Organic families and reactions: TCE Chemistry (Tasmania)

What this dot point is asking

Organic chemistry is the study of carbon compounds. Carbon forms four covalent bonds and can link into chains, branches and rings, giving an enormous variety of molecules. Compounds are organised into families called homologous series, where each member differs from the next by a $CH_2$ unit and shares the same general formula and similar chemical properties, with a gradual change in physical properties such as boiling point.

The functional group is the reactive part of a molecule and determines its chemistry. Alkanes contain only single carbon-carbon bonds and are saturated. Alkenes contain a carbon-carbon double bond and are unsaturated. Other key families include haloalkanes (a halogen atom), alcohols (the hydroxyl group $-\text{OH}$), carboxylic acids (the $-\text{COOH}$ group), and esters (the $-\text{COO}-$ linkage). Knowing the functional group lets you predict both the name and the reactions.

IUPAC naming follows a systematic set of rules. Identify the longest carbon chain containing the functional group to give the stem (meth, eth, prop, but, pent and so on). The suffix indicates the family: -ane for alkanes, -ene for alkenes, -ol for alcohols, and -oic acid for carboxylic acids. Number the chain to give the functional group the lowest possible locant, and name branches as prefixes with their position numbers. For example, $CH_3CH(OH)CH_3$ is propan-2-ol.

Isomerism arises because the same molecular formula can give different structures. Structural isomers differ in the connectivity of atoms, such as a straight chain versus a branched chain, or different positions of a functional group. Isomers can have markedly different physical and chemical properties despite sharing a molecular formula.

Each family has characteristic reactions. Alkanes are relatively unreactive but undergo substitution with halogens in the presence of ultraviolet light, where a hydrogen atom is replaced by a halogen atom. They also undergo complete combustion to give carbon dioxide and water, releasing large amounts of energy.

Alkenes are much more reactive because of the double bond. They undergo addition reactions, where atoms add across the double bond and it becomes a single bond. Examples include hydrogenation (adding $H_2$ with a catalyst), halogenation (adding $Br_2$, which decolourises bromine water and is a common test for unsaturation), and hydration (adding water to form an alcohol).

Alcohols undergo several important reactions. Primary and secondary alcohols can be oxidised by oxidants such as acidified potassium permanganate or acidified potassium dichromate. A primary alcohol oxidises first to an aldehyde and then to a carboxylic acid, while a secondary alcohol oxidises to a ketone. Tertiary alcohols resist oxidation because the carbon bearing the hydroxyl group has no hydrogen to remove. Alcohols also undergo combustion and dehydration to form alkenes.

Carboxylic acids are weak acids that react with bases and carbonates in the usual acid reactions. They also react with alcohols in an esterification (condensation) reaction, catalysed by concentrated sulfuric acid, to produce an ester and water. Esters often have pleasant fruity smells and are used as flavourings and fragrances.

In exams, draw structures clearly, name compounds using correct IUPAC rules with locants, and link each reaction type to the functional group present.