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comparing the cahn-ingold prelog priority of the above groups, I should first compare the atoms attached to the first carbon. So we get:

(A): {O,C,H}

(B): {O,C,H}

(C): {O,C,H}

(D): {O,C,H}

therefore we have to consider the second level, and this is where I get confused: Do I compare it this way:





I did this by rearranging all the "second level atoms" in descending order of atomic numbers.

since A and D are still in a tie, I rearrange all the "third level atoms" in descending order of atomic numbers:



so the priority is C>B>D>A Is that correct?

  • 1
    $\begingroup$ In your examples you have made the decision because you have{CHO}. Three different groups.. If you had{CCO) for example, you would have to go to a second level to break the C C tie. That is if I understand your question correctly. Take a look here: ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/isomers/… $\endgroup$
    – user55119
    Commented May 8, 2021 at 2:50

1 Answer 1


look, all i can say is that $ D > C > A $
why ?
Lets compare A and C
$ OCH$3 and $ H $ are common to both. one has CH3 and the other has CH2F. of course, C > A.
compare A and D in the same way and find that D > A
compare D and C now.
there is clear proof that D > C because it has F in two places while C has H in those two exact positions.

thus it is seen that D > C > A

now see B and C
in B : after O we have CH3 and after C we have { H , H , F }
in C : after O we have H and after C we have { H , H , O }
looking at second layer after oxygens : B > C
looking at second layer after carbons : C > B.

thus even i am confused when it comes to B

but definitely D > C > A

  • 1
    $\begingroup$ You have avoid B on your final grading. What about B? $\endgroup$ Commented May 8, 2021 at 11:53
  • $\begingroup$ i said i dont know about B. this is all i can say and it is pretty fair that i at least provided a solution till here $\endgroup$ Commented May 10, 2021 at 15:20

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