the alkanes

The Alkanes


                      Conformational Isomerism in Substituted Cyclohexanes
In a monosubstituted cyclohexane, the substituent can be either in an
equatorial or axial position. Equatorial positions are more spacious and in
substituted cyclohexanes they are preferred. Cyclohexane rings flip between
chair forms and establish an equilibrium. In the process of flipping, all
equatorial positions become axial and all axial positions become equatorial.
The equilibrium favors the chair in which substituents are equatorial. In
monosubstituted cyclohexanes, the conformation in which the substituent is
equatorial is favored. In disubstituted cyclohexanes where one group is axial
and one equatorial, the equilibrium favors the chair form where the larger
group occupies the more spacious equatorial position

                   Geometric Isomerism in Cyclic Compounds
Disubstituted cycloalkanes exhibit geometric isomerism, a type of
stereoisomerism. If both groups are on the same "side" of the ring (both up
or both down) the isomer is termed cis. If they are on opposite "sides" (one
up and one down) the isomer is trans.

 Hydrocarbons: Relationship of Structure and Physical Properties
The solid, liquid, and gas states of a compound differ in arrangements of
molecules, not in molecular structure. In the solid state the molecules are orderly
arranged and immobile with maximum intermolecular attractions. In the liquid
state, molecules are mobile but still there are intermolecular attractions. In the
vapor phase molecules are mobile and ideally there are no intermolecular
attractions. For these reasons, solids have a constant shape and volume, liquids
have a constant volume and variable shape, and gases assume the size and
shape of the container. Physical properties of hydrocarbons are related to
structure.
    A. Melting Point, Boiling Point, and Molecular Weight
Melting points and boiling points generally increase with molecular weight
within a homologous series (a series of compounds in which each
succeeding member differs from the previous one by a CH2 group).
   B. Melting Point, Boiling Point, and Molecular Structure
Branched chain hydrocarbons have less surface area and thus less
opportunity for intermolecular attractions; as a result, their boiling points are
lower than the straight chain isomers. However, their compact nature causes