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This does not compute for me:

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Why would the four-carbon butyl group have a lower relative priority than the 3 carbon isopropyl group?

If we compare the carbons one by one between the isopropyl and the butyl group, we'd run out of carbons in the isopropyl group first! How then can isopropyl possibly have a higher priority than the butyl group?

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Comparing IsoPropyl and n-Butyl

  • On first position both have C.
  • On second position IsoPropyl has two C's whereas n-Butyl has only one C.
  • Thus IsoPropyl has higher priority and we do not see the further chain.
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  • $\begingroup$ Thank you. I referred again to a text (a different one) and it became more clear to me that we first compare the ones attached to the carbon one away from the chiarlity center, rather than simply span outward. $\endgroup$ – Dissenter Jun 5 '14 at 3:47
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If two groups have the same first atom, then compare the second atom from the chiral center. If there are multiple second atoms, then compare them in order of atomic number.

from http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/5a.vii.html, step 1, part 2

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