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268                               ORGANIC CHEMISTRY                              [CHAP. 18


                                             Table 18-3 Some Common Radicals

                              Parent Hydrocarbon                    Radical

                             Name     Line Formula  Name     Line Formula   Structural Formula

                                                                                H

                            Methane   CH 4          Methyl   CH 3 —         H   C
                                                                                H

                                                                               H   H
                            Ethane    CH 3 CH 3     Ethyl    CH 3 CH 2 —    H   C  C
                                                                                H   H

                                                                               H   H   H

                            Propane   CH 3 CH 2 CH 3  Propyl  CH 3 CH 2 CH 2 —  H  C  C  C
                                                                                H   H  H

                            Benzene   C 6 H 6       Phenyl   C 6 H 5 —






               from an alkane, the radical is called an alkyl radical; and if it is derived from an aromatic hydrocarbon, it is
               called an aryl radical.
                   The names of alkanes with radical branches use the name from Table 18-2 for the longest continuous chain
               of carbon atoms. The radicals are named using the -yl ending presented in Table 18-3, and their positions along
               the carbon chain are denoted by a number. For example,
                                           CH 3  CH 2  CH   CH 2  CH 2  CH 3

                                                       CH 3
               is called 3-methylhexane. The longest continuous carbon chain is six carbon atoms long, therefore it is a hexane.
               The single-carbon branch is the methyl group, which is on the third carbon atom from the left end of the chain.
               We actually start counting from the end of the chain that gives the lower number. In this case, the branch is on
               the fourth carbon atom from the right, so the smaller number is used. Be sure to use the longest continuous chain
               of carbon atoms; they are not always presented on a horizontal line.
                   If there are two or more branches, each is named. If there are two or more identical branches, the prefixes
               from Table 6-2 are used. Thus the following names:

                        CH 3  CH    CH   CH 2  CH 2  CH 3     CH 3  CH    CH   CH 2  CH 2  CH 3
                              CH 3 CH  2  CH 3                      CH 3 CH  3
                                 2-methyl-3-ethylhexane                 2,3-dimethylhexane

                   When a hydrogen atom of an alkane or aromatic hydrocarbon molecule is replaced by an atom of another
               element or group of atoms containing another element, the hydrocarbon-like part of the molecule is relatively
               inert, like these hydrocarbons themselves. Therefore, the resulting compound will have properties characteristic
               of the substituting group. Specific groups of atoms responsible for the characteristic properties of the com-
               pound are called functional groups. For the most part, organic compounds can be classified according to the
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