Organic Chemistry

What is organic chemistry?

Organic chemistry is the study of compounds containing carbon. Carbon is special because each atom can form four covalent bonds, allowing it to build long chains, branches and rings. This single property gives us millions of different organic compounds, from the fuels in our cars to the proteins in our bodies.

Key terms Hydrocarbon — a compound made of hydrogen and carbon atoms only.

Saturated — a molecule with only single carbon–carbon bonds.

Unsaturated — a molecule containing at least one carbon–carbon double bond (C=C).

Crude oil and fractional distillation

Crude oil is a finite, non-renewable fossil fuel. It is a mixture of many different hydrocarbons, mostly alkanes. Because the molecules are not chemically bonded to each other, they can be separated by physical means using their different boiling points.

This separation is called fractional distillation. Crude oil is heated until it vaporises and the vapour rises up a tall fractionating column that is hot at the bottom and cooler at the top. Each fraction (a group of hydrocarbons with a similar number of carbon atoms and a similar range of boiling points) condenses at a different height.

As you go down the column, the fractions have longer carbon chains. Longer molecules have stronger forces of attraction between them, so they:

have higher boiling points
are more viscous (thicker, flow less easily)
are less flammable (harder to ignite)
are less volatile (evaporate less readily)

Burning fuels: combustion

When hydrocarbon fuels burn in oxygen they release energy. The products depend on how much oxygen is available.

Complete combustion happens when there is plenty of oxygen. The only products are carbon dioxide and water:

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Incomplete combustion happens when oxygen is limited. It produces carbon monoxide (CO) and/or carbon (soot), as well as water:

$$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$$

Watch out Carbon monoxide is a toxic, colourless, odourless gas. It binds to haemoglobin in red blood cells in place of oxygen, so the blood can no longer carry enough oxygen around the body. This is why faulty gas heaters in unventilated rooms are so dangerous.

Homologous series

A homologous series is a family of organic compounds with similar chemical properties. Members of the same series:

share the same general formula
differ from the next member by a CH₂ unit
show a gradual change in physical properties (e.g. boiling point increases with chain length)
have similar chemical reactions because they share the same functional group.

The IGCSE series you must know are alkanes, alkenes, alcohols and carboxylic acids.

Alkanes

Alkanes are saturated hydrocarbons with the general formula CnH₂ₙ₊₂. The first members are methane (CH₄), ethane (C₂H₆) and propane (C₃H₈).

Alkanes are relatively unreactive because the strong C–C and C–H single bonds are hard to break. Their most important reaction is combustion (burning as fuels), as shown above.

Alkenes

Alkenes are unsaturated hydrocarbons with the general formula CnH₂ₙ. They contain a reactive carbon–carbon double bond (C=C). The first member is ethene (C₂H₄).

The double bond makes alkenes much more reactive than alkanes. They undergo addition reactions, in which the double bond opens up and atoms add across it. For example, ethene reacts with hydrogen (over a nickel catalyst) to form ethane.

Key terms Addition reaction — a reaction in which two molecules join to form a single product, as the C=C double bond opens and atoms add to each carbon.

Exam tip To tell alkanes and alkenes apart, use the bromine water test. Shake the gas with orange bromine water:

- An alkene decolourises bromine water (orange → colourless) because of an addition reaction across the C=C bond.

- An alkane has no effect — the bromine water stays orange.

#### Alkanes vs alkenes

Alcohols

Alcohols are a homologous series containing the –OH functional group. The best-known example is ethanol (C₂H₅OH), the alcohol in alcoholic drinks.

Ethanol can be made by fermentation. Sugar (glucose) from plants is broken down by yeast (which provides enzymes) at about 30 °C in the absence of air:

$$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$$

Ethanol also burns in air to release energy, forming carbon dioxide and water:

$$\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$$

Uses of ethanol include a solvent (for perfumes, inks and cosmetics), an alcoholic drink, and a fuel (sometimes added to petrol).

Carboxylic acids

Carboxylic acids contain the –COOH functional group. The common example is ethanoic acid (CH₃COOH), the acid that gives vinegar its sharp taste.

Ethanoic acid is a weak acid. This means it only partially ionises in water, so it has a higher pH than a strong acid of the same concentration. It still shows typical acid reactions — for example, reacting with metals, carbonates and alkalis.

Addition polymerisation

A polymer is a very large molecule built from many small molecules called monomers joined together. In addition polymerisation, many unsaturated monomers (alkenes) add together: the C=C double bonds open up and the molecules link into a long chain with no other product.

For example, many ethene molecules join to make poly(ethene) (polythene), used for plastic bags and bottles.

Notice that the product has only single bonds — the double bonds are used up in joining the monomers.

Real world Poly(ethene) is cheap, strong and unreactive, which makes it ideal for packaging. But that same lack of reactivity is a problem when we throw it away.

Disposing of polymers

Most addition polymers are non-biodegradable: microorganisms cannot break them down, so they last for hundreds of years. The main disposal methods all have drawbacks:

Landfill — wastes land and the polymers do not rot away.
Burning — releases energy, but can produce toxic gases (e.g. carbon monoxide, or hydrogen chloride from PVC) and adds carbon dioxide to the atmosphere.
Recycling — reduces waste and saves crude oil, but different polymers must be sorted first, which is costly and difficult.

Exam tip If asked to evaluate polymer disposal, give a balanced answer: state a method, then one advantage and one disadvantage. Examiners reward weighing up the options, not just listing them.