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# 4.8 Chapter summary

## 4.8 Chapter summary (ESCM8)

Presentation: 27QS

Presentation: 27QT

• Organic chemistry is the branch of chemistry that deals with organic molecules. An organic molecule is one that contains carbon.

• All living organisms contain carbon. Plants use sunlight to convert carbon dioxide in the air into organic compounds through the process of photosynthesis. Animals and other organisms then feed on plants to obtain their own organic compounds. Fossil fuels are another important source of carbon.

• It is the unique properties of the carbon atom that give organic compounds certain properties.

• The carbon atom has four valence electrons, so it can bond with many other atoms, often resulting in long chain structures. It also forms mostly covalent bonds with the atoms that it bonds to.

• An organic compound can be represented in different ways, using its molecular formula, structural formula, semi-structural formula or condensed structural formula.

• A functional group is a particular group of atoms within a molecule, which give it certain reaction characteristics. Organic compounds can be grouped according to their functional group.

• The hydrocarbons are organic compounds that contain only carbon and hydrogen. They can be further divided into the alkanes, alkenes and alkynes, based on the type of bonds between the carbon atoms.

• The alkanes have only single bonds between their carbon atoms and are unreactive. The alkenes have at least one double bond between two of their carbon atoms and the alkynes have at least one triple bond between two of their carbon atoms. They are both more reactive than the alkanes.

• An alcohol is an organic compound that contains a hydroxyl group $$(-\text{OH})$$.

• The alcohols have a number of different uses including their use as a solvent, for medicinal purposes and in alcoholic drinks.

• The alcohols share a number of properties because of the hydroxyl group. The hydroxyl group affects the solubility of the alcohols (in water). Those with shorter carbon chains are generally more soluble, and those with longer chains are less soluble. The strong hydrogen bond between the hydrogen and oxygen atoms in the hydroxyl group gives alcohols a higher melting point and boiling point than the hydrocarbons.

• A carbonyl group consists of an oxygen atom that is double-bonded to a carbon atom. In a ketone the carbonyl group is in the middle of the carbon chain. In an aldehyde the carbonyl group is at the end of the chain.

• The carboxylic acids are organic acids that contain a carboxyl group with the formula $$-\text{COOH}$$. In a carboxyl group a carbon atom is double-bonded to an oxygen atom, and it is also bonded to a hydroxyl group.

• The carboxylic acids have weak acidic properties because the hydrogen atom is able to dissociate from the carboxyl group.

• An ester is formed when an alcohol reacts with a carboxylic acid. Esters contain a carbonyl functional group as well as an oxygen atom bonded to the carbonyl carbon atom and another carbon atom.

• A molecule is said to be saturated if it contains the maximum possible number of hydrogen (or other) atoms for that molecule. The alkanes are all saturated compounds.

• A molecule is unsaturated if it does not contain the maximum number of hydrogen atoms for that molecule. The alkenes and alkynes are examples of unsaturated molecules. If a double or triple bond is broken, more hydrogen (or other) atoms can be added to the molecule.

• If two compounds are isomers, it means that they have the same molecular formulae but different structural formulae.

• Organic compounds are named according to their functional group and its position in the molecule, the number of carbon atoms in the molecule and the position of any double and triple bonds. The IUPAC rules for nomenclature are used in the naming of organic molecules.

• Many of the properties of the hydrocarbons are determined by their molecular structure, the bonds between atoms and molecules, and their surface area.

• The melting points and boiling points of the hydrocarbons increases as their number of carbon atoms increases.

• The molecular mass of the hydrocarbons determines whether they will be in the gaseous, liquid or solid phase at specific temperatures.

• The weaker the intermolecular forces between molecules the more volatile and the higher the vapour pressure of that compound. Haloalkanes become less volatile the more halogen atoms they contain. This is due to increased intermolecular forces.

• Three types of reactions that occur are addition, elimination and substitution. The alkenes undergo addition reactions because they are unsaturated.

• A polymer is a macromolecule that is made up of many repeating structural units called monomers, which are joined by covalent bonds.

• Polymers that contain carbon atoms in the main chain are called organic polymers.

• Organic polymers can be divided into natural organic polymers (e.g. natural rubber) or synthetic organic polymers (e.g. polystyrene).

• The polymer polyethene for example, is made up of many ethene monomers that have been joined into a polymer chain.

• Polymers form through a process called polymerisation. Two examples of polymerisation reactions are addition and condensation reactions.

• An addition polymerisation reaction occurs when unsaturated monomers (e.g. alkenes) are added to each other one by one. The breaking of a double bond between carbon atoms in the monomer means that a bond can form with the next monomer.

• In a condensation polymerisation reaction a molecule of water is released as a product of the reaction. The water molecule is made up of atoms that have been lost from each of the monomers.

• The chemical properties of polymers (e.g. ability to withstand stress and melting point) are determined by the types of atoms in the polymer, and by the strength of the bonds between adjacent polymer chains. The stronger the bonds, the greater the strength of the polymer, and the higher its melting point.

• Polyethene, polypropene, polyvinyl chloride, polyvinyl acetate and polystyrene are all polymers formed through addition polymerisation.

• Polyethylene terephthalate and polylactic acid are polymers formed through condensation polymerisation.

• Plastics are a subgroup of organic polymers that can be molded. They can contain more than one polymer.

• It is not easy to recycle all plastics and so they create environmental problems.

• Some of these environmental problems include issues of waste disposal, air pollution and recycling.

• Polylactic acid is an example of a biodegradable polymer made from renewable resources.

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