Alkanes and Cycloalkanes; Conformational and Geometric Isomerism - Cengage Learning - page 19

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Alkanes and Cycloalkanes; Conformational and
Geometric Isomerism
Chapter Summary
Hydrocarbons contain only carbon and hydrogen atoms. Alkanes are acyclic saturated
hydrocarbons (contain only single bonds); cycloalkanes are similar but have carbon rings.
Alkanes have the general molecular formula C n H 2 n+ 2 . The first four members of this
homologous series are methane , ethane , propane , and butane ; each member differs
from the next by a -CH 2 -, or methylene group . The IUPAC (International Union of Pure
and Applied Chemistry) nomenclature system is used worldwide to name organic
compounds. The IUPAC rules for naming alkanes are described in Secs. 2.3-2.5. Alkyl
groups , alkanes minus one hydrogen atom, are named similarly except that the -ane ending
is changed to - yl . The letter R stands for any alkyl group.
The two main natural sources of alkanes are natural gas and petroleum . Alkanes
are insoluble in and less dense than water. Their boiling points increase with molecular
weight and, for isomers, decrease with chain branching.
Conformations are different structures that are interconvertible by rotation about
single bonds. For ethane (and in general), the staggered conformation is more stable than
the eclipsed conformation (Figure 2.5).
The prefix cyclo - is used to name cycloalkanes. Cyclopropane is planar, but larger
carbon rings are puckered. Cyclohexane exists mainly in a chair conformation with all
bonds on adjacent carbons staggered. One bond on each carbon is axial (perpendicular to
the mean carbon plane); the other is equatorial (roughly in that plane). The conformations
can be interconverted by “flipping” the ring, which requires only bond rotation and occurs
rapidly at room temperature for cyclohexane. Ring substituents usually prefer the less
crowded, equatorial position.
Stereoisomers have the same order of atom attachments but different
arrangements of the atoms in space. Cis - trans isomerism is one kind of stereoisomerism.
For example, two substituents on a cycloalkane can be on either the same ( cis ) or opposite
( trans ) sides of the mean ring plane. Stereoisomers can be divided into two groups,
conformational isomers (interconvertible by bond rotation) and configurational isomers
(not interconvertible by bond rotation). Cis-trans isomers belong to the latter class.
Alkanes are fuels; they burn in air if ignited. Complete combustion gives carbon dioxide and
water; less complete combustion gives carbon monoxide or other less oxidized forms of
carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or
light. One or more hydrogens can be replaced by halogens. This substitution reaction
occurs by a free-radical chain mechanism .