Assigning Priorities in Ochem

[mario0923]

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in Qvault I thought it was 1R-2R-3S

but Qvault says 1R-2S-3S

for assigning R,S for C2
C1 (left of C2) has OH and R group and hydrogen
C2 has on its own an ethyl group and hydrogen
C3 (on the right of C2) has methyl group and R group and hydrogen

so C1 has the 1st priority, ethyl group 2nd, C3 3rd and hydrogen (in the back) 4th

it will be clockwise so C2 is R

but if I assign ethyl group as 1st and C1 as 2nd then I will have S which is the correct answer according to Qvault??

 

[ballyhoo]

Hm...I just assumed that CH2CH3 was the first priority, and now that I look at it, you're right it is weird...

 

[mario0923]

seriously no one can answer this...? come on guys!!

 

[biomolecmed]

choice E is correct. OH has a priority, so from carbon with OH it is R, S,S 2 Ethyl, 3 methylcyclohexanol

 

[mario0923]

choice E is correct. OH has a priority, so from carbon with OH it is R, S,S 2 Ethyl, 3 methylcyclohexanol

what do you mean by OH has a priority?
can you explain me how you got S for second carbon?

I get the R for second carbon because carbon with OH has the first priority, ethyl the second, carbon with methyl third and hydrogen in the back fourth

for assigning priority, first you compare the atom number of the neighbor atoms (in this case for the second chiral center we have all carbons)
if you have all same atoms then you compare the next closest atoms' atom number which C1 has O and ethyl group has C which means C1 has first priority and ethyl group has second. then I will get R
am I wrong?

here is the picture for clarification
http://forums.studentdoctor.net/attachment.php?attachmentid=20168&stc=1&d=1340594110

 

[TailorMade]

I'm hoping you've just found an error because I'm getting R as well haha

 

[kl352]

the second carbon is an S because you need to compare the substituents on each of the carbons around C2.

C1 has O, C, H
C2 (the ethyl) has C, H, H
C3 has C, C, H attached to it. (methyl group and part of the cyclehexane group)

when ranking substituents, always compare using highest weight so thus C1 is ranked 1st since the oxygen higher priority. now we look between C2 and C3. C2 is bonded to 1 carbon, 2 hydrogens and C3 is bonded to 2 carbons, 1 hydrogen so C3 has higher priority.

C1 > C3 > C2 so it's S

hope that helps

 

[mario0923]

the second carbon is an S because you need to compare the substituents on each of the carbons around C2.

C1 has O, C, H
C2 (the ethyl) has C, H, H
C3 has C, C, H attached to it. (methyl group and part of the cyclehexane group)

when ranking substituents, always compare using highest weight so thus C1 is ranked 1st since the oxygen higher priority. now we look between C2 and C3. C2 is bonded to 1 carbon, 2 hydrogens and C3 is bonded to 2 carbons, 1 hydrogen so C3 has higher priority.

C1 > C3 > C2 so it's S

hope that helps

wow right....lol
I feel so stupid
thank you so much🙂