Alkanes

Overview:

Alkanes begins a study of the first homologous group and addresses nomenclature (including cyclics and bicyclics), physical properties, reactions, Baeyer Strain theory, types of isomers, and conformations.

In nomenclature of alkanes, the terms important to organic chemistry are introduced: homologs, structural isomers, IUPAC and systematic nomenclature, branches, and assigning addresses. The student is given numerous opportunities to name alkanes using the rules of systematic nomenclature. A very clear form of determining the parent chain is shown, even when the chain appears to take twists and turns. It is the beauty of animation that allows an unbroken line to trace the backbone of the compound. Some of the more common prefixes are introduced: iso, sec (secondary), tert (tertiary), n(normal), and neo, to name a few. The use and non-use of these prefixes in alphabetizing are also explained.

Nomenclature of cyclic alkanes seems a perfect way to really address the use of complex nomenclature, and this is where complexes are discussed. Secondary chains are introduced and terms such as bis, tris, and tetrakis become part of the organic student’s vocabulary.
Physical properties of alkanes are addressed and the prediction of patterns is strongly emphasized. Students become acquainted with the impact adding another methylene group can make on the boiling point of a compound, as well as the impact of branching within a specific number of carbons. How compounds are ranked with regard to physical properties is stressed. The interesting phenomenon in progression of melting point of alkanes is addressed.

The general characteristics of alkane reactions is discussed and a separate section is devoted to combustion, cracking (and hydrocracking), and halogenation. Halogenation is only mentioned briefly, but is addressed extensively in a later unit entitled “Chemical Reactions.”

Constitutional, conformational, geometric, and stereoisomers are introduced in this unit. Special emphasis is placed on conformational isomers and the energy needed to allow rotation about single bonds to occur. The nature of energy transitions producing staggered, eclipsed, butane gauche, and such conformations is described, diagrammed, and explained. Rotation about a single bond is animated using Newman projections in such a manner that the energy and torsional strain corresponding to each point becomes clear. The concept of cis and trans structures is introduced with respect to cyclic compounds.

A study of alkanes is not complete with addressing Baeyer strain theory and the importance of “pucker” in cyclic molecules. Characteristics of conformers, particularly chair, boat, and twist conformers of cyclic alkanes are addressed and diagrammed extensively. The use of digital photography of molecular models, coupled with line-angle structures helps bring clarity to this interesting and sometimes difficult topic in alkanes and serves to reinforce topics addressed earlier in the unit.

The nomenclature of bicyclics includes fused rings, bridged bicyclics, and spirocyclics. Students will find this an interesting and challenging variation on the usual rules of alkane nomenclature.