Organic chemistry SA - (CCEA)

• Organic compounds contain carbon atoms. They can be sorted into groups called homologous series, such as alkanes and alkenes.

• Alkanes and alkenes have different functional groups, which affects the reactions they can undergo.

• Organic compounds are obtained from crude oil, which is separated into useful fractions by fractional distillation.

• Addition polymerisation happens when many alkene monomers with a C=C double bond join together to form one long polymer. (Higher tier only)

Organic chemistry is the study of carbon compounds.

Carbon can form four covalent bonds, allowing it to form a wide range of compounds.

It is easier to study these compounds by putting them into groups known as homologous series.

For organic compounds in a homologous series:

• they have the same general formula,

• their molecular formulae differ by CH₂,

• they show a gradual variation in physical properties, such as their boiling points; and

• they have similar chemical properties.

For GCSE Single Award science you will study 2 different homologous series:

• alkanes

• alkenes

The alkanes are saturated hydrocarbons.

Hydrocarbons are compounds which contain only hydrogen and carbon.

Alkanes are described as saturated because their carbon atoms are joined by C–C single bonds.

They do not contain C=C bonds.

The general formula for the alkanes is C_nH_2n+2

Example

Decane is an alkane.

A molecule of decane contains 10 carbon atoms.

How to predict the molecular formula of decane

n = 10

∴ 2n + 2 = (2 × 10) + 2 = 22

Formula: C₁₀H₂₂

What are the first four alkanes?

The table below shows the first four alkanes.

Alkanes are obtained from crude oil, which is drilled up from under the Earth’s crust.

Crude oil is made of organisms (plants and animals) that lived and died millions of years ago.

These organisms were changed under the heat and pressure of the Earth to form crude oil.

Crude oil is a complex mixture of hydrocarbons.

Crude oil is an important source of fuels and is used as a raw material (feedstock) for the petrochemical industry, which makes plastics, rubber, solvents, lubricants and detergents.

Fractional distillation separates crude oil into simpler, more useful mixtures, because different hydrocarbons have different boiling points.

Each simpler mixture is called a fraction.

During fractional distillation:

• Heated crude oil enters a tall fractioning column, which is hot at the bottom and cooler at the top.

• Vapours from the oil rise through the column.

• Each fraction condenses when it becomes cool enough, each at a different position in the column.

• Pipes lead each fraction away.

Small hydrocarbon molecules have weak intermolecular forces, so they have low boiling points.

They do not condense, but leave the top of the column as gases.

Long hydrocarbon molecules have stronger intermolecular forces, so they have higher boiling points.

They leave the bottom of the column as hot, liquid bitumen.

Hydrocarbons ability to combust, or burn, depends on the amount of oxygen available.

Alkanes can undergo either complete combustion or incomplete combustion depending on the amount of oxygen available for combustion.

What is complete combustion?

Complete combustion occurs when there is a plentiful supply of oxygen.

The carbon and hydrogen atoms react with oxygen in an exothermic reaction.

• Carbon dioxide and water are produced.

• The maximum amount of energy is given out.

In general:

hydrocarbon + oxygen → carbon dioxide + water

Here are the equations for the complete combustion of propane, which is used in bottled gas.

propane + oxygen → carbon dioxide + water

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (Higher tier only)

Incomplete combustion happens when there is less oxygen available, the details are not required for GCSE Single Award science.

What are the problems with combustion?

Combustion is a major source of atmospheric pollution as burning hydrocarbons produces large volumes of carbon dioxide gas.

Carbon dioxide is a cause of the greenhouse effect that can cause:

• climate change,
• rising sea levels; and
• increased flooding of low-lying areas.

What is the greenhouse effect?

• Carbon dioxide in the atmosphere forms a blanket around the Earth.

• This causes the greenhouse effect that helps to keep the temperature of the Earth steady.

• Too much carbon dioxide in the atmosphere thickens this blanket and causes more heat to be trapped, this warms the Earth in an effect called global warming.

• Global warming leads to polar ice caps melting, sea levels rising, flooding and climate change.

The alkenes are unsaturated hydrocarbons.

Hydrocarbons – because they contain hydrogen and carbon only.

Unsaturated – because they contain at least one C=C.

Each alkene containing one C=C has two fewer hydrogen atoms than the corresponding alkane.

What is a functional group?

Alkenes have a functional group, C=C - this is the part of the molecule that takes part in reactions.

Alkanes, in contrast, do not have a functional group and are less reactive as a result.

The general formula for the alkanes is C_nH_2n.

The table shows the first four alkenes:

Note: But-1-ene and but-2-ene have the same molecular formula.

The only difference between the two molecules is the position of the double bond.

The number within each name describes the position of the double bond in the carbon chain.

A polymer is a long chain molecule made up of small, repeating units called monomers.

These small molecules, called monomers join together in a long chain to make polymers.

For example: polyethene is a polymer made from a very large number of ethene molecules (monomers) bonded together.

The reaction is called an addition polymerisation reaction.

• Ethene is the monomer

• Polyethene is the polymer.

The number of molecules (n) joining up is variable, but is in the region of 2000 to 20000.

The C=C double bond in ethene is involved in the polymerisation reaction.

It allows ethene molecules to join, so it is an example of an addition reaction.

Poly(ethene) is an addition polymer.

The table shows the structure of ethene and its polymer.

Polychloroethene is commonly known by the initials of its old name, PVC (polyvinyl chloride).

Polychloroethene is made by polymerising chloroethene.

Equations use repeating units to model addition polymerisation reactions.

The letter n stands for a large number.

How to dispose of polymers

Polymers are unreactive, so they are suitable for storing food and chemicals safely.

Unfortunately, this also makes them difficult to dispose off.

They are often buried in landfill sites or incinerated.

Landfill

Waste polymers are disposed in landfill sites, but this uses valuable land and suitable sites often fill up quickly.

Incineration

Polymers release a lot of energy when they burn, which can be used to heat homes or generate electricity.

But burning polymers produces carbon dioxide, adding to global warming, and can release toxic gases.