Organic Chemistry I

More notes about the branched alkyl groups:
The common names of the branched alkyl groups have been used broadly, and are adopted as part of the IUPAC system.
Understanding the origin of these common names is very helpful in distinguishing and memorizing the names.
Three-carbon branched alkyl groups
Both of the two 3-carbon branched alkyl groups come from propane. Since propane has two types of hydrogens, primary
(1°) and secondary (2°), so there are two alkyl groups depends on which H is removed.
Figure 2.2g The primary and secondary hydrogen of propane
Four-carbon branched alkyl groups
Out of the four 4-carbon branched alkyl groups, two come from butane and the other two come from isobutane (or
2-methylpropane).
2.2 Nomenclature of Alkanes | 53

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Figure 2.2h The primary and secondary hydrogen of butane
Figure 2.2i The primary and tertiary hydrogen of isobutane
IUPAC name of branched alkyl groups
The branched alkyl groups can also be named by the IUPAC rules. To do that, they are treated as if it were a compound
itself. Begin numbering at the point of attachment to the parent chain, and number the branches the same as before
to avoid confusion. The complex substituent name is put in parentheses when the name of the complete molecule is
written.
For the example of isobutyl below, the part that connect directly onto the parent chain has 3 carbons, so it is “propyl”.
There is another CH
3
on the 2
nd
carbon of propyl, therefore the whole group is called “2-methylpropyl”.
54 | 2.2 Nomenclature of Alkanes

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Figure 2.2j 2-methylpropyl or isobutyl
Naming of Cycloalkanes
Cycloalkanes are alkanes that contain a ring(s) as part of the structure. For the cycloalkane that contain one ring, there
are
two fewer
hydrogens than the non-cyclic alkane, so the general formula of cycloalkanes with one ring is