2: Alkanes and Cycloalkanes:
The numbering pattern of the ring is chosen so as to give the lowest number to the substituent at the
first point of difference between
them. Thus, the carbon bearing
two methyl groups
is C-1, and the
ring is numbered counterclockwise,
placing the isopropyl group on C-4 (numbering clockwise would
place the isopropyl on C-8). Listing the substituent groups
in alphabetical order, the correct name is
4-isopropyl-l,l-dimethylcyclodecane. Alternatively, the systematic name for isopropyl
(1-methylethyl) could be used, and the name would become
I ode c ane.
(c) When two cycloalkyl groups are attached by a single bond, the compound
is named as a cycloalkyl-
substituted cycloalkane. This compound
2.12 The alkane
that has the most carbons (nonane) has the highest boiling point (151'C). Among the others,
all of which have eight carbons,
isomer (octane) has the highest boiling point (126"C)
and the most branched
the lowest (106'C). The remaining alkane,
boils at 116"C.
All hydrocarbons burn in air to give carbon dioxide and water. To balance
combustion of cyclohexane (C6H12),
the carbons and the hydrogens on the right side.
the oxygens on the left side.
6co2 + 6Hzo
(Table 2.2) is CzoHqz.
It has four more methylene (CHz) groups than hexadecane,
in Table 2.3. Its calculated heat of combustion
is'therefore (4x653 kUmol)
Heat of combustion of icosane
heat of combustion of hexadecane
+ (4 x 653 kUmol)
Two factors that influence the heats of combustion of alkanes are, in order of decreasing
(1) the number of carbon atoms and (2) the extent of chain branching. Pentane, 2-methylbutane, and
are all CsHrz; hexane
is C6H1a. Hexane has the largest heat of combustion.
to a lower heat of combustion; 2,2-dimethylpropane
is the most branched and has the
lowest heat of combustion.