Hydrocarbons and functional groups

 

Aliphatic compound. The family of carboxylic acids contains a carboxyl

(-COOH) functional group. Acetic acid is an example.Classification normally starts with the hydrocarbons: compounds which contain only carbon and hydrogen. For sub-classes see below. Other elements present themselves in atomic configurations called functional groups which have decisive influence on the chemical and physical characteristics of the compound; thus those containing the same atomic formations have similar characteristics, which may be: miscibility with water, acidity/alkalinity, chemical reactivity, oxidation resistance, and others. Some functional groups are also radicals, similar to those in inorganic chemistry, defined as polar atomic configurations which pass during chemical reactions from one chemical compound into another without change.

     Some of the elements of the functional groups (O, S, N, halogens) may stand alone and the group name is not strictly appropriate, but because of their decisive effect on the way they modify the characteristics of the hydrocarbons in which they are present they are classed with the functional groups, and their specific effect on the properties lends excellent means for characterisation and classification.

     Referring to the hydrocarbon types below, many, if not all of the functional groups which are typically present within aliphatic compounds are also represented within the aromatic and alicyclic group of compounds, unless they are dehydrated, which would lead to non-reacting co-optional groups.

Reference is made here again to the organic nomenclature, which shows an extensive (if not comprehensive) number of classes of compounds according to the presence of various functional groups, based on the IUPAC recommendations, but also some based on trivial names. Putting compounds in sub-classes becomes more difficult when more than one functional group is present.

     Two overarching chain type categories exist: Open Chain aliphatic compounds and Closed Chain cyclic compounds. Those in which both open chain and cyclic parts are present are normally classed with the latter.

Aliphatic compounds

     The aliphatic hydrocarbons are subdivided into three groups, homologous series according to their state of saturation: paraffins alkanes without any double or triple bonds, olefins alkenes with double bonds, which can be mono-olefins with a single double bond, di-olefins, or di-enes with two, or poly-olefins with more. The third group with a triple bond is named after the name of the shortest member of the homologue series as the acetylenes alkynes. The rest of the group is classed according to the functional groups present.

     From another aspect aliphatics can be straight chain or branched chain compounds, and the degree of branching also affects characteristics, like octane number or cetane number in petroleum chemistry.