Organic Materials (VCE SSCE Chemistry): Revision Notes

Organic Materials

Introduction to organic materials

Carbon is an extraordinary element that forms the basis of over 90% of all known compounds. This remarkable ability has enabled the development of an immense diversity of living organisms, all composed of carbon-based molecules. The term 'organic' originally reflected the belief that only living things could produce these carbon-containing compounds.

What is organic chemistry?

Organic chemistry is the branch of chemistry that studies carbon-containing compounds. However, this definition comes with important exceptions. Some simple carbon compounds are classified as inorganic rather than organic:

• Carbon monoxide ($\text{CO}$)
• Carbon dioxide ($\text{CO}_2$)
• Carbonic acid ($\text{H}_2\text{CO}_3$)
• Carbonate compounds such as calcium carbonate ($\text{CaCO}_3$) and sodium carbonate ($\text{Na}_2\text{CO}_3$)

This distinction dates back to the chemist Jöns Joseph Berzelius (1779-1848), who defined inorganic compounds as those originating from non-living systems.

Composition of organic compounds

Organic compounds are found everywhere - both around us and within our own bodies. While carbon is their defining element, most organic compounds also contain:

• Hydrogen - almost always present
• Oxygen - very commonly present

Additionally, organic compounds may contain:

• Nitrogen ($\text{N}$)
• Sulfur ($\text{S}$)
• Phosphorus ($\text{P}$)
• Halogens (fluorine, chlorine, bromine, iodine)

Sources of organic compounds

Organic compounds can be obtained from two main sources: fossil fuels (particularly crude oil) and plant-sourced biomass. These sources differ fundamentally in their sustainability and environmental impact.

Crude oil - a non-renewable source

Crude oil formed over millions of years through a fascinating geological process. When prehistoric marine microorganisms such as bacteria and plankton died, they settled to the ocean floor and were buried by accumulating sand and sediment. Over time, these organic remains became part of the Earth's crust. As they were buried deeper, they experienced increasingly high temperatures and pressures. These extreme conditions converted the oils and fats in the dead organisms into hydrocarbons (compounds containing only hydrogen and carbon) and other organic compounds. The resulting mixture is what we call crude oil.

Fractional distillation of crude oil

Crude oil is not used in its raw state. After extraction from oil fields, it is transported to refineries where it undergoes fractional distillation. This process separates the complex mixture of hydrocarbons based on their different boiling points (the temperature at which a liquid becomes a gas).

The various fractions obtained have different uses:

• Gasoline ($\text{C}_5$ to $\text{C}_{12}$): used to make petrol
• Kerosene ($\text{C}_{10}$ to $\text{C}_{18}$): used for jet fuel and can be processed into diesel
• Diesel/gas oil ($\text{C}_{12}$ to $\text{C}_{20}$): used for diesel fuel

Currently, over 90% of crude oil products are used as fuels.

The sustainability problem

Crude oil is a fossil fuel - so called because it originates from the fossilised remains of ancient organisms. As such, it represents carbon that has been locked away in the Earth's crust for millions of years. This makes crude oil a non-renewable resource: we are consuming it far faster than it can possibly form through natural geological processes. Once depleted, it cannot be replaced on any human timescale.

Plant-sourced biomass - a renewable alternative

Plant-sourced biomass refers to carbon-based materials that come from plants. Unlike fossil fuels, plants represent a renewable source of organic compounds because they can be continuously regrown and harvested.

The key to this renewability is photosynthesis, the process by which plants capture carbon dioxide from the atmosphere and convert it into glucose using energy from sunlight:

$6\text{CO}_2(\text{g}) + 6\text{H}_2\text{O}(\text{l}) \xrightarrow{\text{UV light}} \text{C}_6\text{H}_{12}\text{O}_6(\text{aq}) + 6\text{O}_2(\text{g})$

Through photosynthesis, plants continually fix atmospheric carbon dioxide into organic molecules. This makes plant biomass a renewable carbon source, as the carbon can be replenished through new plant growth.

Making bio-polyethene from sugar cane

An excellent example of plant-sourced biomass in action is the production of bio-polyethene from sugar cane. Polyethene is a widely-used polymer (plastic) found in products such as clingwrap, food containers, and toys. When polyethene is derived from plant sources rather than crude oil, it is called bio-polyethene.

Case study: everyday products from organic compounds

Many everyday objects around us are made from organic compounds, particularly polymers. A common example is ABS (Acrylonitrile Butadiene Styrene), which is used to make computer keyboards, mice, calculator bodies, mobile phone covers, light switch surrounds, electrical wall sockets, car interior parts, LEGO bricks, and even medical equipment.

What is ABS?

ABS is a polymer made from three different monomers (small molecules that join together to form polymers):

• Acrylonitrile ($\text{C}_3\text{H}_3\text{N}$)
• 1,3-Butadiene ($\text{C}_4\text{H}_6$)
• Styrene ($\text{C}_8\text{H}_8$)

The recycling challenge