ideal monomers because their double bonds can break and join together in a process called addition polymerisation.

Example: Polymerisation of Ethene

Many ethene monomers join to form poly(ethene), commonly known as polythene.

`n(CH₂=CH₂) → -(CH₂-CH₂)-ₙ`

Uses of Polymers in Pakistan:

• Poly(ethene): Used for plastic bags, packaging film, and bottles.
• Poly(propene): Used for stronger ropes, crates, carpets, and car bumpers.

Exam Trap: A common mistake is confusing the properties of a monomer with its polymer. Ethene is a reactive gas, but poly(ethene) is an unreactive, solid plastic. The double bond is gone!