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4.1 What are organic molecules?

Chapter 4: Organic molecules

4.1 What are organic molecules? (ESCK3)

This section studies simple organic molecules, functional groups and the physical properties associated with these functional groups. Only molecules with one type of functional group, and no more than three of the same functional group, should be studied. This section is a basis for any organic chemistry they may study beyond school so it is important that learners have a thorough understanding of this chapter. Some common polymers are also covered in this chapter, learners should understand the environmental impacts of these polymers as well as their connection with simple organic molecules through the monomers they are formed from.

This section of work is the first chemistry chapter of the Grade 12 year. Learners should have an understanding of intermolecular forces and chemical bonds from Grade 11. These will be particularly important in the physical properties section. Only 12 hours are allocated in CAPS for this section. If possible more time could be given here to ensure a thorough understanding, as there is a lot of work to cover.

Emphasis should be placed on the different representations of organic compounds: macroscopic, sub-microscopic, symbolic representations and the links between all three. Where possible, use atomic model kits to help explain reactions, physical properties and the structure of molecules.

The following topics are covered in this chapter.

  • What makes a molecule organic, and organic molecular structures

    This chapter starts with a brief introduction to what makes a molecule organic (containing carbon atoms). This then leads in to the properties of carbon that make it so unique. Learners are introduced to structural, semi-structural, condensed and molecular formula representations for molecules. It is important that they have a thorough understanding of this before moving on as they will use these representations throughout the chapter. It is also important that learners understand that molecules are not two-dimensional, if possible do some demonstrations with atomic model kits so they can get a better feel for the shape of an organic molecule. Semi-structural representations are not required by CAPS, but will be shown in many textbooks and so it would be good for the student to understand them.

  • Functional groups

    Understanding functional groups is essential to understanding organic molecules. An introduction to each of the required basic functional groups, the homologous series to which they belong, and their general formulas are covered in this section: hydrocarbons (alkanes, alkenes, alkynes), alcohols, alkyl halides (specifically haloalkanes), aldehydes, ketones, carboxylic acids and esters. The concept of saturated and unsaturated compounds is also covered, this will become more important later in the chapter when studying reactions. Isomers of compounds with the same functional groups, and compounds with different functional groups are also covered.

  • IUPAC naming

    A good knowledge of IUPAC naming is very important in organic chemistry. This section walks the learners through the naming of the functional groups, with many worked examples. Going through those worked examples and the associated exercises will help the learners understand this section. There are a number of in-class activities provided that will also help with understanding. The naming of compounds with more than three of the same functional group, or more than one functional group, is not required by CAPS. Chain lengths of no more than eight carbon atoms are allowed, and esters may not have branched groups.

  • Physical propeties and structure

    A revision of Grade 11 intermolecular forces would be useful before starting this section. The important IMF this year are hydrogen bonds and van der Waals forces. The physical properties covered include: viscosity, density, melting and boiling points, flammability and vapour pressure, volatility, physical state, smell. It would be good for learners to revise molecular shape from Grade 11 as well. The changes in physical properties are linked to the intermolecular forces of the molecule, which is in turn linked to functional groups, chain length and chain branching. Although these are seperated into sections the learner should understand that they are linked (different intermolecular forces are due to functional groups, chain length, etc.).

  • Applications of organic chemistry

    In this section learners will explore the applications of organic molecules. Specifically the cracking of hydrocarbons, and the (complete) combustion of alkanes. Esters are also covered in more detail in this section, with industrial uses.

  • Addition, elimination and substitution reactions

    The learners need to know the addition, elimination and substitution reactions mentioned in this section, including reaction conditions and the major and minor products that will be formed. They should understand the difference between an addition reaction, an elimination reaction and a substitution reaction and the reactants required in each specific reaction.

    No mechanisms of reactions are required, only the reaction equations.

  • Plastics and polymers

    The polymers covered in this section are: polyethene, polypropene, polyvinyl chloride, polyvinyl acetate, polystyrene, polyethylene terephthalate and polylactic acid. Learners should understand what makes a compound a polymer, the difference between an addition and a condensation reaction, and how to determine the polymer from the monomer and the monomer from the polymer.

There are several experiments in this chapter. The learners will be using dangerous chemicals and should be properly instructed on the correct use of safety equipment, including safety goggles, gloves and protective clothing. They should also be reminded not to sniff any chemicals as the fumes can be dangerous as well. More information on laboratory procedures as well as safety precautions is provided in Chapter 1 (Science skills).

These experiments are also an excellent opportunity to get the learners to research organic molecules. Before any experiment the hazards of the chemicals being worked with should be studied. Have the learners do the research for at least some of the experiments before completing them as a way of better understanding the molecules.

Do not confuse organic compounds with naturally produced food. Organic compounds are often produced in a laboratory.

Organic chemistry is the branch of chemistry that deals with organic molecules. An organic molecule is one which contains carbon, although not all compounds that contain carbon are organic molecules. Noticeable exceptions are carbon monoxide (\(\text{CO}\)), carbon dioxide (\(\text{CO}_{2}\)), carbonates (e.g. calcium carbonate), carbides (e.g. calcium carbide) and cyanides (e.g. sodium cyanide). Pure carbon compounds such as diamond and graphite are also not organic compounds. Organic molecules can range in size from simple molecules to complex structures containing thousands of atoms!

Although carbon is present in all organic compounds, other elements such as hydrogen \((\text{H})\), oxygen \((\text{O})\), nitrogen \((\text{N})\), sulfur \((\text{S})\) and phosphorus \((\text{P})\) are also common in these molecules.

Organic molecule
An organic molecule is a molecule that contains carbon atoms (generally bonded to other carbon atoms as well as hydrogen atoms).

Figure 4.1: A simple organic molecule, propane, can be used in a gas lamp (left). The complex organic molecule DNA carries the genetic code of a person and can be used to identify them.

Organic compounds are very important in daily life and they range from simple to extremely complex (Figure 4.1). Organic molecules make up a big part of our own bodies, they are in the food we eat and in the clothes we wear. Organic compounds are also used to make products such as medicines, plastics, washing powders, dyes, along with a long list of other items. There are millions organic compounds found in nature, as well as millions of synthetic (man-made) organic compounds.

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