If a decision can't be reached by ranking the first atoms in the substituent, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. A -CH2CH3 substituent and a -CH3 substituent are equivalent by rule I because both have carbon as the first atom. By rule 2, however, ethyl receives higher priority than methyl because ethyl has a carbon as its highest second atom, while methyl has only hydrogen as its second atom. Look at the following example to see how the rule works:
If a decision can't be reached by ranking the first atoms in the substituent, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. A -CH2CH3 substituent and a -CH3 substituent are equivalent by rule I because both have carbon as the first atom. By rule 2, however, ethyl receives higher priority than methyl because ethyl has a carbon as its highest second atom, while methyl has only hydrogen as its second atom. Look at the following example to see how the rule works:
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